How long does plant fertiliser stay safe to use once mixed?
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.
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 all the 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 food 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. There are some exceptions - like the impressive Dyna-Gro - who go the extra step of tracking batches, so you'll see a manufacturing date, lot number, or both stamped or printed on the pack.
But 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?
Find out how to store and keep each type of plant fertiliser fresher for longer...
Liquid inorganic or synthetic plant fertiliser
If you're feeding a liquid inorganic or synthetic fertiliser, like 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!).
This category of fertilisers tend to stay stable and usable the longest. Ideal storage for them is not too cold, not too hot. I'd avoid storing in direct sunlight too (even if the bottle is opaque, not see-through). For most of these, you want to aim for between about 10 degrees and 26 degrees, not too different to what most of our indoor plants can tolerate.
Watch the lower end though! If they freeze, the dissolved minerals usually freeze at a lower point than the water does, so you end up with crystals forming. That can stuff up the balance in the nutrients. That doesn't mean you have to throw out the food, but if it's still good to go, you may need to change your dose or add some hot water into the mix. If that happens to yours, check with me (or the retailer you bought it from, or the manufacturer direct).
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. Otherwise one freezing cold night in the shed and you may have to chuck a whole bottle of fertiliser out.
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 powder formula - like the fantastic growth booster Groconut (made from freeze-dried coconut water) - has a shorter shelf life than inorganic or synthetic powder and granule formulas. Even Groconut is a good 12 months when stored correctly though, so it's not panic stations to use it up. 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.
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. Groconut for example is best stored sealed, in the fridge.
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 just wouldn't do that without checking with me (or the retailer you bought it from, or the manufacturer direct).
Exploding organic plant food (when 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, that baby's likely got to go.
Warning out of the way, these types of products still have a really decent shelf life unopened, in 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 this family of 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 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. Some 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 it 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 fizziness, green algae growth, a bad smell, a film across the surface, 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 stable for longer even once mixed, but storage still matters. Store them 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, 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 anything organic, I'd push it no more than about 48 hours in winter, or 24 hours in summer. Light and heat are no-no's, so when storing an already made-up mix, storing in a cool, dark place is the go. For inorganic and synthetic, I'd push it out no more than one week given good storage conditions. Always check stored pre-mix for those danger signs 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!
If you ever need a hand, please do ask. If your question's about a fertiliser I sell, and I don't know the answer, I'll ask the experts for you. Even if it's not about a product I sell, or 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.