Why your houseplants will love worm poo

Worms are fascinating creatures. They can eat almost any decayed matter and breathe through their skin.

Gardeners love them too because they give so much back to the soil. When they wriggle their way underground, they leave air pockets in their wake, keeping plant roots happy with a regular supply of oxygen.

The horticultural benefits of worms don’t end at soil oxygenation. Many studies have confirmed that worm poo (otherwise known as worm castings) is one of the best organic fertilizers. They contain a high concentration of nitrates, phosphates, and potassium, the mineral nutrients that plants require in the greatest amounts. They also contain calcium and magnesium, minerals that plants need in trace amounts.

The term for using worm castings as fertilizer is ‘vermicomposting’. The practice is so effective at improving soil quality that it’s used in large-scale agriculture in Canada, Italy, Japan, India, Malaysia, the Philippines, and the US. Gardeners can get worm castings from many horticultural shops and there's a huge range of choice online.

Worm castings have one distinct advantage over synthetic fertilizers and even animal manure. Their nutrient content is more dilute meaning that using worm casting fertilizer is less likely to produce an excess build-up of minerals in the soil, which can kill plants. An additional benefit is that, unlike animal manure, worm castings do not have an odor. Using worm castings will not lead to your house smelling like a farm. 

Apart from providing your plants with food, worm castings increase the metabolic activity of microbes that live in the root zone. These beneficial microbes help your plants take up any soil nutrients more efficiently.

Another way that worm castings improve the condition of the soil is through their fungicidal properties. Adding the stuff to your houseplant soil can ward off fungal growth, which can occur in humid settings.

Below are two methods for infusing houseplant soil with all the goodness of worm poo.

Adding worm castings directly into the soil 

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You can add worm castings straight into the soil like you would any other soil amendment. Bear in mind that worm castings naturally have a slightly alkaline pH of between 6.5 – 7.0. Adding it might raise the pH of your soil enough to damage acid-loving houseplants like African violets, Boston ferns, and Crotons.

If you happen to be re-potting your plant, take the opportunity to mix worm castings into the fresh potting medium, using about 0.6 ml of worm castings for every gallon of organic soil.

If you are not repotting, just spread the same ratio of castings-to-soil evenly over the surface of the soil.

Add worm castings to the potting medium in small amounts and never use it as the only potting medium! They are rich in nutrients and salts so treat them like you would any other type of fertilizer. Your potting medium should be a maximum of 5 percent worm castings.

Make worm tea 

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Worm tea is a diluted solution of worm castings that you can use to water your plants. You can make it up at home if you have the right ingredients and tools.

Use the tea to water your plants every two weeks or so during the spring and summer growing season.

To make worm tea, get a 5-gallon bucket, non-chlorinated water, three to five handfuls of worm castings, a muslin bag (or pantyhose), and one tablespoon of horticultural molasses (a type of sugar), and a fish aquarium bubbler.

Fill the bucket with water ¾ of the way up. Bubble the water with an air bubbler for two hours. Add the castings and molasses. Let the bucket sit for 24 hours, keeping the bubbler going. Finally, pour the mixture through the muslin bag or pantyhose into a new container. You’ve made your worm tea! You will need to use the worm tea as soon as possible because it can start decomposing quite quickly.

Getting the amounts right 

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As with any organic substance, the amount of nutrients inside your worm castings will vary. The ratios of worm castings to potting mixture above are only approximate guides.

For more precise numbers, you can do a little experiment with your plants. Add three different amounts of worm castings on three houseplants of the same species and pot size. Label the amounts used for each one and observe their growth over one or two months. Whenever you top up the worm castings, do so at the same time for all three plants. You'll be able to see which ratio of worm casting works best for improving your plant's health.