Harvesting Worm Castings (and Worm Tea): When It’s Ready + How to Use It
When a worm farm is ticking along, it quietly turns everyday scraps into two seriously useful garden boosters: worm castings and worm tea.
Both can help lift soil health, support steady growth, and reduce your reliance on synthetic inputs. The trick is knowing when it’s ready, how to harvest without stressing your worms, and how to use each product safely.
We get asked about harvesting worm castings all the time, especially by people who are new to worm farming and don’t want to accidentally scoop up half the worm population.
Below is our practical, no-fuss approach, with options that suit different set-ups, including multi-tray worm farms like our Maze Worm Farm with Legs, plus the usual worm farm accessories such as bedding blocks (like coco peat), worm blankets (like Hessian), and collection trays.
If you’re building (or upgrading) your set-up, you can browse our worm farm range here:
What are worm castings, really?
Worm castings are what worms leave behind after digesting organic matter. They look like dark, crumbly soil, but they behave differently to standard compost. Castings are typically finer, more uniform, and designed by nature to help plants access nutrients steadily.
In other words, they’re a brilliant fertiliser alternative when you want to improve your soil without going overboard.
Worm tea (sometimes called worm wee or worm liquor) is the liquid collected from a worm farm’s drainage system, or a brewed liquid made by steeping castings in water. Because the term gets used loosely, we focus on safe, conservative use either way: dilute worm tea before applying, and apply to soil rather than splashing it on edible leaves.
When are worm castings ready to harvest?
You’ll know castings are ready when the materials in your farm have shifted from “bits of food and bedding” to “rich, finished compost-like material”.
Signs your castings are ready
Look for a combination of these:
- Dark colour: deep brown to near black.
- Crumbly texture: like damp coffee grounds, not slimy or stringy.
- Earthy smell: fresh and soil-like, not sour or rotting.
- Less recognisable food: most scraps are gone (except tougher bits like onion skins or avocado skins).
- Worm behaviour changes: worms spend more time in newer feeding zones, and the older area looks “finished”.
Typical timing (realistic expectations)
Timing depends on temperature, what you feed, moisture, and worm numbers. In warmer months, farms often process faster. In cooler weather, things slow down. Instead of chasing a calendar date, we recommend harvesting when the look, texture, and smell are telling you the castings are mature.
How to separate worms from castings
This is the part people worry about, but you’ve got a few easy options. Pick the method that suits your worm farm style and your patience level.
Method 1: The migration method (best for tray-style farms)
If your worm farm uses stacked working trays (common in many systems), the simplest approach is to encourage worms to move up to fresh food.
- Stop feeding the tray you want to harvest for about a week (optional, but helpful).
- Add fresh bedding (for example, coco peat) and food to the tray above.
- Keep that new tray comfortably moist and appealing.
- After 1 to 2 weeks, most worms will migrate upward.
- Harvest the lower tray, which should now be largely castings.
This approach pairs nicely with multi-tray set-ups and accessories like bedding blocks and a worm blanket to maintain stable conditions.
Method 2: Sieve it
If castings are too wet, sieving is frustrating. If they’re lightly dried and crumbly, a sieve can separate out worms and any chunky bits quickly. Just keep it gentle and don’t let castings bake in the sun.
How to harvest worm tea
Many worm farms include a drainage area and a way to collect liquid. In our range, you’ll see set-ups designed for collection via trays and other accessories that make it simple to capture and decant.
A few important safety notes
Because “worm tea” can mean different liquids, here’s our safe baseline:
- Always dilute worm tea before use.
- Apply to soil rather than spraying onto edible leaves.
- Use it fresh. Don’t store it for long periods.
- If it smells rotten, don’t use it. Tip it onto a compost pile instead.
How to dilute worm tea
A practical, conservative range:
- For seedlings: very weak, about 1:20 (1 part tea to 20 parts water).
- For pot plants and garden beds: 1:10 is a common strength.
- If you’re unsure, go weaker. You can always apply again next week.
Worm castings use: pots vs garden beds
Castings are powerful, but they’re also gentle enough to use in small amounts regularly. Think of castings as a soil improver and microbial booster, not a “dump a whole bucket on” fertiliser.
Pot plants: worm castings
For pot plants worm castings use, we like these options:
- Top dressing: sprinkle 1 to 2 cm on the surface, then water in. This is classic top dressing with castings and works well for indoor plants, herbs, and patio pots.
- Potting mix upgrade: mix castings through at roughly 10 to 20 percent of the total mix. For very small pots, keep it closer to 10 percent.
- Revive tired pots: remove the top few centimetres of old mix, replace with fresh mix plus a handful of castings, then water well.
Garden beds: steady, seasonal improvements
For veggie beds and ornamental gardens:
- Pre-plant soil boost: spread a thin layer over the bed and lightly fork it into the top few centimetres.
- Side dressing: place a small handful around heavy feeders (tomatoes, corn, cucurbits), keep it a few centimetres away from stems, then mulch and water.
- Seedling safe compost approach: for new seedlings, use castings sparingly. A light dusting in the planting hole or a gentle top dress is plenty.
How we store harvested castings
Worm castings are best when they’re biologically active.
- Dry slightly, don’t bake: air-dry in the shade until just crumbly.
- Store breathable: a bucket with a lid that is not airtight, or a breathable bag.
- Keep out of sun and heat: a cool shed shelf is ideal.
- Use within a few months for best results.
If castings dry out completely, they’re still useful as organic matter, but they lose some of that living “spark” gardeners love them for.
Common mistakes we see
- Harvesting too early: if it still looks like bedding and scraps, give it more time.
- Too wet conditions: soggy farms slow down and can smell. Aim for “wrung-out sponge” moisture.
- Overfeeding: add food gradually. If scraps linger and stink, scale back.
- Using worm tea too strong: when in doubt, dilute more.
FAQs / Common Questions
When are worm castings ready to harvest?
They’re ready when the contents are dark, crumbly, and soil-smelling, with very little recognisable food left. Most worms will prefer newer feeding areas, making separation easier.
How do I use worm castings in pots vs garden beds?
For pots, we recommend top dressing with castings (1 to 2 cm) or mixing castings into potting mix at 10 to 20 percent. For garden beds, use a thin spread before planting or a handful as a side dressing around established plants.
Is worm tea safe, and how should I dilute it?
Used sensibly, yes. We recommend applying it to soil, using it fresh, and always diluting. Start at 1:20 for seedlings and around 1:10 for general use. If it smells pungent, skip it.
