Worm casts, which you will also see called worm castings or vermicast, are the dark, crumbly material earthworms leave behind after digesting organic matter, and they are one of the gentlest and most useful things you can give your plants because they feed the plant and the soil at the same time. The two names mean exactly the same thing, so whichever one you have been searching for, this guide covers what worm casts are, how we make ours, and why so many organic and no-dig gardeners would not grow without them.
By Kel Dickson, Senior Brand Marketing Manager at Gro Worm, organic, peat-free grower. Reviewed by Brian McCuskey. Updated October 2026.
What are worm casts, exactly?
Put plainly, worm casts are worm droppings, and while that sounds unglamorous, it is one of the oldest and best soil improvers on the planet. As earthworms work their way through organic matter they digest it and leave behind a dark material with the look and feel of fresh coffee grounds, which ends up richer than whatever the worms started with.
What sets worm casts apart from an ordinary fertiliser is that they are a living product, because as the worm processes organic matter it enriches it with beneficial bacteria, fungi and enzymes, and it leaves the nutrients in a form that plant roots can take up straight away. You are not only feeding the plant but also the whole soil ecosystem around its roots, which is why gardeners have long called worm casts "black gold" and why our own Black Gold compost range carries the name.
Worm casts or worm castings: is there a difference?
There is no difference at all, since "worm casts" is the term most UK gardeners grew up with while "worm castings" is more common on product labels and in American growing guides, and you will also see vermicast and vermicompost used for the same material. Whatever it is called, what matters is that it is pure, well-processed and free from anything synthetic.
How we make our worm casts
Our process for producing worm casts is simple and natural, and we repeat it every four weeks so that there is always a steady supply of fresh, high-quality casts for our customers.
- Mixing: we begin by mixing our worms with an organic substrate in specially designed trays, which gives them a natural environment to thrive in.
- Feeding: the worms are fed a nutrient-rich organic meal along with water, which provides everything they need to produce high-quality casts.
- Waiting: over the following four weeks the worms digest their meal and naturally produce worm casts that are rich in beneficial bacteria and organic matter.
- Separating: at the end of the cycle we screen the trays to separate the worms from the casts, and the worms go back into fresh trays so the process can begin again.
Because nothing else is added along the way, our worm casts are 100% natural and produced in a way that is sustainable for the environment, and they are Soil Association approved and OF&G certified organic.
The benefits of worm casts
Once you understand what worm casts are, most of their benefits explain themselves, but they are worth setting out properly.
They feed gently and slowly. Worm casts are lower in nutrient concentration than a synthetic feed, which sounds like a drawback until you realise it is the whole point, because they release their goodness gradually over weeks, so there is no risk of scorching roots or burning tender seedlings and you can be generous without worrying.
They help plants grow bigger and stronger. The microbes in worm casts help unlock nutrients that are already in your soil but held where roots cannot reach them, which supports stronger roots, lusher foliage and better harvests, and our own growing trials have shown that Gro Worm worm casts can help produce yields up to eight times bigger.
They hold on to moisture. The fine, crumbly structure of worm casts retains water well, so the soil or compost they are mixed into stays evenly moist for longer, which is particularly useful in containers, hanging baskets and dry summer spells when plants need consistent watering.
They improve the soil itself. Worked into a heavy clay bed, worm casts help open it up so roots can push through more easily, and mixed into a light, sandy soil they help it hold water for longer, so you get a soil conditioner and a feed in the same handful.
They are completely natural and peat-free. There is nothing synthetic in our worm casts and nothing that harms the UK's peat bogs, which makes them an easy win for anyone trying to garden in a way that is kinder to the planet.
How to use worm casts
Worm casts are almost impossible to get wrong, and the most popular ways to use them are scattering a thin top dressing around plants and watering it in, blending them into potting compost at up to about one part casts to four parts compost, adding a handful to the planting hole when you plant out, and steeping them in water to make worm tea. We have put all of the detail, including how much to use and how often, in our guide to how to use worm castings, and there are dedicated guides on using worm castings for seedlings and worm castings for houseplants. If you are wondering whether your particular plants will benefit, our article on whether worm castings are good for all plants covers it in full.
Frequently asked questions
Are worm casts and worm castings the same thing?
Yes, they are two names for exactly the same material, along with vermicast and vermicompost, so a bag labelled with any of these should contain the nutrient-rich droppings that earthworms produce as they digest organic matter.
Are worm castings better than compost?
They do different jobs and work best together, because compost is your bulk growing medium while worm castings are a concentrated booster that is richer in microbes and plant-available nutrients than the material the worms started with, so mixing a little castings into your compost gives you the best of both.
How are worm casts made?
Earthworms are mixed with an organic substrate and fed organic matter and water, and over around four weeks they digest it and leave behind their casts, which are then screened to separate out the worms so they can start again on a fresh batch.
Where can I buy worm casts in the UK?
Our pure worm casts are made in the UK, certified organic and delivered to your door, and for allotments, market gardens and larger plots we also offer a 900 litre bulk bag of pure worm casts.
Give your plants the black gold treatment
If you want to see the difference a living soil makes, our pure worm casts are the simplest place to start, and if you would rather have the worm casts already blended in for you, our Black Gold peat-free compost is ready to use straight from the bag.
