Fertilising Your Houseplants: Everything You Need to Know

Fertilising your plants doesn’t need to be that complicated.

You can either:

1. Research all your individual plant’s needs and come up with some kind of elaborate spreadsheet that documents the type of fertiliser, frequency, etc etc. Not really for me!
2. Water in fertiliser every X weeks, regardless of, er, anything. Cut it down in winter
3. Chuck worm castings in when you repot and hope for the best
4. 3., but without the worm castings.

Personally, I like a bit of 2. when I’m feeling fancy, but I’m leaning more towards 3 at the moment.

But which fertiliser to choose?

You can go to the garden centre and get something like Miraclegro, but I would recommend getting a specific hydroponic fertiliser (one that can be used with soil) because they contain ALL the nutrients a plant will need.

A regular house plant fertiliser may only provide nitrogen, phosphorus and potassium.

I use General Hydroponics Flora System, but it can be an expensive initial outlay (though it’ll last for YEARS).

It’s generally assumed that plants get their energy from the sun. All they need is a bit of bright, indirect light and the odd water and they’ll be a-ok!

And that’s largely true.

The majority of house plants will grow ok with no fertiliser, especially if they’re regularly repotted – by which I mean yearly, not like, every month.

But in order for your plant to really thrive, it needs food and nutrients. Sure it'll be ok without it, but you might not get the full range of colours, or the massive leaves, or the blooms, or whatever it was that attracted you to that kind of plant.

If you want giant, mature leaves, like the ones of the Monstera pictured above, you’ll need fertiliser. It’s not as important as good light and a good watering regime, but it’s definitely still pretty key.

I have a video on fertilising house plants here.

Houseplant fertiliser for dummies

I could make this post extremely long and sciencey, but that would require a lot more research than I can be arsed to do have time for, so I’m just going to cover the basics.

If you wish to research which of your plants requires which ratio of N-P-K in their fertiliser go all out.

I personally use one fertiliser and hope that’s ok.

None of my plants have complained in a way that is noticeable enough to me.

So, what is houseplant fertiliser?

Houseplant fertiliser is plant food. But you can get lots of different kinds:

Chemical fertiliser

This is, as you might have guessed, a concoction of synthetic chemicals held together with petroleum products, or sometimes rock.

Some of the chemicals may be naturally occurring, but they will be in a purer form than they are in organic fertiliser

Pros of chemical fertiliser:

• Cheap
• The nutrient availability is high, so plants respond quickly
• The macronutrient values are measured and fairly accurate
• A lot of research has gone into making them, so they’re pretty effective
• Easy to navigate for beginners
• You can buy them from most garden centres and, of course, Amazon.

Cons of chemical fertilisers:

• Not particularly eco-friendly, since they’re usually made from non-renewable materials
• They only help the plant, not the soil
• Over-fertilisation is more likely since the chemicals are so concentrated
• They tend to leach away quickly
• They can kill beneficial bugs

Organic fertiliser

Usually made from plant and animal waste – though many contain fish and bone, so we vegans avoid them. Organic fertiliser can include anything from liquid fertilisers made from seaweed to manure and compost

Pros of organic fertilisers:

• Cheap if you make your own
• They actively benefit the soil as well as they plant, leading to healthier plants overall
• Manure and compost are slow-release, so they won’t burn your plant’s roots.
• More environmentally friendly
• Less chance of harmful salt and mineral deposits building up in the soil

Cons of organic fertilisers

• If you can’t make your own, they can be expensive
• It’s pretty hard to tell exactly what macronutrients are contained within the fertiliser
• Er, it might smell
• FUNGUS GNATS

DIY fertiliser

It’s highly likely that you produce food waste that can be repurposed into plant fertiliser. The pros and cons are the same as for organic fertilisers, but make sure that you don’t accidentally give an alkaline-loving plant too much acid and vice versa.

• Coffee grounds – the jury’s out here on how much these actually work – I wrote all about using coffee grounds as fertiliser here
• Banana peels – bury them in the pot for some added potassium and nitrogen. Beware of the flies. Honestly, don’t do it yourself. Is it great for your plants? Maybe. Eco-friendly? Yes. But so. Many. Gnats.
• Rabbit poo – I have a ready supply of this, and it’s…fine. To be honest, I don’t really notice an improvement when using this, but I only did it for a couple of months (I go sick of keeping buckets of poop)
• Aquarium water – if you have a freshwater aquarium, simply water your plants with the old water when you do water changes. It’s gentle enough to use on every water, but is packed full of fish poop nutrients. I have a whole article on using aquarium water for house plants here.

Worm castings

I’m giving worm castings their own category because you can either buy them or get a wormery and use that.

Worm castings are worm poop, which is like compost, but has been really, really thoroughly, er, composted. They’re very nutritious but veeery gentle on your plant’s roots. They also don’t smell, and don’t attract flies.

Here is a picture of a bag of worm poo:

So I took a picture of the worm castings so you’d know what they look like, and then realised I could have just said that they look like dark soil.

WARNING: most bags of worm castings come will a healthy population of worms. Some dead, some alive. It’s quite traumatic the first time you get them.

When not to fertilise houseplants

It’s pretty difficult to tell when plants need fertilising, so first I’ll go through times when you shouldn’t be fertilising your house plants:

• When you bring them home 

Ok, so this does depend on where you’ve bought your plant from, but as a general rule of thumb, you shouldn’t have to fertilise your plant for a year (one growing period and one period of dormancy/quiet time i.e. winter).

The reason for this is that most commercial house plant producers use a nutrient-rich potting mix in which to grow their plants. Often you can see little blue balls, which are slow-release fertiliser crystals.

Similarly, if you buy a plant, for example, online and it’s shipped to you bare-roots, then the potting mix you use is likely to have some form of fertiliser in it, whether it’s slow-release, compost, worm castings.

There are cases when you’d need to fertilise your plants from the get-go, for example, if you know it’s potted in a nutrient-free potting medium, such as pure coir. I’d be tempted to repot in something a bit more nutritious though.

• When they’re not growing

Some plants stop growing in winter, so you don’t need to fertilise them. Some keep on going as if nothing’s happening, so you can feed those as normal.

• When they’re sick

This is closely related to ‘when they’re not growing’ but things like pests can sap a lot of a plant’s energy and they don’t need feeding at that time.

I know it sounds counterintuitive, but great light and humidity and much better sources of energy for them than fertiliser. Fertiliser helps boost leaf size and growth rate, but won’t help against pests.

You can use silica though.

When to fertilise your houseplants

Frequency is entirely up to you. Honestly. For every article claiming that, for example Monstera are heavy feeders, there is another saying they don’t need fertilising at all.

If your plants are in good health, and are growing well, then you can fertilise them as often as every other time to water, to every two months.

If they’re in bad health, fertiliser won’t really help. It’s highly unlikely that your plant’s bad health is caused by under-fertilising. Instead, work on improving their light and humidity situation, and their root health. Once that’s sorted….fertilise as frequently as you like.

This year, I fertilised every time I water throughout May and June (except for Pothos Njoy who hates it and gets brown marks. The only one out of 100+ plants), then laid off for a month because growth slowed (I assume down to overfertilising) and then fed every other time I water.

My plants are thriving and I’ve got some good sizing up of leaves.

Equipment needed to fertilise houseplants

Ok, I don’t use them, so I can’t make you, but I would recommend wearing gloves. A lot of fertilisers can stain and most of them stink to high heaven. Especially if you use something gross like fish emulsion.

Past that, you don’t really need any specialist equipment – it’s just your usual watering routine with a bit of fertiliser added in. I would just be a wee bit more careful with, say, the plants in carpeted rooms, because some fertilisers do stain/will make your house smell like fish.

How to fertilise your houseplants

Try not to wait until your plants are bone dry – that can cause damage to the roots. That being said, I have fed plants with dry roots and they were fine.

I make up nutrient water by adding 5ml of each of the Flora series chemicals into a 5 litre bottle of water. Then I add it to my teapot and water as normal.

I use this setup:

I reuse any of the nutrient water that falls into the pot. I don’t care if you’re not meant to. I don’t like wasting water if I can help it, and I’ve suffered no ill effects.

I also add nutrient water to any of my plants in water, and soak my bare root irchid in nutrient water for an hour or so.

Should you vary fertilisers?

I mean, won’t the plants get bored?

If you were to take ph readings and have the soil analysed etc, then yeah, you’d probably want a few different fertilisers on hand to make sure everything stays balanced.

But I’m not that plant parent. As long as my babies are healthy and happy, there’s no reason to go all sciencey on them.

THAT BEING SAID

If you’re using a plant fertilizer that’s very gentle, such as aquarium water or worm castings, and you think your plant is looking a bit sad or stunted, you could give them a stronger fertiliser a couple of times a year.

That doesn’t mean to say that gentle fertilisers aren’t effective – they are – but as someone that routinely forgets to fertilise her plants, I assume others will too. If you’re always using aquarium water (plus some extra filter gunk from when you clean that out) or add more worm castings every 6 months or so, you’ll be fine.

What do the numbers mean on fertiliser bottles?

On the back of the bottle it’ll say something like 5-5-5 or 10-10-10.

That’s the amount and ratio of macronutrients. So in 5-5-5 there’s equal amounts of the three nutrients, but in lower quantities than 10-10-10.

Worm castings are 1-0-0 but you use more than would with chemical fertilizer, because it stays in the soil.

That makes no sense, but think of it like this: the fertiliser is 10-10-10 but you use a tiny capful, and water it down a LOT. Then most of the water drains away from the plant pot. Whereas the worm castings is only 1-0-0, but you use a lot more (how much doesn’t really matter, I make a potting mix that’s about 10% worm castings) and it stays put.

Though we’ll all agree that even if we put a TONNE of worm castings on, we’re only ever gonna get nitrogen, since the other nutrients are 0-0.

Ok, but what are the macronutrients

Nitrogen

Helps plants make the proteins they need to create new tissues and grow properly. In layman’s terms, it makes good leaves. Yay.

Phosphorus

Encourages healthy root growth, as well as helping plants to produce buds, blooms, and fruit.

Potassium

Helps plants make carbohydrates and help them fight disease.

There are other micronutrients that you might find in fertilisers, such as:

Calcium – improves the soil

Magnesium – aids in photosynthesis by processing sunlight

Sulphur – helps plants make proteins

Using a hydroponic fertiliser ensures you have the full spectrum of macro and micronutrients.

Signs that your plants are over fertilised

I’ve read a lot on this but thought I would over-fertilise my plants just to see what happened.

All that happened was they stopped growing. My Pothos got brown marks on the leaves, but that was it.

All of my plants were growing like stink, and then they stopped. I laid off the fertilising for a month and they all went back to growing super fast.

I was fertilising every time I watered. Unless I was adding slow-release food into the soil I couldn’t have over-fertilised them any more.

What I'm saying is that if your plant is unhealthy and you suspect over-fertilising, there's probably something else wrong with it as well. Does it need better light? Better humidity? Are its roots healthy? 

All these things are WAY more important than fertilising.

How to remedy over fertilising

You can flush your plants. I didn’t.

I just stopped fertilising them. I kept a close eye on the N-joy, because it looked sad and sad looking plants attract pests.

Don’t panic. It’s not the end of the world.

Can you fertilise plants when you bottom water?

Yes, sure.

HOWEVER

If you’re using chemical fertilizer, rather than aquarium water, you’re going to make sure it’s reeeeally diluted. I’d go with at least half of what the manufacturer recommends, otherwise, you might end up with root burn.

And I’d worry that terracotta pots would soak up the fertiliser and start to smell. That may be an unfounded worry, but my house is tiny and I would prefer it not to smell of seaweed.

Stick and automatic fertilising systems.

By which I mean those little sticks of slow-release fertiliser you stick in the top of the soil.

I’m only going to mention them briefly, because from what I’ve read, they’re not very popular with professionals, for a few reasons:

• The slow-release system is…erratic as best, and it’s the general consensus that they’re just not very reliable when it comes to releasing fertiliser. Maybe in the future they’ll improve
• They’re chemical. A lot of houseplant experts favour organic fertilisers like seaweed or fish emulsion.
• They look weird. Ok, I made this one up, but they do. They’re about as aesthetically pleasing as those yellow things some people use to trap fungus gnats.

If you’re going on holiday for an extended time, then, by all means, use one of those slow release sticks, but chances are, your plants will be fine so long as you feed them before you go.

Are there any plants you shouldn’t fertilise?

Yes, actually.

You don’t have to fertilise carnivorous plants because they’ll catch their own food. And I can confirm: they’re EXTREMELY efficient.

My sundew actually has a bit of an aphid infestation at the mo, so I keep having to remove some, which I hate doing because I don’t want to damage him.

How useful though?

It’s also a way of keeping down numbers of fungus gnats down that doesn’t compromise my vegan values. It’s nature, y’all.

(If you didn’t know, vegans don’t object to anyone eating meat if they need to do it to survive – it’s the people that eat it gratuitously that piss us off).

Which plants need fertilising a lot?

The one indoor plants that will DEFINITELY need a lot of feeding is anything citrus. They can look really sad and spindly pretty quickly if you don’t feed them enough. Luckily, they’re one of those plants that’s popular enough to get its own type of fertiliser, so just get one of those, and read the instructions.

I’ve heard good things about this one, but any 7-3-3 should do the job.

The only other houseplants I’ve experienced needing more food than any other is Monstera adansonii. I have absolutely no idea why. If you notice the leaves yellowing in a stripey pattern, she’s probably hungry. Or has thrips – mine was both!

I actually don’t think adansonii necessarily need feeding more often than other plants, I think they just show it on their leaves the fastest. Monstera deliciosa NEVER show hunger on their leaves, but that doesn’t mean they don’t need feeding regularly.

What about fertilising plants in leca/semi-hydroponics?

Fertilising plants with water (rather than soil) roots is a bit different because the water can lock in certain nutrients and root uptake and blah blah blah.

All you need to know is that you need a specific hydroponic fertiliser. The good news is that most hydroponic fertilisers can be used in soil, so you only need one.

I would actually recommend getting a hydroponic fertiliser over a regular one because they contain micronutrients as well. Hydroponic fertilisers provide 100% of the plant’s nutrition, whereas soil fertilisers can assume the soil will provide some.

If you’re in the US, go for Dynagro. It’s a one-step system, which is easier than the General Hydroponics Flora system, which is what I use because you can’t get Dynagro in the UK.

I actually don’t know which is better, I just know that getting started with leca is hard enough without having to concoct potions, fun as it is.

You also need to measure the pH of the nutrient water, which is a ballache.

What about foliar sprays?

I have a dedicated article on foliar sprays here, and to be honest, I’m not sure how effective they are. They’re extremely popular for epiphytic plants like orchids, but they’re definitely not a requirement.

I have an orchid one that I use for a couple of reasons, neither of which have to do with fertilising:

1. It’s fun. I love spraying stuff.
2. The spray I have claims to deter pests. I have no idea if it does, but it doesn’t hurt!

If you have any specific questions on fertilising plants feel free to leave a comment. If you’re after a way to keep track of your fertilising schedule, I have resources for that here.