Great question, and important too, not just for your plant's health, but also for your plant's life! You wouldn't be the first person to lose a plant from a fertiliser stuff-up. Yes, I'm guilty of that one. Don't be me!
I know most manufacturers say 'prepare and use fresh' but is that just sneaky marketing to make you go through the product faster? Or is there really a risk to your plants if you use 'old' plant fertiliser you mixed up the day, or week, or month before?
Let's take a look at how long plant food stays fresh in the bottle, which the safest ones are, and how long you can safely store and use plant fertiliser later after it's already been mixed.
*CAVEAT* This has to be said: this advice is general, not specific or applicable to every single plant fertiliser ever made, or even to all the indoor plant fertilisers I sell. Always follow the manufacturer's advice on storage and use.
How long does plant fertiliser stay fresh in the bottle?
First up, consider the shelf life of what's in the bottle. How long does plant fertiliser actually stay stable in the bottle or pack anyway? No point worrying about how long it stays fresh once mixed, if you're already starting off with fertiliser that's gone bad!
And yes - this does vary. Most foods won't have a best before on the label. Many don't have date of manufacture either. This isn't human food after all, plus keeping fertiliser still sealed and stored right, many stay good for years.
So in the right conditions, some types of fertiliser can stay fresh indefinitely in theory. So what are those 'right conditions'? And what types of plant fertilisers stay fresh longest? What about powder and granules versus liquid fertilisers? And organic versus synthetic?
Inorganic or synthetic plant fertilisers:
If you're feeding a liquid inorganic or synthetic fertiliser, ones like Growth Technology and Superthrive Dyna-Gro, they tend to have the longest shelf life. They also tend to be the most forgiving of less than ideal storage conditions (not that I'm suggesting you ignore their advice!).
In general, this category of fertilisers tend to stay stable and usable the longest. Ideal storage for them is not too cold, not too hot. Always store indoors to avoid the temperature swings outdoors. Always avoid storing in direct sunlight for the same reason (yes, even if the bottle is opaque vs see-through).
Temperature is a biggie too. For most liquid fertilisers, you want to aim for between around 10° and 25° Celsius (50° to 77° Fahrenheit), a similar to range to what most of our indoor plants can tolerate.
How to avoid crystals forming in your fertiliser
Watch the lower end of the temperature range! This can happen when fertiliser is stored in cold temperatures, freezes then thaws, or swings too much in temperature (such as a heated home during the day then temperature drop at night when the heating is off in winter), or when you store fertiliser in the garage or shed outdoors. When crystals or sediment forms in fertilser it's called 'salt out' (as that is quite literally what is happening).
It's like trying to dissolve sugar in a cold cup of water vs a hot cup of tea. The mineral salts in the fertiliser can drop out of suspension and form crystals or sediment at the bottom of the bottle.
Although the better quality fertilisers also tend to be the most concentrated (so you're paying for the nutrients not the water), they can also be so concentrated that they're just on the edge of what the solution can hold without them falling out of suspension. That's why you may notice this happening more with better quality, more concentrated liquid fertilisers.
Although normal and not often anything to worry about if it's a small amount, it can cause a slight imbalance in the balance of nutrients. That doesn't mean you have to throw out the fertiliser. It hasn't gone bad. It's just separated. use the solutions below to get those salts back into suspension.
What to do when you get crystals or sediment at the bottom of your fertiliser
One solution if it's just minor is a quick shake before every use. Another is putting the closed bottle in a warm water bath to dissolve those salt crystals and sediment, so it goes back into suspension (then a good shake after a warm bath).
Another is to wait till the end, mix any sediment or crystals with an equal amount of hot water to dissolve the crystals, then triple-dilute with water. So if you'd normally use 1ml per 1 litre water, once dissolved, use 1ml per 3 litres water (as the salts will be more concentrated).
Probably the best tip to avoid it in the first place is to only get what you need! It's very tempting to buy in bulk as there are some significant savings from most brands when you get bigger sizes.
BUT unless you feed over winter (which I do, this is why), you're better to get a fresh bottle of fertiliser at the start of the growing season (around late-winter to early-spring), and aim to use it up by the time winter comes around, or within a year of opening.
Can you store fertiliser outdoors?
In this guide we're talking about indoor plant food, but temperature is the reason both indoor and outdoor plant fertiliser is always best stored indoors. And no, that doesn't mean in a shed or garage (unless it's heated in winter).
Watch out for powder or granule plant fertiliser
The good thing is water-soluble powder or granule fertilisers can also have an almost indefinite shelf life in the right conditions, BUT that depends on what they're made of.
An organic powdered or granule formula tends to have a shorter shelf life than inorganic or synthetic powder and granule formulas. Inorganic or synthetic powder or granules tend to stay fresh longer depending on what protection's included in the formula and manufacturing process.
What to watch out for with these, oddly enough, is to beware of water. Be paranoid about storing them somewhere dry. Even high humidity (where there's more water dissolved in the air), can be enough to turn them to sludge or solid. Like that Milo you thought you had sealed perfectly in the pantry, only to discover it's gone rock hard.
That's one reason you'll see the advice to avoid storing fertilisers in your laundry. Both the high humidity and fluctuating temperatures can play havoc with your fertiliser.
Also, be careful to only use perfectly dry tools to get it out of the pack and to measure (like a dry teaspoon), and to follow the storage instructions on the label for this type of food. Some powder fertilisers and growth boosters are best stored in the fridge for example.
Like with synthetic and inorganic fertilisers, if things to do wrong with storage, you might still get away with using your new solid powder formula by mixing with hot water and changing the dose. But I'd still check with the retailer or manufacturer before you do.
Exploding organic plant fertiliser (when plant food goes boom!)
Um, a little word of caution with these. Liquid organic products - like fish and seaweed emulsion, blood and bone etc - can be the most temperamental, even in the bottle. Don't let that put you off though. Just get the storage right and you'll be sweet. Heat and sunlight are the big baddies when it comes to organic goodies.
Don't assume these types of fertilisers (or elixirs or tonics), use preservatives like inorganic or synthetic foods do to stay fresh. For some, to get that coveted organic status, it can mean manufacturers are prevented from using preservatives (this varies a bit depending on the country's rules).
Instead, a clever trick is to play with the pH of the liquid mixture in the bottle to keep them stable. When you drop the pH, it helps suspend the biological bits, keeping them from becoming active. That reduces the chances of bacteria or fungus throwing a party and inviting everyone over.
Do watch out for bulging containers with these types of foods (something to be wary of with all food really). That can be an initial sign of impending doom (or rather, impending 'boom' when they explode).
A bulging bottle isn't always a red alert though. Open it slowly and carefully (best opened outdoors), and check out what's going on inside. If it smells like it's gone off, it's fizzy, or there's fungus growing inside, it's got to go.
Warning out of the way, these types of products still have a really decent shelf life unopened, stored the right conditions. A good 3 to 5 years is pretty standard. But, once you open them and start using it, air gets in. Air plus storing it at home instead of in a temperature controlled retail store both mean the clock starts ticking. Check the pack, but a typical shelf life once opened averages 12 months
Once opened, just avoid being a dummy and you'll be fine. Store sealed, in a cool, dry place, out of direct sunlight, and you're good to go. Also, and this may seem obvious, but don't leave the top off any longer than you have to. I'd do this with any food, but with organic ones it's extra important as exposure to air (or direct sunlight) can shorten shelf life.
Too hot can be more a concern than too cold for organic fertilisers. Anything that isn't preserved you want to take extra care to follow the storage instructions.
The moral of the story (so far)? Don't assume the storage of all plant fertilisers is the same!
What about plant fertiliser after it's been made up?
Okay, back to what probably bought you here in the first place. Trust me, all that stuff you just learned will be worth it for making the safe call for this next bit...
How long does fertiliser stay fresh after you've mixed it up?
If you want the 'easy answer' a safe bet is to use up pre-mixed fertiliser in 24 to 48 hours. And to store it sealed, in a cool, dry, dark place over that time. But the more helpful answer (I think) is to apply the stuff you just learned above, to the type of fertiliser and your storage conditions.
Many are safe to store and use way longer than 48 hours. Whether you take that 'risk' is up to you (and yes, there is a risk).
If you're using an organic fertiliser, whether it started off in liquid or powder or granule form - personally I'd play it super safe and use it fresh only (I normally give leftovers to the garden or grass outside), rather than storing or saving leftovers for later use.
Once out of its protective bottle, and exposed to air, light, heat, and water, the pH can change, and it will become biologically active. Bacteria and fungi can send out party invites. The warmer the ambient temperature, the shorter it will stay fresh before it's party time and unwanted guests start arriving.
I'd use any organic fertiliser right away myself, but you should have a bit of leeway of up to 24 hours in warmer temperatures, or about 48 hours max at cooler times of year. Check for any bubbles, green algae growth, a bad smell, a film across the surface, a sediment on the bottom, or fungus. Warmth helps both goodies and baddies replicate.
If in doubt, throw it out.
As for inorganic or synthetic fertilisers, these are normally safer and more stable for longer even once mixed, but storage still matters.
Once mixed, store sealed in a cool, dry dark place. I'd aim to use the mix within a week max, given good storage conditions.
The same things to watch out for apply. Look for fizzing, green algae growth, a bad smell, a film across the surface, sediment at the bottome or cloudiness in the water, or fungus. Same as before: if in doubt, throw it out.
Keep in mind any fertiliser, elixir or tonic - whether organic or inorganic - will be subject to infection from airborne pathogens once it's been diluted. Your mix could also be infected with water-born pathogens. Either way, not good. Pathogens is the fancy name for bacteria, viruses or other microorganisms that can cause diseases.
Also give thought to what happens over time once mixed then stored. At worse some nutrients may degrade or disappear altogether, leaving you with an incomplete food or a possible nutrient deficiency. At worse, some products can ferment or even become toxic. At the wrong pH, even the most beneficial nutrient can become toxic to a plant.
The bottom line
In an ideal world, we'd make up only what we need, and always use fresh. Realistically, we're going to have extra left over we don't want to go to waste.
For any organic fertilisers, I'd push it no more than about 48 hours in winter, or 24 hours in summer. and be smart with your storage!
For inorganic and synthetic fertilisers, I'd push it out no more than one week given good storage conditions. Always check stored pre-mix for those danger signs above before use.
And a last heads up warning before you go. This article is talking about commercially available, packaged products. But if whatever you've made up needs to brew or ferment - like compost tea or banana skins - don't store it any beyond than when it's ready to use.
At best, those precious micro-organisms you brewed or fermented will die, the container might go boom, or at worse, you might lose a plant. I've personally had plant loss from a banana skin concoction, I think because the pH went bananas (sorry, couldn't resist the pun - but honestly was pretty sad at the time). The reality is natural does not mean safe!
Help!
If you ever need a hand, please do ask. Even if it's not about a product I sell, or if your questions is just about plant care in general, I welcome your questions. A lot of people helped me along the way in my plant parenthood journey and I love to pay it forward by helping others. Questions are most welcome anytime. Just get in touch.
Happy growing,
Anna :)
Keep learning
Should you fertilise indoor plants in winter? >
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident writer for NZ Gardener magazine on the topic of indoor plants. You'll see her houseplant care articles in magazines in-stores NZ-wide, and online on Stuff, The Post, NineHoney and more.