Worm castings, which many UK gardeners know as worm casts, are one of the few soil improvers that is almost impossible to get wrong, because they are what earthworms leave behind after digesting organic matter, they end up richer than the material they started with, and they are packed with billions of beneficial microbes, chelated minerals and plant-available nutrients. Since they release everything slowly and gently, you cannot scorch a plant with them the way you can with synthetic feeds, so here is how to use worm castings in every part of the garden, and roughly how much you need.
By Kel Dickson, Senior Brand Marketing Manager at Gro Worm, organic, peat-free grower. Reviewed by Brian McCuskey. Updated October 2026.
What worm castings actually do
Worm castings work in two ways at once, because they feed the plant directly with nutrients in a form roots can take up straight away, and they feed the soil by adding beneficial microbes and a better, more open structure that keeps roots healthy and growing strongly. It is this living, microbial side that sets them apart from a bag of ordinary fertiliser, and it is why no-dig growers, including our partner Charles Dowding, rate worm-worked compost so highly. If you would like the full background on what they are and how we make ours, our guide to what worm casts are and why to use them covers it.
Six simple ways to use worm castings
In the planting hole. When you plant out, mix a generous handful of worm castings into the bottom of the hole and the soil around the roots, which gives new plants an immediate, gentle boost right where they need it and helps them settle in with less transplant stress.
In your potting mix. For containers, hanging baskets and houseplants, blend worm castings through your compost at around 10 to 20 percent by volume, which works out at up to one part castings to four parts compost and is enough to enrich the mix without making it too rich. For plants that like sharper drainage, add a little perlite or grit to the blend as well, and if you would rather skip the mixing altogether, our Black Gold peat-free compost already has the worm castings blended in.
For seeds and young plants. Mixing a small amount of castings into your seed compost gives delicate roots a nutritious start without any risk of burning, and the beneficial microbes help young plants build a strong root system early, which is covered in more detail in our guide to worm castings for seedlings.
As a top dressing. For established plants, pots and beds, scatter a thin layer of castings over the surface and water it in, so that every time you water a little more goodness washes down to the roots, and repeat a few times through the growing season.
As a feeding mulch. Across larger areas such as vegetable patches and flower beds, a layer of Black Gold compost spread over the surface works as a mulch that keeps the soil moist and protected while slowly releasing nutrients and improving soil structure as it breaks down, which suits no-dig beds particularly well.
As worm tea. Steep a couple of handfuls of castings in a bucket or watering can of water for around a day, give it a stir, and use the liquid to water your plants for a quick, gentle pick-me-up, which is a lovely way to stretch a bag further and perk up hungry houseplants, vegetables and flowering plants. Our guide to making worm tea has more detail.
How much should you use, and how often?
This is the easy part, because it is very hard to overdo it, and since worm castings are gentle and non-scorching there is no burn risk the way there is with strong synthetic feeds, so you do not need to measure to the gram. A handful per planting hole, a thin top dressing every few weeks in the growing season, or 10 to 20 percent through a potting mix is all you need, and you can ease off over winter when plants are resting and taking up far less.
Frequently asked questions
How much worm castings should I use?
A little goes a long way, so use a handful per planting hole, blend around 10 to 20 percent through potting compost, or scatter a thin top dressing over pots and beds, and there is no need to be precise because castings are gentle enough that rough amounts work fine.
Can you use too much worm castings?
It is very hard to, because unlike synthetic feeds worm castings release nutrients slowly and will not scorch roots, so at most you are being generous with a good thing, and that gentleness is one of their biggest advantages.
How often should I apply worm castings?
Work them in at planting, then top-dress every few weeks through spring and summer or water them in as worm tea, and reduce or stop over autumn and winter when growth slows and plants need less.
Can I use worm castings for seedlings?
Yes, and they are one of the safest things you can give young plants, so mix a small amount into your seed or potting compost and the gentle, slow-release nutrients and microbes will support strong early root growth without any risk of burning.
How do I make worm tea?
Steep a couple of handfuls of worm castings in a bucket of water for around a day, give it a stir, then use the liquid to water your plants, which makes a quick, gentle feed and a great way to make a bag of castings go further.
If you are wondering whether a particular plant will benefit, our article on whether worm castings are good for all plants goes through it in detail.
Ready to feed your garden the gentle way
However you use them, our pure worm castings are Soil Association approved and OF&G certified organic, with billions of beneficial microbes in every handful and gentle enough for every plant you grow, and for allotments and larger plots there is a 900 litre bulk bag. If you would prefer it ready-mixed, our Black Gold peat-free compost has the worm castings already blended in.
