Having spent a decent amount (let's not get into exactly how much), on your indoor jungle, of course you want to look after your green buddies as best you can. Soon after most of us start the hobby, we also start the hunt for 'the perfect fertiliser'. And boy can that get confusing real fast.
How do you figure out which fertiliser is 'best'?
Every fertiliser company seems to claim the same things. But surely they can't all be the same, right? Do you just buy the big brand you've heard of? Or the one that geeks you out with the most impressive sounding science? Maybe you get a second opinion and ask what your fellow plant besties feed their jungle?
Not to mention what plant-tubers and planty-influencers use (no, not the sponsored stuff, what they actually use), especially if they grow their plants to sell. After all, they rely on plants for their livelihood, so what they feed really matters, especially considering the value of their plant collections! Plus they often get gifted all the fertilisers to try, so if one works better for them than others, they have done the testing for you.
...or you could go down the education route, and find out what's really going on underneath the pretty label and behind the clever marketing
If you prefer to do the research and then 'test the best' for yourself - or you're just a plant-geek like me and want to know what your plants need and why - this guide is for you. But if you just want me to help you pick the best one to match your plants and your needs, email me instead.
But be warned, this is one of those guides you'll probably skim through right now, but will want to bookmark for later, as there's so much to learn. But by the end of this I hope you'll feel upskilled enough to make an educated decision for yourself on the best fertiliser for your indoor plants.
Has this happened to you too?
When I started in the hobby all was well for the first growing season. Easy peasy. Light, warmth, water. Job done. Okay, there may have been an infestation of fungus gnats that got a bit out of control (here's how I got rid of them >), but other than that, plain sailing.
Then something happened. I started noticing the occasional yellow leaf. Then more of them. My prized Watermelon Peperomia started giving me split, cracked leaves. My Brasil started randomly putting out smaller leaves than normal. I noticed a growing list of little niggles. Nothing major at first. But I couldn't think of anything I'd changed. And I hadn't. That was the problem.
It was because of what I WASN'T doing, not what I was
All were signs my plants were running low on the nutrients they had come packed full of from the nursery. Of course commercial growers fertilise. After all, they want their plants to grow big and healthy as fast as possible so they are ready to sell sooner. But my plants had 'run out of fuel' because I wasn't fertilising.
Ready? Set? Grow!
So let's do this. Go grab a cuppa. Chuck on a lab coat (I'm one step ahead of you as I already wear glasses), and let's sort out what to look for - and to watch out for - to find the best fertiliser for indoor plants together.
Just kidding. Well, about the lab coat bit. Put down that dictionary. Don't worry. I won't actually go hardcore plant-geek on you. I prefer to simplify, and take the guesswork and confusion out of plant parenting.
One of my (many) favourite sayings is:
"If you can't explain it simply, you don't understand it well enough"
Can the real fertiliser please stand up?
Like nutrition for people, we're always learning more about what our plants need. And like us, almost any food is better than no food at all. And yes, fad diets definitely do the rounds in the plant world too, just like they do in the human world. So before we geek out (a little bit), on the science, let's get a few things straight.
Firstly, not everything is fertiliser. And just because something IS a fertiliser, does NOT mean it gives plants everything they need.
Secondly, it's easy to call everything 'food' and ask 'what do you feed?' but our clever plants actually make their own food.
Elements and minerals from air, water, fertiliser, soil and other sources, give our plants the ingredients they need to make their own food. I tend to shift between saying 'fertiliser' and 'food' but I don't want to upset the purists, so thought I'd get that distinction out of the way. And yes, I am simplifying all this (as you will soon find out, analogies are my friend).
One of these things is not like the others
But here's the thing. A lot of what we think are fertilisers, are actually 'additives' and by themselves, are not enough for our plants to thrive. That's another reason it gets confusing. You think you're fertilising when you're not and wondering why your plant is still not living its best life.
If you imagine a good fertiliser as a complete and balanced meal, then almost everything else is like a supplement. Do you remember what the vitamin and mineral supplement adverts always say really fast at the end of every ad? "Does not replace a balanced diet" Yep, that applies to our plants too.
Groconut? Not a fertiliser. HB-101? Not a fertiliser. Seaweed, worm tea, epsom salts, banana skin tea, dolomite lime, Clonex, Crazy Keiki, Urban Botanist, Mag-Pro... all not a fertiliser.
So all these growth boosters and root boosters, cloning pastes and health tonics are more like supplements, not a 'complete and balanced diet'. That's why a lot of plant hobbyists 'mix-feed'.
What is mix-feeding?
Mix-feeding is simply when you feed your plants more than one thing. For most of us indoor plant hobbyists that will be a combo of a growth booster plus a fertiliser. My plants get a growth booster and complete fertiliser every time I water, all year-round, plus a seaweed feed about once a month (or when I remember). More about how often to feed, whether to feed in winter, and about seaweed, all coming up.
4 of the most popular combos houseplant hobbyists mix-feed here in NZ are either HB101 or Groconut (those are both growth boosters), combined with Growth Technology (GT) (fertiliser), also Groconut + Plant Runner Indoor Plant Food or Groconut + Dyna-Gro (yes, Groconut is very popular)... and the last is seaweed + everything!
Side note for seaweed fans: A unique benefit of Plant Runner Indoor Plant Food is that it combines an NPK fertiliser plus seaweed all in one formula.
But you do NOT have to mix feed. Find a great fertiliser and stop there by all means. That by itself will make the biggest difference to your plants - no growth boosters needed - and if you choose a good one, that will give your plants all the essential minerals they need.
Plus your plant cupboard fills up really fast if you buy everything that's beneficial for your plants. You should see mine (a common side-effect of owning a plant store). And yes, knowing the difference between beneficial and essential is key. More about that coming up too.
Can growth boosters replace fertiliser?
Growth boosters and other plant supplements offer our plants goodies like vitamins, minerals and growth hormones that can boost growth and work all sorts of magic. They promise results such as helping plants get bigger faster, be greener, flower, flower longer, be stronger, grow roots faster, fix cracked leaves, grow bushier, activate dormant plants, be less likely to be a bug's next snack (and more).
However the answer is no, they are not complete fertilisers and do not offer all the essential minerals, in the right amounts, that our plants HAVE to have to be their best selves long-term.
That's why one thing you'll find fertilisers do claim, that growth boosters don't, is that they are 'complete and balanced'. That's a pretty big claim by the way, which we'll cover too in this guide.
Why growth boosters 'stop working'
Imagine growth boosters like putting your foot down on the accelerator in your car. Growth boosters like Groconut and HB-101 do boost growth, sure, but your plant still needs all the essential elements to fuel that faster growth. If you run out of fuel, it doesn't matter how much you slam your foot down on the accelerator, eventually you'll be running on fumes, then come to a stop.
Using growth boosters by themselves often starts out amazing. What a difference! Rave reviews all round. But over time that growth slows down, then stops. Did the growth booster stop working? No. Your plant just ran out of fuel! Fertiliser fills the tank back up with essential minerals, ready to keep fuelling more growth.
On the topic of amazing results: Some clever marketing to watch out for. You know those amazing 'with versus without' photos and testimonials companies love sharing about their products? Keep in mind almost anything is better than nothing. If a company shows you a plant given nothing but water compared to being given their product, of course the plant given their product will be doing better.
Using that sort of 'test' would make a bag of lollies the ultimate food compared to starving yourself
Instead, look for 'like with like' comparisons (such as comparing how much better a plant does on their fertiliser or growth booster, versus the competitor's product that claims the same benefits as theirs).
Fertiliser fact or fiction?
Speaking of clever marketing, understanding what's underneath the label, inside that bottle, can require a little bit of work separating fact from fiction. Probably the most confusing word used to describe fertilisers is 'complete'.
We know there are 12 essential minerals, yet most fertilisers list 3 on the label, and others list more than 12, and yet they all say they are complete. When it comes to fertiliser, complete doesn't mean what you think it means.
What 'complete' really means
To be able to claim a fertiliser is 'complete' it needs to provide plants with the 3 primary macro-minerals: nitrogen, phosphorus and potassium.
See those 3 numbers on your fertiliser bottle? Like 12-2-12 or 7-9-5? They tell you the 'NPK' levels in the bottle. N is the symbol for nitrogen, P is for phosphorus and K is for potassium. So 12-2-12 tells you the amount of nitrogen, (12), phosphorus (2) and potassium (12) in the bottle, before it's diluted.
For plant-geeks like me now wondering why Potassium gets the symbol K: Potassium, also called potash, gets the symbol K from 'Kalium' which is Mediaeval Latin for 'pot ash'.
Why 'complete' doesn't give you the whole story, is because there are not just 3 essential minerals. There are 12 essential minerals all plants need (and a handful of extras called beneficial minerals that some plants need).
It's not that calling a fertiliser complete is false advertising, it is technically correct, but it can get confusing unless you check the label (and know what else to look for).
What are the essential minerals?
The 12 proven essential minerals include 6 macro-nutrients that plants need more of, and 6 micro-nutrients they need less of (but that are still essential).
- Top of the list are nitrogen, phosphorus and potassium. They are the 3 primary macronutrients.
- Then you have 3 secondary macronutrients: calcium, magnesium and sulfur.
- Then last are the 6 micronutrients plants only need small amounts of, which are iron, boron, copper, manganese, molybdenum and zinc.
Since saying a fertiliser is 'complete' doesn't necessarily mean a food offers all 12 essential minerals, rather than stop there, check the label or supplier's website for the guaranteed analysis. That will tell you if all 12 essential minerals are in the fertiliser. Some fertilisers include extra non-essential nutrients also.
That's another benefit mix-feeding can offer. Depending on what you choose to add with your fertiliser, mix feeding can 'fill in the blanks' to help provide what your fertiliser might be missing.
My go-to is seaweed. A superfood that offers over 70 essential and beneficial elements. The brand I use is BioPower Organic Seaweed. I give my plants a seaweed boost in their water about once a month (or when I remember), all year round. Seaweed's also handy to help plants recover after repotting (repotting for plants, is like surgery is for us), so if you need to repot, do this before and after repotting > for a speedy recovery with less risk of complications.
The calcium controversy
We know calcium is an essential macronutrient - but what's not well known is most fertilisers do NOT include calcium, even those that are by definition 'complete'.
Different plants vary in their requirements for calcium depending on a range of factors, such as the type of plant, stage of growth, soil pH and levels of other minerals, but we do know some averages.
How much calcium do plants need?
Calcium requirements vary from around 50 to 200 ppm (ppm means parts per million). In soil-less media the range varies from around 40 to 100ppm. In hydroponics (growing in water) it varies from around 80 to 140ppm and in soil, up to 200ppm of calcium is needed (note these are averages only).
Your tap water likely has a little calcium in it already. If you're also in New Zealand like me, you can look up your water supply. For example, I'm in Auckland, and depending on where you live in Auckland, your tap water might come from Onehunga or the Waikato, and averages around 9 to 19 ppm calcium. Better than nothing, but far short of the range plants need, so for our indoor plants, we need to supply what's missing.
What are the signs a plant might have a calcium deficiency?
Without enough calcium, you may see symptoms such as:
- Split, cracked leaf edges (that turned out to be a problem for my Watermelon Peperomia)
- weak stems
- stunted or deformed new growth
- peduncles and flowers buds that die and fall off before flowering
- curled, cupped leaves not caused by watering
- foliage that starts by browning along the edges, spreads and dies early
- edema, causing foliage to go translucent or have brown or black spots appear after watering (learn more about what is edema and how to fix it >)
The bad news (and the good news) about calcium
The reason calcium is the problem-child of the macronutrients family is because calcium is not mobile. In fact it's the only macronutrient that isn't mobile. And macronutrients are what our plants need the most of. Not good.
If there's a calcium deficiency in a new leaf when it's forming, the bad news is that's a deficiency for the often shorter or damaged life of that leaf (or root, or flower bud).
Luckily, many essential nutrients are mobile. Nitrogen, phosphorus, potassium, magnesium, sulfur... all mobile. Let's say your plant is deficient in nitrogen. First thing you'll probably notice is yellowing of the older leaves.
That's because nitrogen IS mobile, so what the roots can't provide, your plant will take from older leaves, sacrificing them to keep newer leaves alive and to fuel new growth. But our plants can't do that with calcium.
Never cut off yellowing leaves!
On the topic of yellowing leaves, put down the snips. Leave those yellow leaves on. They are your plant's life support system right now. Learn more about the causes and fixes for yellowing leaves >
How much calcium is in fertiliser?
Most fertilisers don't include calcium, so there are other solutions below for ways to add calcium if your fertiliser doesn't. However to help answer this one, I checked a couple that do include calcium: Dyna-Gro Foliage Pro and GT Foliage Focus.
Dose of course makes a big difference. Like most fertilisers, both those brands give you a range to use their products at depending on your plant's needs. Calcium levels varied from 20ppm (in Dyna-Gro, based on the higher end of the dose range, of 1ml per 1 litre), up to 100ppm (in GT, again based on a higher dose of 10mls per litre).
For the plant geeks: To compare minerals ppm levels for yourself, you'll need to work out ppm. An online ppm calculator makes this easy (this is the one I use). You need the calcium as %w/v (the fertiliser label will normally tell you that), and the dose as ml's per litre (again, the label should help you work that out also), and the calculator will tell you ppm to help you compare different fertilisers with different doses.
How to fix a calcium deficiency
The good news is you can correct a calcium deficiency for future growth, however unless your fertiliser is one of the few that does include calcium, a simple solution is a calcium supplement like a sprinkle of dolomite lime once a season, or a cal-mag supplement to mix-feed with your fertiliser. I would recommend staying away from eggshells or milk-water for our indoor plants.
What about the risk of fertiliser burn?
Yes, this is a risk that comes with fertilising. However 'starving' your plants and not fertilising at all is much worse for them, and much more likely to harm your plants, compared to fertiliser burn.
Don't let worry about fertiliser burn stop you from feeding your plants. There are a few easy ways to avoid fertiliser burn that will protect your plants while giving them everything they need.
What causes fertiliser burn?
Fertiliser burn happens when minerals salts from fertiliser get to high concentrations in the soil. As salt levels build-up, a whole range of symptoms appear, including loss of lower leaves, dry, brown 'burnt' leaf tips and leaf edges; and below the surface, drying and dying roots, as well as changes to soil pH.
When soil pH goes outside the ideal healthy zone for our indoor plants (a slightly acidic pH of 6 is ideal for most houseplants), that can also turn some nutrients toxic and prevent others from being absorbed, called 'nutrient lockout'. For example, the wrong soil pH can cause a calcium deficiency even if your fertiliser does include calcium.
The reason it's called fertiliser 'burn' is because plants dry up and look scorched
That's because those excess salts are literally sucking the moisture out of your plant!
The good news is there are multiple factors you can control that will decrease the risk. Simply changing the way you water is the easiest way to avoid fertiliser burn, no matter what you feed. Changing the way you feed can also make a big difference. More about all what to do (and not do), to reduce the risk of fertiliser burn coming up.
Variegated plants are more at risk
Some plants are more salt-sensitive than others, including variegated plants. As most variegated plants grow slower than their all-green buddies, they are more prone to fertiliser burn and need a different approach (I use the 'weakly weekly' method), however there are entire plant families that are more salt sensitive than others (and on the flipside, also those that are more salt tolerant, like coastal plants). Learn why care is different for variegated plants >
Take extra care with baby plants
Another risk factor is when a plant is very young. Their more delicate, less established root system is more likely to be damaged by excess fertiliser salts. What you feed, how you feed, and how you water at this stage of a plant's life is extra important. That includes during propagation.
3 ways to avoid fertiliser burn
(1) look for a low-salt fertiliser
(2) top water, not bottom water
(3) use the 'weakly weekly' method
Let's unpack each of these below...
#1 Choose a low salt fertiliser
Hold on. What is salt doing in fertiliser?!
When you think of salt, you might think of table salt (sodium chloride), but you're not likely find sodium chloride listed in your fertiliser ingredients. In fact, fertilisers ARE salts (just not the 'ready salted' kind). A plant can only absorb a nutrient when that nutrient is in the form of a salt. That's why you don't see 'no salt' or 'salt free' complete and balanced fertilisers.
What is the salt index?
However some fertilisers are higher-salt than others. An ingredient with a high salt index can increase salt levels in the soil, faster than an ingredient with a low-salt index. Choosing a fertiliser that uses low-salt ingredients helps reduce the chances of fertiliser burn.
What do you want instead?
What you ideally want is a fertiliser that avoids high-salt ingredients, is in a form that's more easily absorbed by a plant, and is balanced in the amounts a plant needs.
Get all that right and a plant is more likely to use up everything that's in the fertiliser, leaving fewer salts behind to build-up over time. That's why finding a fertiliser that's both complete and balanced matters.
The most obvious risk factor is getting the fertiliser directions wrong and accidentally overdosing. Don't do that. But the least obvious, but most important factor turns out to be how you water. Simply changing the way you water is the easiest way to reduce the risk of fertiliser burn, no matter what you feed.
The highest salt ingredient in fertiliser
The top dog of the salt index is potassium chloride. It sits at #1 with the dubious title of winning 'the highest salt index' rating among common fertiliser ingredients, measuring 116 on the salt index. That gives it the highest potential of increasing salt levels in the soil.
To compare, typically the first most common ingredient in fertilisers is urea, which measures 75 on the salt index. Unlike urea however, potassium chloride is pretty stubborn. It likes to hold on to soil and excess isn't as easily watered away.
The shift to chloride-free potassium sources in agriculture is being driven by the demand for more sustainable production that increases yield, without harming soil health.
The good news for our houseplants is there is a small but growing number of urea-free and chloride-free fertilisers available for indoor plants too. All Dyna-Gro formulas are urea-free. All Growth Technology (GT) formulas are urea-free also.
Chloride-free is more difficult to find in indoor plant fertilisers, but GT is one that is both urea-free and chloride-free (more about urea coming up next).
But before you freak out and tip your fertiliser down the drain, there are plenty of other ways to almost eliminate the risk of fertiliser burn, no matter what's in your current fertiliser. Especially tip #2 coming up.
The problem with urea
We know nitrogen is an essential nutrient for plants. And urea is a popular, affordable source of nitrogen. But it's not the only source of nitrogen. The most common nitrogen sources you'll find on fertiliser labels are urea and ammonium nitrate. Many fertilisers use both.
The key to understanding the issue with urea, is to know that plants can only use nitrogen once it's in nitrate form
For urea to be usable for our plants, it has to first change form to ammonium, then change again in to nitrate. Outdoors this isn't a biggie. Bacteria in organic soil do that job for plants.
But indoors, most of our houseplants are in soil-less mixes. Our indoor plants lack the invisible good guys, making it harder for plants to convert urea into nitrates. That un-used urea can accumulate in the pot. Which - you guessed it - increases the risk of fertiliser burn.
What do urea-free fertilisers use instead?
The go-to is usually ammonium nitrate. Also a source of nitrogen, but in a different form. Plants still need to convert it, but like the name says, ammonium nitrate has nitrate in it already, so plants can get vital nitrogen from it faster and more easily than urea. That makes ammonium nitrate a more available source of nitrogen for indoor plants, leaving less behind to build-up and cause fertiliser burn. However ammonium nitrate is not as affordable as urea.
But don't panic about fertiliser burn even if your fertiliser does use urea for nitrogen. Most fertilisers do. Luckily urea is highly mobile and easily washed away (especially if you both top water, and flush water). Simply changing the way you water will flush out the excess, away from delicate roots.
#2 Top water to reduce fertiliser burn
No matter what fertiliser you use, top watering heavily enough to completely saturate the potting mix, until water freely pours out the drainage holes, reduces the risk of fertiliser burn more than anything else.
This is called flush watering (and no, flush watering has nothing to do with overwatering - and does not increase the risk of root rot - you're thinking of what can happen if you water again too soon). Flush watering can be done with water + fertiliser, or plain water.
Remember even if you DO find a urea-free and chloride-free fertiliser, fertilisers ARE salts, but if you 'flush water' you can help stop those salts from building-up over time. That's because along with the water pouring out, it also flushes out excess fertiliser salts.
Is bottom watering ok?
If you prefer to bottom water, all good, just add a top water once every month or two to flush everything out. Or if you just can't make top watering work for you at all, at least make sure you drain really well after bottom watering.
Never leave plants sitting in drained water
The water that's drained out will have those excess fertiliser salts in it. If you leave it there, the plant will just reabsorb the excess salts, increasing the risk of fertiliser burn. Also keep in mind the water will evaporate, but the fertiliser salts stay put, becoming more and more concentrated over time.
Never top up or re-use water
That's the same reason not to 'top up' water. Whether it's water in a saucer, in a reservoir when growing hydro or semi-hydro, or when propagating. Always tip out what's left and replace with fresh, as over time that water will become higher and higher in mineral salts (because the water evaporates but the salts don't).
That's also why it's best not to 're-use' water that drains out of one plant, to water the next plant. Rather than tip it down the drain, you can save it for your outdoor plants instead.
Don't water before you fertilise
Another benefit of fully saturating the soil is to ensure all the roots get water and the essential minerals they need along with it. Watering too lightly can mean some roots miss out, dry up and die (which later causes root rot). Yes, underwatering causes root rot too.
The practice of watering first with plain water, BEFORE watering again, but this time with fertiliser mixed in, comes from the days when outdoor fertiliser was all us indoor plant hobbyists had. And it tended to be strong stuff, that was often far from complete or balanced.
Not only did it need to be diluted much more than the directions said, you also had to be careful to flush out all those excess salts left-over from the last time you fertilised.
That's also where the practice of only fertilising every second water, or once a month, comes from. Watering with plain water in-between gave indoor plants a an extra buffer to protect from fertiliser burn.
But to understand why you should avoid flush watering first with plain water, instead of with fertiliser, imagine roots like a sponge. There's only so much roots can absorb at once. At first a sponge will absorb all the water, but once it's fully saturated, the rest just runs through and down the drain.
Flush watering with plain water before you fertilise, effectively means the roots get the water, but your plant misses out on the essential minerals they need. Then when you follow that plain water flush with fertiliser water, you just wash most of your fertiliser down the drain!
#3 Use the 'weakly weekly' method
What the 'weakly weekly' method does NOT mean is to water every week, so don't take the name literally. What is DOES mean is to fertilise lightly, every time you water. This method both reduces the risk of fertiliser burn while also reducing the risk of deficiencies. Win win.
How much is 'weak'?
As a general guide, if your fertiliser directions say to use it every second watering, then I'd use half the amount, every time you water. If the label says once a month, then I use quarter the amount, every time you water.
Some fertilisers are already formulated for the 'weakly weekly' method. Just check the directions. If they tell you the dose to use every time you water, you don't need to change anything. Learn more here about the weakly weekly method for fertilising indoor plants >
No drainage holes? Watch out
You can probably guess where this one's going. With no drainage holes, flush watering is pretty much impossible. That means you'll need to be extra careful about choosing the best fertiliser to avoid those harmful salts building-up over time, with nowhere to go, causing fertiliser burn. No drainage holes also increases the risk of root rot from water hanging out at the bottom of the pot, drowning roots without enough oxygen. Speaking of, it also reduces oxygen for roots.
Should you feed in winter?
Actually yes, you should fertilise in winter. I know. This one surprised me too. Unlike many plants outdoors, most of our tropical-loving plants kept indoors don't die back or go dormant over winter.
Many of our favourite indoor plants are native to Southeast Asia and Tropical America (a region that includes the Amazon rainforest), with a 12 month growing season where temperatures average around 25 to 27 degrees and humidity is high all year round. Staying warm and humid all year, with more consistent temperatures and sunlight levels than we offer, means most indoor plants want to grow all year round.
Unless they're dealing with freezing conditions over winter or they die back and go dormant, even when leaf growth slows or stops, roots keep growing in winter. Roots use winter to take advantage of slower foliage growth demands to refuel on nutrients, recovering from the last growing season and 'bulking up' for the next. A deficiency in essential minerals caused by not feeding in winter, can result in stunted growth and yellowing leaves come spring.
There are more reasons that that to feed our indoor plants through winter BUT you do need to change the way you feed. Learn more about why to feed indoor plants in winter (and what you have to do differently)
Are 'bloom boosters' fertiliser?
No, unless they specifically say so, most products that call themselves bloom boosters are just that, a booster, not a complete fertiliser. Dyna-Gro Bloom for example is a complete fertiliser, however Dyna-Gro Mag-Pro which claims to be "...a blossom booster to help initiate flowering..." is designed for mixed feeding along with fertiliser, not to replace your fertiliser.
Can a fertiliser or bloom booster make plants flower?
The short answer is no. This one may turn out to be more about clever marketing by the fertiliser companies. Despite how much we want it to be true, or what companies claim, fertiliser cannot initiate flowering.
Flowering is controlled by plant hormones, which are triggered in response to factors like daylight hours, availability of water, changes in temperature and for some plants, age. But that doesn't mean minerals have nothing to do with flowering (just not in the way we wish they did).
Personally I think some of this is our fault. The desire to get plants like orchids, hoya and peace lily to flower means there's a lot of demand for a product that will 'make my plants flower'. So of course the fertiliser companies want to meet our demand the best way they know how, with minerals.
Multiple minerals are involved in supporting flowering. Commercial growers can manipulate both environmental factors and adjust nutrient levels to encourage a plant to shift from growing leaves to growing flowers, so they can meet the demand for selling plants in flower. However one of the most reliable ways to induce flowering at home (if it's the right time of year) is to change light or temperature.
Some studies into how minerals support flowering, actually suggest it's the reduced nitrogen, not the increase in phosphorus and potassium (like most 'bloom boosters' do), that has more impact on shifting a plant from foliage to flowering. But that still doesn't mean fertiliser can literally make a plant flower.
What are the beneficial minerals?
We know there are 12 proven essential minerals. But there are far more than 12 minerals. What about the rest? Seaweed is a perfect example of this. Seaweed is often called a 'superfood'. A good quality seaweed can offer over 70 beneficial minerals, vitamins and enzymes.
So why isn't seaweed classed as fertiliser? Because (depending on the source) it may well be more complete than most fertilisers, but it's not balanced. Seaweed alone doesn't have enough of the nutrients plants need. It's more like a multi-vitamin, than a complete meal.
That's why mix-feeding seaweed + fertiliser is a popular way to create a more 'complete and balanced diet' for our plants.
There are extra minerals that are beneficial for certain plants, at certain times, under specific conditions or at particular stages of growth. That's what we call beneficial minerals.
A beneficial mineral by definition is not essential to life. They may benefit only a small number of plant species, or only in specific situations. They don't meet the criteria of being classed as essential because plants can live without them.
The list of beneficial nutrients changes depending on the plant, however generally includes silicon, nickel, chloride (not to be confused with chlorine), selenium and vanadium - sometimes cobalt appears on the list too.
The little things are a big deal (if you're a plant)
What I found fascinating when I first started down the rabbit hole of plant nutrition, is that our plants grow based on what they have the least of, not the most of. Called Liebig's Law of the Minimum.
If one of the essential nutrients is too low or missing when the plant needs it, even if it's one plants only need small amounts of, our plants can't take in all the other available nutrients until the one it needs the least of is provided.
Like a chain that's only as strong as its weakest link, for our plants to live their best lives they need ALL essentials present and available, when they need them. Just like giving a plant the perfect conditions for light, temperature and humidity, won't help if you never water it!
Should you give indoor plants the beneficial minerals too?
The answer is yes, maybe and no all at once, as it depends on what the plant is that you're wanting to feed. For this guide my focus is on our indoor plants and the general consensus for them seems to be no - but unlike the essentials, not everyone agrees on the beneficial minerals actually being beneficial.
Silicon for example is a beneficial mineral that I personally think indoor plant hobbyists should consider adding (I use a silicon supplement by Dyna-Gro called Protekt), but others on the list? Based on my research so far, that's a no from me.
Nature's bodyguard for your plants
Silicon
Silicon is the mineral I call nature's bodyguard because it's been shown to make a long list of different plants species stronger. Both physically stronger, and by strengthening their defences (imagine that like their immune system). It helps increase a plant's resistance to pests and helps them cope better with biotic and abiotic stresses in general, like drying out (here's a brief explanation with examples of abiotic and biotic stresses). Sounds good to me!
But despite the 800+ articles and studies showing the benefits of silicon across a wide range of plants - one of which analysed silicon's effect on 735 plant species - silicon isn't recognised as essential. So the jury is out on silicon (but the evidence is very compelling). At the end of the day, it doesn't meet the criteria for being essential. Plants can live without it.
For the plant-geeks: What does meet the criteria of being an essential element? There are 3 must's...
(1) A deficiency in the element would make it impossible for the plant to complete its life cycle
(2) the symptoms of a deficiency are specific to the element in question, and can only be corrected by supplying the element
(3) the element is directly involved in the nutrition of the plant (and not merely correcting some unfavourable condition like temperature or drought).
Peas are plants too
Nickel
Nickel is another interesting one. It has been officially added as an essential nutrient for some cereal crops like barley, and legumes like beans, alfalfa and peas. But indoor plants don't appear to need nickel, and in the wrong amount, nickel is also phytotoxic (a fancy way of saying 'toxic to plants').
Nickel is a nano-nutrient. Even crops that do need nickel only need tiny amounts, so it doesn't need to be added to most fertilisers - plus nickel is a common contaminant in soil, water and fertiliser anyway, luckily in very small amounts.
But if our indoor plants don't need nickel, is it safe for them? You should still be safe in low amounts, which is just as well since nickel is probably in your fertiliser anyway, although likely in such tiny amounts that it doesn't meet the minimums to be listed on the label.
Cobalt
Back to peas and beans again. Cobalt is another one that appears to benefit legumes (it actually seems to benefit the good bacteria that live on the roots of legumes, rather than the plant directly, but I digress).
Cobalt is also another common contaminant in fertilisers, so is probably already in what you feed even if - like nickel - cobalt's not at high enough levels to be listed on the label. However like nickel, it doesn't take much cobalt to cause toxic effects.
Why is sodium in fertiliser?
I know what you're thinking. Isn't sodium salt? Actually sodium chloride is table salt, so no, not exactly - but since fertilisers essentially are salts, you are right in a way (but again, just not in a 'ready salted' food kinda way).
And yes, some companies do list sodium on the label. Like cobalt and nickel, sodium is another mineral that it doesn't take much of much to cause toxic effects. Sodium is also a common contaminant already in fertilisers anyway. Sodium deficiency (as such), doesn’t appear to cause any symptoms in indoor plants, it is not essential after all, however sodium toxicity can occur even at low levels.
Like cobalt and nickel, there are some plants for which sodium appears to improve growth. Instead of legumes though, this time it's a subset of the 'C4' plants which benefit, which includes celery, spinach, sugar beet and turnips. But so far I haven't found any research showing indoor plants need sodium.
Why can't anyone agree?
Actually, they can agree on the essential minerals, but it depends on your source. The reason some companies and universities add some of these beneficial minerals to their list of essential nutrients, comes down to what you're growing. If you're reading agricultural research, and growing cereal crops and legumes, it's a different story to the essentials needed by our indoor plants.
So if these 'beneficial minerals' are not essential, and potentially not beneficial for our indoor plants - and some are potentially toxic even in low amounts - why would sodium, cobalt, nickel and more be in your fertiliser?
The explanation seems to be one of two things: The first reason could simply be because the fertiliser we use for our indoor plants was also formulated for crops and vegetables such as legumes (eg: peas and beans), or C4 plants (like spinach).
The second reason might simply be because some beneficial minerals are common contaminants of fertilisers anyway, so they are probably in the fertiliser as a by-product already, however probably in such small amounts they are likely safely under toxic levels for our indoor plants.
So what IS the best fertiliser for indoor plants?
After all that learning and 'behind the label' insights, we've come full circle, back to the original question that likely brought you here. However here's where it gets tricky. That's because it's not just science, but also experience, that makes the best decision for you and your plants, in your conditions, to suit the way you prefer to care for your plants.
For me? I take into account science first, and in a close second I value the opinions of professionals and growers who have more experience than me. Then I get to testing the best from there for myself, before I settle on what works best for me.
I never do completely settle though. I have my staples I return to, but I never stop testing. When something new comes on the market I can't resist trying it (yes, my plants are my guinea pigs), otherwise how will I know I'm really doing the best for my plants?
Of course it's a bit easier for me. Since I sell such as big range of indoor plant fertilisers here in New Zealand, I get to not only test them on my own indoor jungle, but also see what my customers buy most often.
The End?
After getting to the end (congratulations!), if you were hoping this is the part where I simply tell you the #1 fertiliser you should feed your indoor plants, you're out of luck.
However I'm very happy to help you one-on-one if you want to email me. There's no obligation to buy anything, and there no silly questions! Include a bit about the plants you have, how you prefer to water, any problems you're having and what water source you use, and I can give you a hand choosing one from the many indoor plant fertilisers available these days. Our plants are spoiled for choice!
Or, why not go test out your newly acquired knowledge to shop the best selling indoor plant fertilisers here, and decide for yourself.
My 'why' (for the Simon Sinek fans out there), is to help take the stress and guesswork out of caring for your indoor plants. I do that both through my free plant care guides - where I hope to upskill, educate and simplify plant care - and also through the products I sell to help you care for your indoor jungle.
I hope this guide helped you with all those things, but if you do need a hand please ask - Anna :)
PS: I also hope I made this obvious multiple times along the way, but this still has to be said: Just like human nutrition, there are lots of opinions on what's best for plant nutrition. You only need to compare a line-up of 'orchid fertiliser' or 'tomato fertiliser' on the shelf at your local garden centre to see how different each one is, even when they are all for the same plant family! This guide was written by me, based on my research, combined with my experience with my 100+ indoor plants. Please do take this as intended - as a guide - not 'the one and only truth' of indoor plant nutrition :)