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Black soldier fly compost is one of the most efficient ways to turn kitchen scraps into a nutrient-rich fertilizer for your herb garden. It’s a genuinely useful option for herb growers who want a faster, lower-odor alternative to traditional composting. This guide covers how the process works, how to set it up, and specifically how to use the finished compost to feed a medicinal herb garden.
How Black Soldier Fly Composting Works
Unlike traditional composting, this method relies on black soldier fly larvae (BSFL) to break down organic waste. The larvae are voracious eaters, consuming fruit and vegetable scraps, grains, and even meat and dairy. As they feed, they produce a nutrient-dense byproduct called frass, which works as a fertilizer.
Black soldier flies are native to North America and are often mistaken for wasps, but they have no stinger and are harmless to humans. Adult flies don’t even have functional mouthparts; their only role is to mate and lay eggs in decaying organic matter. The eggs hatch into larvae within a few days, and those larvae do the actual composting work. They typically mature and are ready to harvest around 18 to 21 days after they’re introduced to the bin.
Why herb growers might prefer this method:
- Produces compost faster than traditional composting
- Generally low odor and low maintenance when managed properly
- Frass is rich in nitrogen, phosphorus, and potassium, the same nutrients many nitrogen-loving herbs respond well to
Setting Up a Black Soldier Fly Bin
Container: An 18-gallon plastic bin is a reasonable size for a home setup, large enough to be worthwhile without becoming difficult to manage.
Environment: The bin needs to stay moist and warm, with good humidity. That is why drainage holes in the bottom prevent water buildup, and a lid helps regulate temperature and moisture.
Feeding: Larvae will eat most food scraps, with a particular preference for protein and fat-rich materials like meat scraps and dairy. They’re generally slower to break down onion, garlic, citrus peels, and other pungent, sulfur-rich scraps. Larvae research shows they tend to avoid or eat these more slowly rather than being harmed by them, so it’s worth composting those separately or in smaller amounts to avoid them sitting and rotting in the bin.
A useful reference point comes from Purdue University’s guidance. Roughly 250 larvae can process about 0.09 pounds of food per day, scaling up to around 3.6 pounds per day for a bin of 10,000 larvae. Start with a modest amount of food waste and increase gradually as your larvae population grows, since overfeeding leads to odor and pest problems.
Using the Compost in Your Herb Garden
This is the part most black soldier fly guides skip, so here’s what matters if the end goal is feeding herbs specifically, not just managing waste.
Herbs that respond well to BSF compost’s nitrogen content:
- Basil – a heavy nitrogen feeder that benefits from the boost
- Mint – vigorous and tolerant of nutrient-rich soil
- Cilantro and parsley – both benefit from the organic matter and moisture retention
Use with more restraint on:
- Rosemary, thyme, and lavender as these Mediterranean herbs generally prefer leaner, well-draining soil. Heavy nitrogen application can push them toward excessive leafy growth at the expense of the essential oil concentration that gives them their characteristic scent and flavor
How to apply it:
- Mix finished compost into soil before planting, rather than applying it directly around mature plants in large amounts
- Use it as a light top dressing for established herbs, keeping it away from direct stem contact
- For container-grown herbs, blending it into potting mix works well, since it also helps with moisture retention
A caution worth taking seriously: Purdue’s own extension guide notes that BSF digestate can sometimes carry high salt levels, which can harm certain plants, and it isn’t a substitute for regular fertilizer. They recommend testing the finished compost before relying on it heavily. A basic soil or compost testing kit is inexpensive and worth using, especially before applying it generously to herbs that are more sensitive to salt buildup.
Harvesting the Compost
When larvae are ready to pupate, they naturally crawl out of the compost and toward the edges of the bin, where a collection container can catch them. Once collected, they can be used as a high-protein feed for chickens or other animals, or simply left to complete their lifecycle. To harvest the finished compost itself, sift it through a screen to remove any remaining larvae, which can go back into the bin. What’s left behind is a nutrient-rich amendment ready for the garden.
Troubleshooting
Cold climates: Larvae may mature slowly or fail to survive in cold conditions. Placing the bin in a sunny spot, using a heat lamp, or insulating it with straw can help.
Hot, dry climates: Watch for the compost drying out. Misting it periodically or covering the bin helps retain moisture.
Pest and odor control: Bury food scraps a few inches into the compost rather than leaving them on top. Also, avoid overfeeding, and keep the bin covered to deter rodents and unwanted flies.
Before You Go – Black Soldier Fly Compost
Black soldier fly compost is a practical, relatively low-effort way to turn kitchen waste into a genuine nutrient source for your herb garden. Used thoughtfully, it’s a sustainable addition to a home herb-growing setup. Match nitrogen-loving herbs like basil and mint with generous applications, and go lighter on Mediterranean herbs that prefer leaner soil.
Sources
- Terrell, Catherine, and Laura Ingwell. “Black Soldier Fly Composting Guide,” Purdue University Extension, Department of Entomology (E-276): read the publication
- Research on black soldier fly larvae feeding behavior and pungent/sulfur-rich food avoidance, Discover Applied Sciences (Springer Nature)
References – Black Soldier Fly Compost
Little Herb Encyclopedia, by Jack Ritchason; N.D., Woodland Publishing Incorporated, 1995
The Ultimate Healing System, Course Manual, Copyright 1985, Don Lepore
Planetary Herbology, Michael Tierra, C.A., N.D., Lotus Press, 1988
Handbook of Medicinal Herbs, by James A. Duke, Pub. CRP Second Edition 2007
The Complete Medicinal Herbal, by Penelope Ody, Published by Dorling Kindersley
Check the Following Articles
Easiest Things to Grow in a Garden: Tips for Beginners
Raise pH Level in Soil: Methods for Optimal Plant Growth
Frequently Asked Questions – Black Soldier Fly Compost
How do I set up a compost bin for black soldier fly larvae?
Start with a plastic bin around 18 gallons, with drainage holes and good ventilation. You can attract larvae naturally or source them from a supplier. Once established, monitor the bin regularly to avoid overfeeding and make sure moisture levels stay consistent.
What are the benefits of using black soldier fly compost specifically for herbs?
The finished compost, or frass, is rich in nitrogen, phosphorus, and potassium. Nitrogen-loving herbs like basil, mint, and parsley tend to respond particularly well, while Mediterranean herbs that prefer leaner soil (rosemary, thyme, lavender) should get it more sparingly.
Will onion or garlic scraps harm the larvae?
Not directly. Research indicates larvae tend to avoid or eat pungent, sulfur-rich scraps like onion, garlic, and citrus peels more slowly rather than being harmed by them. This can cause those materials to accumulate if fed in large amounts. Composting them separately or in smaller quantities works better.
Can black soldier fly compost be used on all herbs?
Not without some consideration. It works well for nitrogen-hungry herbs but should be used more sparingly on Mediterranean herbs that prefer leaner, well-draining soil, since excess nitrogen can reduce the concentration of the essential oils that give those herbs their flavor and aroma.
Is black soldier fly compost safe to use without testing it first?
It’s worth testing before relying on it heavily. Purdue University’s extension guide notes that the finished compost can sometimes have high salt content, which can harm certain plants, and cautions that it shouldn’t replace regular fertilizer outright. A basic compost or soil testing kit is a low-cost way to check before applying it generously, especially to herbs that are more salt-sensitive.
What are some effective DIY bin designs?
A plastic tote with drainage holes and a mesh screen cover is a simple, effective option. Some growers use PVC pipe for a vertical composting design. Whatever design you choose, proper drainage, ventilation, and easy monitoring matter most.