Fertilising your house plants: everything you need to know

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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 6 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.

I’m kinda lucky in this respect. I don’t spend much of my time lusting over certain plants. If I see a reasonably priced cute (or ugly) plant in the shop that I don’t already own and think I could do a decent job of caring for, I get it.

This is why I have a house full of plants. And plan one day to adopt a monkey that needs a home.

Am I joking? Who knows? Maybe.

If I can provide a suitable home for a monkey that can’t be returned to the wild and hates other monkeys and loves chilling among the philodendron I’m THERE.

Anyway, if I want massive monkey-supporting leaves, I’m going to need to fertilise my plants so that they can get big and strong and incredible.

I have a video on fertilising house plants here.

House plant 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 plant fertiliser?

House plants fertiliser is basically 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 us 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 stick 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 house plants

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.

• Over winter

Or summer. Depending on where you are. Don’t fertilise your plants outside of the growing season. They don’t need it, and it could do them more harm than good – you run the risk of burning the roots.

Actually, if your plants are still shooting out new growth, feel free to fertilise them if you think they need it. Use worm castings or something equally gentle if you’re unsure.

When to fertilise your house plants

Plants won’t necessarily tell you when they need fertilising as they tell you when they need watering.

Unfortunately, they’re far more dramatic when they’re thirsty than when they’re hungry, so there’s no concrete signs plants give out to suggest they’re hungry, but you should see them perk up once you start feeding them.

Other things to look out for are:

• Weak or stunted new growth
• Yellowing leaves (though check you’re not overwatering first)
• Leaves dropping
• Pale leaves
• No blooms

I would recommend fertilising your plants every six weeks. I use a very diluted liquid seaweed feed. It smells ok, but not great. I only use a tiny amount – usually about a capful per watering can.

When it comes to feeding plants less is definitely more.

It’s generally recommended that you use half of the amount the manufacturer recommends.

Some house plant owners never fertilise their plants. Some (like Amanda from Planterina) are massive believers in fertiliser. She has massive plants so I trust her judgement.

Equipment needed to fertilise your house plants

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 house plants

For one thing, make a note of when you fertilise them. Hopefully, the stars will align and all the plants you want to fertilise will need watering on the same day, but you also don’t want to fertilise onto bone dry soil, because that may lead to root burn

Maybe pick a week and fertilise all your plants that week.

If you fertilise plants that are thirsty, they can absorb it too quickly and get root burn, so make sure your plants are moist before you add fertilise them.

I fertilise my plants over a two day period – I water them one day, then fertilise them the next – just add fertiliser to your water.

I use half the recommended amount of fertiliser to be sure my plants won’t be damaged.

I’ve not fertilised my orchid yet, but it’s been recommended to me that I should bottom water my orchid and add orchid feed to the water. By ‘bottom water’ I really mean ‘soak’ when it comes to orchids, because their roots are as likely to be above the bark as below it. So yeah, I’d have to soak it in fertiliser.

Fishkeeping folks, bottom watering your plants in aquarium water is a great way to water them.

I actually don’t know how old my orchid is, but it was very over watered (and potted in normal compost) and is only just starting to recover. It isn’t recommended that you fertilise plants that are weakened in any way, so, er, I haven’t.

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 personally 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 it fight disease.

The one I use is 19-0-1.5, which is cool because that’s good for foliage. 

It does say on the label that amino acids have been added to promote healthy root growth, so don’t worry too much about the lack of phosphorus.

There are three other nutrients that you might find in fertilisers:

Calcium – improves the soil

Magnesium – aids in photosynthesis by processing sunlight

Sulphur – helps plants make proteins

Like I said, I use seaweed because it’s vegan and natural, so I’m not bothered that the ratios aren’t the same. All my plants look healthy, though I’ll start looking into adding banana skins and rabbit poo to the soil if anyone starts to look a bit peaky.

Signs that your plants are over-fertilised

Over-fertilising and over-watering are similar in that they’re far more likely to kill your plant than underdoing it.

Fertilising plants is a fairly unnatural thing to do. In the wild, plants spread their roots around and take up nutrients through them. There’s no concentrated chemicals or chance that the chemicals could be too powerful.

The ground is massive, so the concentration of plant food will be spread out.

But your plant can’t get away from the chemicals in its pot. So you need to make sure you’re not either fertilising too often or using too strong of a concentration.

Here’s what to look for if you suspect you’re over-fertilising:

• Leaf drop
• Yellow leaves
• Wilting leaves
• Blackened roots
• Slow growth
• Death (nightmare)
• Crust of fertiliser on the top of the soil
• Brown leaf tips

Remedying over fertilisation

Cards on the table, I can’t find any concrete advice on this, so let’s just use our heads.

Start by flushing your plant through with water. If the symptoms persist, then it might be in the best interests of the plant to repot it. It’s not great practice to repot distressed plants, but sometimes it’s kill or cure. Repot the plant in a well-draining potting mix with as little fertiliser as you can find. Thin it down with coir or sand or whatever if that’s all you have.

Then wait.

And hope.

And don’t go overboard with the nurture and accidentally overwater it. The poor thing’s been through enough.

Can you fertilise house plants through bottom watering?

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 the 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 house plants 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!

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 Hydropinc Florasystem, 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.

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