Silica - Nature's secret weapon our indoor plants are missing out on
No, silica isn't considered an essential nutrient. But once you find out what it does and see the difference it makes you might consider it essential for your indoor plants. This powerful nutrient is nature's bodyguard for our plants. Except being indoors isn't exactly 'natural' for our house-plants. So indoor plants need us to give them the protection that nature would normally provide.
Let's take a look a this little powerhouse nutrient, what it does for plants, why there's a shortage (even though it's the 2nd most abundant element in the Earth's crust), and why such a common nutrient is a secret us indoor plant hobbyists are 'behind the times' on finding out about...
Amazon links in this post may be affiliate links which means I may receive a commission for purchases made through links but doesn't change the price you pay. Learn more
What does silica do for plants?
In short? Strength! It makes plants stronger in two ways. Physically stronger, and it supports stronger defences, increasing plant resistance to pests, diseases, and environmental stress.
How does silica make plants stronger?
Silica is involved in cell wall strength, as well as what we might think of as 'immune strength'. It builds a protective barrier against biting and sucking insects, and against diseases like fungus that cause everything from root rot to brown leaf tips and brown patches on leaves with tell-tale yellow halos. It also builds broader stems that can better absorb water and nutrients and strengthens weak stems.
Broader stems also helps with nutrient transport. Stems can more easily and efficiently get nutrients from roots to leaves. Stronger stems can support bigger, stronger leaves, fruits and flowers (yes, silicon is not just for indoor plants - it's superb for producers of heavy fruit and veges too).
How does silica fight insects?
This might be my favourite benefit. Silicon is a key part of nature's defence system. Think of it like giving your plant its own personal bodyguard. Big, tough and ready to fight. Not just stronger, tougher stems and leaves, but the roots too. Silicon helps strengthen your plants physical and mechanical barriers against attack from both chewing and sucking pests.
Common pests we struggle with for our indoor plants include fungus gnats (larvae in excess will eat roots, stunting plant growth), mealy bugs (they pierce your plant and suck out sap), aphids and spider mites (they both suck too - in both senses of the word!).
Interesting side-fact: Diatomaceous Earth (which often recommended sprinkled on soil to aid control of fungus gnats), is a very rich source of insoluble silica. It's up to 85% silica dioxide and used as a natural insecticide. However being insoluble, it's not a form available to plants.
Hold your horses... Before we jump into more about this clever super-nutrient and how it works inside our plants, if you're already in the know and just came for the solution, here are the 3 products I've used with great results for my indoor jungle...
What are the best silica products for indoor plants?
Unless it's panic stations, root feeding little and often is the best way to add silica to your plant's 'diet'. And no, you WON'T find silica in your fertiliser unfortunately. This particular nutrient doesn't play nicely with liquid fertilisers, so has to be added separately. When adding silica to your water, always add Silica first, stir, then add fertiliser and water as usual.
However, if there's something wrong, such as a plant under attack from pests or suffering stress, you absolutely can spray silica to the leaves for super-fast uptake. Great as a short-term boost while root feeding gets to work, as leaves absorb nutrients faster than roots, but the nutrients stay more local. Roots absorb a wider range of nutrients, for the benefit of the entire plant, but does take longer than feeding the leaves.
Silica for the roots (every time you water)
For my New Zealand plant buddies: GT Silica and Dyna-Gro ProTekt are my top picks for root feeding, every time you water. A little goes a long way.
And for my international plant friends: You can get Dyna-Gro ProTekt on Amazon too.
Silica for emergencies
My favourite silica fixer-upper is GT Foliar (available in New Zealand), but for those overseas, I haven't found GT Foliar on Amazon yet, however another I have seen rave reviews for is Bloom City Silica Boost which is multi-purpose, so can be used as a foliar spray or applied via the roots when you water.
Silicon helps defend against bugs in 2 ways
The first is proactive defence, by strengthening plant tissues in stems, leaves and roots. That barrier makes it more difficult for insects to chew or penetrate (that's how the sucking insects feed - think of them like mosquitoes). If the plant is eaten, silicon also makes plants harder to digest, as well as making the plant taste worse by reducing palatability.
Reducing digestibility has the added benefit of slowing insect growth and reproduction. Studies found larval survival was reduced from the eggs of insects fed silicon-supplemented plants. In one study, rice supplemented with silicon showed a ten times increase in its physical barrier to insect pests.
Consider it from a bug's perspective. Why try to chomp in to a silicon-strengthened 'rock' of a leaf, when you could munch on something soft and easy? Move on bugs. Nothing to eat here.
Silicon is considered natural pest control, used alone, with, or instead of chemical alternatives. It's common to see the recommendation in plant forums and groups of applying silicon with neem oil to infested plants.
The second way silicon helps defends against bugs is reactive, by helping the plant's natural defence reactions, which are triggered when a plant is under attack from insects chewing or piercing it.
Silicon strengthens a plant's defence responses, as well as how quickly a plant reacts to threats. Silicon also works to more quickly block the flow of sap, in a similar way to our blood clotting when we cut ourselves - but for the plant under attack that reduces sap loss from sucking insects like aphids and mealy bugs. Plants have also been shown to continue to send silicon to the damaged area after the threat is gone which appears to to assist with faster healing.
How does silicon help plants resist disease?
Around 85% of plant diseases are caused by fungi or fungal-like organisms. Symptoms of fungal infections can vary depending on the type of fungus, but can include powdery mildew or mould, leaf wilting (even when watering is fine), spots on leaves, chlorosis (yellowing of leaves), reddish-brown leaf or stem rust, and black or discoloured rotting patches (usually close to the soil).
The same proactive and reactive defence mechanisms that silicon assists with in defence against pests, also come into play with pathogenic diseases caused by fungi. Silicon both increases a plants resistance and recovery.
When a fungal nasty comes along, it must first drill through the plant's cell wall to get to the nutritious cell centre. Once the centre is reached, the fungus gets the food it needs to fuel it spreading through your plant.
By strengthening the cell wall, silicon helps protect from the disease getting in, so it can't spread. Applied to a diseased plant, silicon also helps reduce further spread and gets to work to assist healing and recovery.
How does silicon protect against extremes?
The short but fancy-sounding answer? Silicon helps plants resist abiotic stress.
Abiotic stress is stress from environmental factors like heat shock, limited water, and limited nutrient availability (biotic stress is from living things like bugs). I think of as silicon as protecting plants from both us and nature ;)
Silicon helps plants to better absorb, transport, and retain water, helping plants cope with neglect, drying out between watering, temperature extremes, dry air, low humidity, draughts, and inconsistent watering.
Growers report plants fed silicon need less frequent watering, staying hydrated longer. More water is put to work and less is lost through transpiration (that's water loss through evaporation from the leaf surface). Reduced water loss also reduces the risk of dehydration and water-deficit stress.
An added benefit for our house plants is that helps plants who prefer higher humidity, cope better in less humid, dryer environments - yep, the typical indoor-plant home environment. Especially during winter with heaters blasting or an HRV / DVS system running. It also helps protect from heat stress. Ideal in summer when plants have to cope with alternating between being shut up in an unbearably hot house, then suddenly changing to cool when the air con's turned on.
Basically, silicon helps plants cope with extremes. Depending on where you live in NZ, most areas become either too hot, or too cold multiple times a year - even inside - compared to the temperature range most indoor plants prefer.
When stomata are closed, a plant can't photosynthesize. During extreme conditions, a plant is forced to close it's stomata to limit water loss, leading to the leaf not cooling itself, and causing carbon dioxide levels to accumulate in the leaf (leaves use stomata to 'breathe' and to cool themselves, exchanging water for carbon dioxide). Wind and drafts also increase water loss.
How does silicon protect plants from us?!
Silicon's protection from extremes, also helps plants resist our excesses as a result of too much love! It helps plants cope with heavy metal toxicity and excess sodium, such as mineral salt build-up from us feeding imbalanced or DIY plant food, when we intend well but get plant food doses wrong, when we use plant food with urea, and when we over-fertilise (even with good intentions).
Whether it's extremes we can't control, or neglect or excesses we can control, silicon helps protect our plants.
How does silicon help plants grow?
Only the most important process on the planet, photosynthesis is sort-of a big deal. Silicon supports photosynthesis in multiple ways.
Think of a leaf like a solar panel. The top captures sunlight, the bottom 'breathes' by exchanging carbon dioxide and water. SiIicon helps keep the solar panel in the perfect position for both, by strengthening the stem, as well as reducing wilting and drooping.
Plants rely on nutrients getting where they need to be, when they're needed. Silicon helps here too. Silicon is the highway that nutrients travel along.
A lot of minerals have a relationship with silicon. They work better together. Calcium and boron are two important ones. Boron helps make silicon available to plants, and also improves calcium uptake.
Calcium is one of those sneaky immobile minerals. It can't re-locate around the plant - so if there's a calcium shortage when a stem or leaf is being formed (causing symptoms like deformed leaves, stunted growth of new leaves, loss of new leaves, and weak stems), then that's it - that deficiency will be for the life of that stem or leaf's cells.
As important as it is, calcium isn't the best traveller. Give it the support of boron, and those nice broad nutrient highways thanks to silicon, and calcium can better travel to where it's needed, when it's needed. Especially important late winter to early spring when plants are gearing up for lots of new growth. The time a calcium deficiency can cause the most long-term damage.
If it's so common in nature, why do growers supplement with silicon?
Silicon is everywhere! It's is the second most abundant element in the Earth's crust. Clays are alumina silicates. Sand is mainly silicon. So why are we facing a shortage of silicon in our soil?
The answer lies in the form of silicon that our plants need. Plants uptake silicon as silicic acid and this is the type missing in many soils. It's possible conventional agricultural practices have compromised insoluble silicon from turning into the form that's soluble by plants. Maybe it's a mineral imbalance, or it could be caused by an imbalance in the soil microbes.
In short, we don't know why there's a widespread silicon deficiency in our soil, but as a result of the shortage, it's lead to a huge amount of research and resource put in to proactive pest and disease management through supplementing with silicon. We now know far more about silicon than we ever did.
How often do indoor plants need silicon?
Plants need varying amounts of silicon, but they all need it. Some can store silicon reserves in their tissues, but others (non-accumulators), rely on their soil for silicon. Either way, the protective effects of silicon benefits all plants.
How much silicon do plants need?
It will vary depending on factors like the stress a plant is under, but soil water in areas with sufficient silicon average 20ppm - research suggests maintaining a constant level of 50 ppm (parts per million) - or for a plant under stress, up to 100 ppm is recommended. However, for proactive maintenance, you'll often see 20ppm recommended.
What silica products are best for indoor plants?
First thing before you jump into adding silica to your plant's fertilising routine, is that outdoors your soil may already be sufficient in silica. I'd recommend getting your soil tested. Yes, silica is abundant in nature, but that doesn't mean your soil water is automatically also providing the silica your plant needs. But for our indoor plants our plants rely on us to be 'Mother Nature' and provide them with the silica they need.
You can apply silica either as a leaf spray in the short-term when a plant is under attack, or the best way to apply for long-term protection is to the roots. I use both methods depending on the plant's needs. For an immediate boost, my go-to is GT Foliar (a ready-to-use leaf spray). For the roots, either GT Silica or Dyna-Gro ProTekt are the two I personally use on my indoor jungle (those two you mix with water and apply when you top or bottom-water your plants).
Those 3 products are readily available in New Zealand, but for my US and other international fans, you should be able to find Superthrive Dyna-Gro ProTekt (as it's an American brand), or another I've seen rave reviews for is Bloom City Silica Boost which is multi-purpose, so can be used as a foliar spray or applied via the roots when you water.
Can you overdose on silica?
Yes, you can go overboard, although it'll take some doing to get to toxic levels, where you may see symptoms such as flower deformities in more silicon-sensitive plants. It's generally a safe mineral that's difficult to overdose. For most plants, it's suggested more than twice the maximum recommended dose would be needed to get to concerning levels - over 200 ppm. Some plants are more sensitive to excess silicon than others.
Non-accumulators are plants that don't hold or accumulate silicon reserves in their tissue. They're more sensitive to excess silicon than accumulators, but more reliant on it being available in their soil.
Common non-accumulators include begonia, geranium, gerbera, pansy, petunia, sunflowers and tomatoes. Common accumulators (at different levels), include ferns, roses, chrysanthemum, marigold, cucumber and zinnia.
Not enough silicon is the bigger concern
When do plants need silicon?
Continually. This isn't a 'wet and forget' nutrient you dose once, then walk away. Plants require silicon during their entire life-cycle for growth and proactive protection, plus it needs to be on-hand when a plant's under stress, and during times of attack, for reactive protection.
Keep in mind however, once silicon's gone where it's needed, that's it - it can't relocate within the plant - so the same as in nature - silicon should always be available to plants via their substrate.
Can silicon be fed along with your usual plant food?
Definitely, yes. That's the ideal way to 'feed' it. You ideally want to add silicon to your existing feeding schedule. For me, that's the 'weakly weekly' method. There are many plant silica supplements available.
A tip to keep in mind (for the silicon supplements I've researched anyway), is in order to keep silicon soluble in liquid form, it'll be in a high pH, alkaline base. When mix-feeding, you want to dilute anything high pH first, then add your plant food, which are normally lower pH. So always add silicon to the water first, give it a mix, then your plant food. Some advise waiting up to 20 minutes before adding your fertiliser but I've not had any problems myself adding fertiliser right away after a good stir in-between. I add silica to half my water, give it a stir, add the rest of the water, then add my fertiliser.
Doing it the wrong way around can cause the silicon to revert to a gel form, and you just end up with a blob your plant can't absorb.
What method's best to give indoor plants silicon?
It depends. I prefer to be proactive about it and soil feed with every water. However if you've only found out about silicon late in the game - and your plant's already under attack from pests or disease - then you need to act fast to support your plant's natural defence strategy. Some growers recommend an immediate foliar feed (where you apply it by spraying on the leaves directly), followed by a root feed.
When a disease or bug attack begins, a plant sends all available silicon to the attack site. The hiccup with this clever response however, is once it's on-site, silicon can't go elsewhere. That's why frequent feeding is best so a plant has silicon in reserve.
If I've got a plant already in dire straits (great band), that's when I'd foliar feed either the attack site, or even better, spray the entire plant when there's an infestation or fungal infection.
Keep in mind though that foliar feeding doesn't replace soil feeding, but in emergencies can give the fastest uptake of deficient nutrients needed for protection. The one I personally use is GT Foliar (available in NZ), but for my overseas plant friends I hear Bloom City Silica Boost is superb. For the long-term (applied to the roots), my current go to is GT Silica but I used to use (and still happily recommend) Superthrive Dyna-Gro ProTekt which is also excellent.
You can get silicon fertiliser in solid form too, but liquids are easier to control and dose for indoor plants, and have the fastest uptake and response by plants.