Issue: Novem1er 3, 2001

Fungus gnats when bring plants in or repotting

I recently purchased potting soil at my neighborhood nursery. I used this soil to repot several houseplants. It appears every time I water these plants, small black gnats start flying around and I mean a lot of them. Is it possible the soil had bugs and now they are hatching? How do I get rid of them in the soil? I have over 50 houseplants. -Judy DG

These two questions are similar and have the same answer, so I will use them together. Fungus gnats are a problem with house plants. They may come in potting soil, especially if the bag has been opened or if it was torn when you bought it. Houseplants kept outside during the summer may also become infested while outdoors.

There are various insecticides (organic and synthetic) which may be used effectively. Check your local nursery or garden center for the ones available in your area. Read the label before purchasing to be certain it is designed to control fungus gnats indoors and to be sure you understand and can follow the directions. It is very important to understand and follow the directions when using indoor pesticides. These products will be most effective if they can be used as a soil drench to kill the gnat larvae. It is also possible to reduce the problem, but not eliminate it, using "yellow sticky traps." These are yellow because gnats are attracted to yellow. They are sticky to trap the flying adult insects and remove them from the home environment. They are non-toxic but can be messy. They should be well illuminated to be most attractive to the gnats. These traps are often employed in conjunction with the indoor pesticides to give immediate reduction in the number of flying gnats while the soil drench pesticide will reduce the number of larvae which will otherwise produce more flying adults.

Covering materials when frost expected

What is good to cover plants with so they don't freeze? Plastic? Fabric? -Jeanne

You can use either fabric or plastic, but fabric may protect a little better. Clear plastic only protects if there is a layer of moisture on the underside. Clear plastic is transparent to the infrared wavelengths of heat. Plastic will also cook the plants in the morning unless it is moved soon after the sun comes up; however, the temperature may still be freezing outside.

It is hard to get the plastic to work to your advantage. Black plastic may be a little better, but it will also heat up in the sunlight. Fabrics may insulate better and won't heat as rapidly in sunlight. Light fabrics may not insulate very well.

In either case, tie down any covering materials to protect from wind blowing it away. Moist soil under the covering helps protect the plants too.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!