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Changing house plant potting mix/soil is one of those areas in house plant care where there’s a lot of leeway between best practice and ‘don’t do that, it’s gross and your plants will die’.
A lot of online sources will tell you that soil needs replacing every x months and that all the soil needs to be discarded and fresh potting mix used.
I understand where they're coming from, and an argument could be made it's best for your plants (I think it's unnecessarily disruptive tbh), but I think that it's unnecessarily wasteful.
Picking the right potting mix is important but it’s nowhere near as important as the care you give your plant.
A lot of people make the mistake of using a great, well-draining potting mix as recommended by the experts, and then accidentally underwater because they’re so worried about overwatering.
I actually think it can be easier for beginners to use a denser mix because it’s easier to tell when it’s wet or dry. But don’t tell other house plant people.
So, how often should you change your house plant soil?
Honestly? It depends on the plant. It also depends on how often you fertilise the soil.
The internet says you should change the soil every 12-18 months, but here’s the thing:
You don’t need to change the soil completely. Just add more when it’s time to repot.
I have a hoya bella that's been in the same soil for three years. She needs repotting, the soil is pretty compacted. But she's fine. In fact, I have to wait to repot her because she's about to bloom and if I do it now she'll drop all her peduncles.
How do you know when to change house plant soil?
Most of the time, you never have to completely change it.
The only time you need to chuck it out and start again is if the soil is totally unsuitable and retains too much water (for example if you used outdoor soil for an indoor plant).
Soil breaks down and gets compacted over time. Most of the time you can uncompact it by soaking it or breaking it up with a chopstick.
However, over time the soil can break down into tiny pieces that then merge together into one big lump.
This has happened with my hoya bella (hence needing a repot). When I water her the water runs over the top of the top and doesn’t permeate it AT ALL so she needs to be soaked in a pasta bowl for a couple of hours.
I still won’t change all of the soil when I repot her. It’s just too wasteful for me. I’ll chuck out whatever’s genuinely unusable – i.e. the powdery stuff that’s compacted into lumps.
What happens next depends on how much that disturbs her roots.
If they’re disturbed a lot and there’s a lot of breakages, I’ll just replace the missing soil and leave her be.
However, if the roots are pretty untouched, I’ll replace about half of the soil with fresh stuff and keep the stuff I removed to either mix in with fresh potting mix (we don’t waste) or if it’s in really bad shape I add it to my garden outside.
Should you change house plant soil when it’s mouldy?
No. Mould is usually harmless to house plants and is usually just a sign that the soil is too damp on the top.
I get a lot growing in March/April when I start watering more, the humidity starts to increase, but it’s still pretty cold. If a plant is particularly prone to mould, I bottom water.
Mould will attract fungus gnats, and I see a lot of people (on Facebook especially) claiming that you need to change the soil to get rid of fungus gnats. Not only is this supremely wasteful, but also pretty pointless.
Unless you learn what attracted the gnats in the first place, they’ll just come back.
The best option is to the bottom water (so it's a less attractive environment for them) or to add a cleanup crew like springtails to eat the fungus. Springtails are beneficial for soil and stay in the plant, fungus gnats are pretty benign with regards to plants but very annoying.
Do you need to change all of the soil?
No. Rather than adhering to a strict repotting schedule wait until the plant is ready to be repotted, and then just add new soil around the sides.
If the old soil is still pretty structurally sound (ie. hasn’t turned into powder) but you worry about its nutritional value, you can add extra worm castings into the new potting mix.
Potting mix ingredients like perlite, leca, and even orchid bark take years to break down (even coir takes a loooong time) so as long as they’re still providing a solid structure for the roots you can always add worm castings (or just make sure to regularly fertilise) and keep on using them.
Can you reuse old house plant soil?
I do. All of it. I never chuck it out.
As I’ve been in the hobby longer, I’ve become aware of how wasteful it is, and a lot of the products we use aren’t sustainable.
Best practice suggests we should always use fresh potting mix, and replace it every year, especially if we’ve had pests.
However, it’s largely unnecessary to replace it. Especially if its only crime was to have pests.
Most house plant pests don’t live in soil – they live on the plant.
Sure, they might drop into it, but they can’t live in soil for very long without access to plant matter. Rather than chucking out infested soil, put in a sealed bag and don’t reuse it for a couple of months.
I even reuse soil that’s had root rot in it. I just keep it out in the open air so it dries out properly – root rot is caused by bacteria, and if there’s plenty of oxygen, they can’t survive.
And yes, I reuse soil that's full of old roots. They'll break down over time and release nutrients into the soil.
I also catch my water underneath my plants when I water. Let’s not waste water (especially if we’ve gone to the trouble of adding nutrients).
I have this very professional set up:
Should you remove old house plant soil when repotting?
99% of the time there’s no need.
When I repot, I sit the old pot in the new pot and then fill around the edges with new soil. Then I take the plant out of the new pot and pop the plant in the hole left by the pot. It's extremely satisfying.
I don’t even break up the roots, just slide it straight in.
If your new soil is moist the roots will grow into it by themselves and if you’re lucky, you won’t get the repotting sulks (the term I just made up for when plants throw a hissy fit when they’re repotted).
Does potting mix go bad?
Yes and no.
I’ve found that it keeps pretty well in a container that allows for aeration.
I have kept it in plastic bags and it’s fine, but it can get a bit musty and mouldy over time. This doesn’t make it bad per se, but it will lead it to break down faster.
Though it doesn’t technically go bad, it does lose its nutritional value over time.
When you buy plants from the nursery they usually have slow-release fertiliser in the soil that will last about one growing season, depending on how fast your plant grows.
Once that fertiliser has gone, you’ll either need to replace it with something similar, worm castings, or fertilise regularly.
As some parts of the soil break down, such as orchid bark, it’ll release nutrients into the soil.
Whilst this is good and natural and fine it can change the pH of your soil and cause nutrient lockout.
If you’re fertilising properly but your plants are acting like they’re starved, this could definitely be your issue.
One of the reasons I don’t like to use or recommend DIY fertilisers is that they can cause potting mix to go bad.
The actual mix is fine but it’s filled with rotting matter and it goes all mouldy and the fungus gnats get ridiculous and it’s a huge, annoying mess.
Compost your banana skins if you must, but don’t add them to soil when they’re still, er, banana-y.
Soil does break down over time, but there’s rarely a reason to throw it out and start again. In fact, I can’t think of a situation in which you’d need to do that, bar using outside dirt for your houseplant.
Instead, just add fresh soil every time you repot, and you should be good.
If your plant has shallow roots and rarely needs up-potting don’t worry.
Plants with shallow roots are typically pretty tough and used to a life without much water/nutrition.
Cacti are perfectly happy in the same soil for decades and will continue to bloom as long as you fertilise them. If you notice that the soil is hydrophobic after a few years, remove about half of the soil and replace it with fresh.
It doesn’t matter how you do this. It doesn’t need to be evenly distributed. Just try not to disturb the roots too much.
Hoya aren’t used to arid environments like cacti, but they are epiphytes, so they don’t grow big root systems. Mine seem to be happy with nutrient-free soil as long as I regularly fertilise them. hence why they tend to do well in leca and pon.
I hope this was helpful! Don’t let people scare you into throwing away perfectly good soil!