Composting for

Houseplant compost
should be completely
broken down

By: Sandy Feather 2010
Penn State Cooperative Extension

Sandy's Gardening Columns

Q. There are insects flying around my apartment that I have never seen before. I thought they were termites at first, but their wings seem too short. Perhaps they came out of the soil that I brought into the house from the compost bin? They are about to of an inch long. Their bodies are green and they have large eyes. They buzz around quietly and have not bitten anyone yet. Do you know what they are?


A: You can bring some unwanted guests into your home if you use fresh compost on your houseplants, especially if it has not broken down completely. A number of insects feed on decaying organic matter and find compost irresistible. You would not want to treat your compost for them because insects work in concert with fungi and bacteria to break organic matter down into humus. The writer sent samples that were submitted to Penn State's Insect Lab for accurate identification. They turned out to be black soldier flies. They were probably present as larvae or pupae in the compost the writer used to make a potting mix for her houseplants.

Compost at a landscape supply yard

Black Soldier Flies

Black soldier fly larvae feed on all kinds of decaying organic matter, including compost, manure and carrion. They are not considered plant pests and will not bother the houseplants. Adults feed on pollen and nectar, primarily from plants in the daisy and carrot families. They do not bite, sting, or transmit any kind of disease, nor will they damage household goods. It is just a nuisance to have them flying around your home. No treatment beyond a fly swatter is recommended because they will die out when the organic matter in the potting mix breaks down completely.

Treating compost for houseplants

Although organic matter is extremely beneficial for houseplants, you should screen it and allow it to air dry to reduce the possibility of creating a nuisance in your home. Many of the insects that live in compost will not survive if it is thoroughly dried. To be on the safe side, you might want to pasteurize compost before using it on houseplants. Be warned -- baking compost stinks!

Place it in a shallow baking pan -- inexpensive foil pans are ideal. Heat the compost up to 180 degrees for 30 minutes. Use a meat thermometer to check the internal temperature. You might consider doing this on an outdoor grill to avoid smelling up your house.

Worm Bins

Another option is to get a worm bin. Known as vermiculture, this composting process uses red wigglers and other species of worms to turn vegetable scraps, coffee grounds and other non-meat food waste into a nutrient-rich organic matter that is ideal to incorporate into potting mix for houseplants. The worms live in a sealed bin partially filled with shredded newspaper for bedding. When you "feed" them food scraps, it is important to cover them with a fresh layer of shredded paper to avoid attracting fruit flies.

This a great composting method for apartment dwellers -- they do not smell or take up much space, and the worm castings produced are an excellent source of organic matter and nutrients for indoor plants as well as those in your landscape. The only problem is that once you see the positive effect the worm castings have on your plants, you will want more than a single bin will produce!

The Pennsylvania Resources Council offers training in vermiculture and backyard composting. For a schedule of classes, please visit


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