Worm castings and worm tea: how to use them

A worm farm gives you two things: castings (the solid worm poo) and liquid that drains out the bottom. Both are useful in the garden. They are gentle, they add organic matter and nutrients, and they put your kitchen scraps back to work. Here's how to use them well, and an honest look at what they can and can't do.

Castings, leachate and worm tea

Gardeners use these words loosely, so it helps to be clear.

• Castings are the dark, crumbly material worms leave behind. They look like fine, rich potting soil.
• Leachate (often sold or called "worm wee" or "worm juice") is the liquid that drains through the farm and out the tap. It's mostly water that has run through the castings and rotting food.
• Worm tea, strictly speaking, is made by soaking castings in water. Many people also call leachate worm tea. In this guide, we'll say leachate for the liquid from the tap.

Using the liquid

Leachate is too strong to use straight. Dilute it until it's about the colour of weak tea. For most farms that works out at roughly one part liquid to ten parts water, but colour is a better guide than a fixed ratio because the strength varies.

How to apply it:

• Water it onto the soil around plants rather than over the leaves, especially on anything you'll eat.
• Every two weeks or so through the growing season is plenty for most plants. Pot plants and hungry veggies can have it a little more often.
• Use it fresh. Collect it and use it within a day or two. Liquid left sitting in a bucket can go stagnant and smell.

If the liquid is very dark, smells bad or sour, the farm is probably too wet or overfed. Leave the tap open to drain, add dry bedding and dilute well before using any of it.

Using the castings

Castings are the more valuable product. Ways to use them:

Top-dressing

Sprinkle a thin layer, a centimetre or so, around plants and cover it with mulch. Water it in. This suits veggies, herbs, pot plants and fruit trees.

In potting mix

Mix castings into potting mix at about one part castings to four or five parts mix. Using castings on their own, or in large amounts, can hold too much water and stay soggy in a pot.

Raising seedlings

A small amount of castings in seed-raising mix, or a light sprinkle when you pot seedlings up, gives them a gentle start. Keep it to a small proportion, as seed-raising mix should stay light and free-draining.

At planting time

Throw a handful into the planting hole when you plant seedlings or shrubs, and mix it through the soil.

What's known and what's lore

Worm products have a big reputation, and some of it runs ahead of the evidence.

What's well supported:

• Castings add organic matter, which improves soil structure and water holding.
• They contain plant nutrients in forms that are released gradually.
• They're mild. Used sensibly, they are very unlikely to burn plants the way strong fertilisers can.
• Turning food scraps into castings keeps them out of landfill.

What's less certain:

• Nutrient levels vary a lot from farm to farm, depending on what the worms were fed. Castings and leachate are gentle feeds, not a complete replacement for fertiliser on hungry crops.
• Claims that worm tea prevents plant diseases or acts as a pest spray come mostly from gardeners' experience and small trials. Results are mixed, so don't rely on it to fix a disease problem.
• "Brewed" or aerated worm teas are popular, but whether they work better than simply using castings in the soil is not clearly settled.

The practical view: use worm products as a regular gentle feed and soil improver, and expect steady, modest benefits rather than miracles.

How worm products fit with other feeding

Hygiene

Worm farms contain the same kinds of microbes found in compost and garden soil, and the leachate drains through rotting food. Simple precautions are sensible:

• Wear gloves when handling castings, leachate or bedding.
• Wash your hands well afterwards, even if you wore gloves.
• Don't splash liquid onto leaves of crops you'll eat raw, and wash all produce before eating.
• Avoid breathing in dust from dry castings or potting mix. Keep mixes damp when you handle them.

These are the same habits recommended for handling any compost, soil or potting mix.

Storing castings

Castings keep best slightly moist, in a covered bucket or bag in the shade. Use them within a few months. If they dry out completely, they're still useful, just slower to work.