Worm Castings

Worm Castings FAQ: Everything You Need to Know for Happy Plants

Worm Castings (or vermicompost) are one of nature’s most powerful plant boosters—but they’re also surrounded by confusion. Are they just compost? How much should you add? Can they replace fertilizer? Many plant lovers know they’re good but aren’t sure how to use them effectively for houseplants.

At Sybotanica, we love worm castings for their gentle nutrients and ability to create healthier, more resilient plants. They work for almost every houseplant if used correctly, but there’s a lot of misinformation out there.

That’s why we created this FAQ — to answer everything you might wonder about worm castings.

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General Use

What are worm castings?

Worm castings are the natural by-product of earthworms digesting organic material. They’re nutrient-rich, packed with beneficial microfauna, and act as a slow-release fertilizer for plants.

Are worm castings the same as compost?

No. Compost is decomposed organic matter, while worm castings are processed through earthworms, resulting in finer, more nutrient-dense material with beneficial microbes.

Are worm castings safe for all houseplants?

Yes, almost all plants can benefit. They’re gentle and won’t burn roots like synthetic fertilizers. However, we recommend lighter use for succulents and cacti due to their low nutrient needs. For terrariums and carnivorous plants no worm castings are necessary.

Can worm castings replace potting soil?

No. They’re a soil amendment, not a growing medium. Pure worm castings hold too much moisture and lack structure. But they are great when added to your own or premade soil mix, such as our aroid mix.

Are worm castings organic?

Yes, worm castings are 100% natural and chemical-free. They’re perfect for organic plant care.

Product Use

How much worm castings should I mix into soil?

For most houseplants, we recommend:

  • 10–30% worm castings mixed into your potting soil.

Too much can cause overly dense soil and slow drainage.

Can I top-dress with worm castings?

Yes. Apply a thin 1–2 cm layer on top of your soil and water normally. Nutrients will slowly seep down with each watering.

Can I use worm castings in self-watering planters?

No, we do not recommend doing so, as the worm castings can act as a sponge holding on too much water in this type of setup. This can cause root rot and mould growth on the coir. 

Do Sybotanica soil mixes contain worm castings?

Some of our premium SYBASoil blends include worm castings for natural nutrition and improved microbial life.

How often should I add worm castings?

Add once every 3-4 months especially during spring/early summer, it will slowly release its nutrients over the next few months. Unlike liquid fertilizers, they release nutrients slowly over time.

Can worm castings be used year-round for fertilizing?

Worm castings is a natural, complete fertiliser for your plants. It is the perfect replacement for chemical fertilizers, which can often burn your plant's roots due to their high concentration of minerals and salts.

Unlike liquid fertilizers, worm castings can be applied throughout the year without the risk of burn! 

Can I use worm castings for propagation?

Not alone. They’re too dense for cuttings. For propagation, use a light mix (like coco coir and perlite) and add a small amount of worm castings for nutrients. You can read more on our tips regarding plant propagations here.

Is worm castings safe for my animals / children?

Worm castings is a natural product and completely safe for animals, for example in a terrarium. However we do not recommend eating it, and keep it out of reach of children.

Can I mix worm castings with mineral substrates like SYBAStones?

No. Worm castings work best mixed with organic soil. For semi-hydro, feed nutrients via (liquid) mineral fertilizers instead.

Do worm castings replace fertilizer?

They provide long-term nutrition but are not as concentrated as liquid fertilizers. We recommend combining them with SYBASupport Plant Food for balanced feeding.

Benefits & Results

What nutrients are in worm castings?

They’re rich in:

  • Nitrogen for leafy growth
  • Phosphorus for strong roots and blooms
  • Potassium for plant health and resilience

They also contain trace elements and beneficial microfauna.

Do worm castings improve soil structure?

Yes. They help soil stay aerated while holding the right amount of moisture, which is perfect for tropical plants.

Are worm castings pH neutral?

They have a near-neutral pH (around 6–7), making them suitable for almost all houseplants.

Are worm castings safe for edible plants?

Yes. They’re natural, safe, and non-toxic.

Do worm castings help prevent pests?

Yes. Healthy soil with worm castings supports microfauna that improve plant resilience and can make plants less attractive to pests like spider mites or aphids.

Will worm castings cause fungus gnats?

No. They don’t create pests themselves. Fungus gnats are usually caused by overwatering and decaying organic matter, not clean worm castings.

Can worm castings help stressed or sick plants?

Yes. The beneficial microbes can support root recovery and improve nutrient uptake, especially after pest attacks or overfertilization. Always make sure to not overdo it, as too much worm castings will not be beneficial to your sick plants.

Advanced Use & Edge Cases

Can I brew worm casting tea to water my houseplants?

Yes. Mix about 1 tablespoon of worm castings per liter of water. Let it steep for 12–24 hours (stir occasionally), then strain and water your plants. Use within 24 hours.

Can I combine worm castings with slow-release fertilizer?

Yes. Worm castings offer microbial and organic nutrients, while slow-release fertilizer provides sustained NPK. The combo supports both immediate and long-term plant health.

Are worm castings good for orchids?

Only in small amounts and mixed with airy media. Most orchids don’t like dense soil, so keep it below 5–10% if you add any at all.

Can I use worm castings with hydroponic systems?

No. Worm castings clog hydro lines and introduce organic matter that can rot in water. Use liquid nutrients like our mineral SYBASupport instead.

Can I mix worm castings into my lawn or outdoor containers?

Absolutely. For lawns, top-dress lightly. For containers, use 10–30% in the soil mix for stronger root development and long-term nutrition.

Are worm castings safe for seedlings and cuttings?

Yes. They’re mild enough for seedlings, especially when diluted or mixed lightly into propagation mixes. For delicate cuttings, use only a small pinch.

Can I sterilize worm castings before use?

We do not recommend putting our worm castings in the microwave or oven, as this would kill the beneficial microfauna that make them valuable. Always use them as-is, unless you suspect contamination (which is rare from trusted suppliers like Sybotanica).

Can I use worm castings for bonsai trees?

Yes, but sparingly. Bonsai soil needs to stay airy and drain well, so only mix in a small amount (5–10%) or apply as a light top-dress.

Are worm castings vegan?

Technically no, since they are created by worms. But they’re cruelty-free, and no worms are harmed in the process. It depends on your definition of vegan-friendly.

Can I use worm castings for carnivorous plants?

Generally no. Most carnivorous plants prefer nutrient-poor substrates. Adding worm castings can cause overfeeding and damage the plant.

What’s the difference between worm castings and vermicompost?

Worm castings are the pure excretion from worms. Vermicompost may include partially broken-down organic matter along with castings. At Sybotanica, we only use clean, high-quality castings.

Do worm castings increase humidity in the soil?

Not directly, but they do help soil hold water longer. This can reduce how often you need to water moisture-loving plants.

Can worm castings cause salt build-up over time?

No. Unlike synthetic fertilizers, worm castings have low salt content and won’t accumulate harmful residues.

Should I use worm castings with every watering?

No. They're not a liquid nutrient. Apply every 3–4 months as a top-up, or use worm tea monthly for a gentler liquid boost.

Will worm castings attract worms to my houseplant pots?

Unlikely. Earthworms won’t appear unless you introduce them. Indoor pots generally aren’t suitable environments for worms to survive long-term. But our worm castings can contain eggs and small worms, this is a sign your mix and worm castings are really fresh!

Can I compost old worm castings?

Yes. If you’ve reused them or they’ve compacted too much, you can add them back to your compost bin—they’ll enrich your next batch.

Can I mix worm castings with coconut coir?

Yes. Coir lacks nutrients on its own. Mixing in 10–30% worm castings creates a light, balanced substrate ideal for many houseplants.

Do worm castings work in clay-heavy soils?

Yes. They improve structure, increase aeration, and introduce microfauna that help break down compacted clay.

Are all worm castings the same quality?

No. The quality depends on what the worms were fed. At Sybotanica, our castings come from clean, plant-based worm diets consisting of garden and food waste—no manure, no smell.

Want to know even more about Sybotanica's worm castings?

Check out our worm castings product page or read more here.