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One of the great benefits of having a worm farm is the worm castings, or vermicompost, that the worms produce from organic waste. This wonderful substance is excellent for the soil, but you might be wondering how to tell when it’s time to harvest the worm castings. If you’ve had a worm farm up and running for at least a few months, then it’s probably a good time to start thinking about harvesting the valuable castings. It takes worms approximately three months to process organic matter into castings under optimum conditions, but it will depend on a range of factors, especially whether the conditions in the worm farm are ideal for the worms. The rate at which the worms can process organic waste depends on factors like temperature, humidity, and pH in the worm farm.

There are a number of signs to look out for when deciding whether it’s time to harvest. In general, if you notice that there is very little food and bedding left in the worm farm, there is likely to be a good amount of castings to harvest. Other signs to look out for are the texture and colour of the matter in the worm farm. Mature worm castings have an even, fine texture and a rich, dark brown colour. There should be no chunks of organic matter left. If it looks like the worm farm is mostly castings with some large pieces of food left, you can remove these to put back in the worm farm after harvesting. There are some things such as avocado seeds that are obviously just way too big for the worms to get their little mouths around! As well as sieving the worm castings to sort out any unprocessed organics to put back into your worm farm, you can also put these more difficult to chew on materials through a blender to help your wormy friends to feast on them.

You might also notice fewer worms and slower reproduction when the worm farm is ready to harvest, as the ratio of food and bedding changes. There should not be any strong smell to the castings – they should have a mild, earthy scent. To sum up, castings that are ready to harvest should have a very fine texture, a dark colour, and an earthy inoffensive odour.

As well as the castings being ready to harvest, another consideration is to work out the best time to harvest them depending on what you can do with them in the garden at that time of year. In other words, the castings keep very well in your worm farm, giving you a choice as to when they will be most useful in the garden. The start of spring or end of winter is a great time to harvest, as plants will benefit greatly from the application of castings ahead of the growing season, but autumn is also a good time to nourish the soil, particularly as it is a good time to establish new trees and shrubs before the soil cools down in winter.

If conditions are hot and dry, it is better to make up a liquid fertiliser by putting some castings into a porous bag such as hessian or an old pillow slip and soaking the bag in water for a couple of days. Worm castings are excellent for plants that are getting established or that are flowering and fruiting, and they can also be harvested any time of year in small amounts for this purpose. Plants in containers will also benefit from regular applications of castings to help replace organic matter in the soil, and they can be harvested any time for that. Depending on the type of worm farm you have, a very useful tool for harvesting small amounts of castings from the lower levels is the Tumbleweed Compost Mate, like a big ‘compost corkscrew’ that can be used to extract a core of castings while aerating at the same time.

Knowing when to harvest and what to look out for depends on the type of worm farm you have. If you have a layered worm farm such as the Tumbleweed Worm Café or Can-O-Worms, it’s hard to know what’s happening in the lower layers, but if the worms have eaten all of the food and bedding in the top layer, then it’s likely the bottom layer is ready to harvest. In a three-layer worm farm such as the Worm Cafe, the middle layer might not be fully broken down yet, so don’t harvest this layer yet. It will probably look mostly decomposed and have quite a few worms in it. The best and most mature worm castings will be on the bottom layer of the worm farm. Sometimes they can have quite a damp spongy texture if there is a bit more moisture in the worm castings, or if they are drier, they have a fine texture and look like coffee grounds. Either way, they should have a fine uniform texture and a rich brown colour.

Another tip when you harvest castings, if they are very moist and spongy you can put them on a tarpaulin to dry out to make them easier to disperse for when you want to dig them into your soil or use them as a potting mix ingredient. Alternatively, you can put a few scoops of the moist castings into a bucket or watering can and make them up into a slurry that can be watered onto the soil or pot plants, an excellent way of getting them down to the roots of established plants. Make sure to take the rose off the end of the spout of your watering can to stop it getting clogged up.

The bottom line with worm castings is that you have one of the world’s best fertilisers to add back the nutrients that plants are constantly removing. Your kitchen scraps have pretty much the exact balance of nutrients you need to replace, especially if you are using the castings to feed your fruit and vegetable garden. So if you are casting around for a job to do in the garden this week, look no further than your worm farm!

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