Harvesting worm castings

When to harvest

A worm bin is ready to harvest when most of the original bedding has been converted into dark, crumbly, coffee-ground-like material. This typically takes 3–6 months depending on bin size, worm population, and feeding frequency. You'll notice the bin smells earthy rather than like decomposing food.

Method 1: Migration (easiest)

Push all the finished material to one side of the bin. Add fresh, moist bedding and food scraps only to the empty side. Over the next 2–3 weeks, worms will migrate toward the food. Once the migration is complete, scoop out the finished castings from the depleted side — most worms will have moved on.

Method 2: Light sorting

Dump the bin contents onto a tarp in bright light or sunlight. Worms avoid light and will burrow toward the centre of each pile. Scrape off the outer layer of castings, then wait a few minutes as worms retreat again. Repeat until you're left with a small ball of worms. Return them to the bin with fresh bedding.

Method 3: Screen sifting

For cleaner castings, pass the material through a ¼-inch hardware cloth screen. Castings fall through; worms, partially decomposed material, and cocoons stay on top. Return everything that doesn't pass through to the bin. This method gives you the finest finished product.

How to store and use castings

Castings can be stored in a breathable bag or container (avoid airtight sealing — they need some airflow) and remain potent for 6–12 months. Use them as:

• Potting mix amendment — blend 10–20% castings into your mix
• Top dressing — sprinkle around the base of plants and water in
• Seed starting — mix with perlite and coco coir for a gentle, nutrient-rich starter medium
• Worm tea base — steep a cup of castings to make liquid fertilizer