Reusing potting soil on mature plants possible, but ill-advised with seedlings

Q: Last spring I made my first attempt at starting my own plants from seed. I sowed some seed in large, open flats. After I transplanted those seedlings, there was some potting soil left over, which I saved. Can I use that soil for this year’s seed sowing endeavors, or should I not reuse it? – K.M., Hermon

A: In general, it is not a good practice to reuse potting soil. Most premixed potting soil used for germination is sterilized and free from insects or pathogens. Sterilizing soil helps minimize exposure of vulnerable seedlings to potentially deadly soil-borne pests. Reusing potting soil compromises its quality. It is possible to sterilize soil by slowly heating it in an oven, but the task is time-consuming and may not be a reliable method of sterilizing soil.

While reusing soil for starting seeds can lead to pest problems, it may be acceptable to reuse potting soil on more mature plant specimens that are healthy and not vulnerable to hard-to-treat diseases such as dampening off. For example, it probably would be fine to use your soil to transplant houseplants into larger containers.

Q: I’m interested in learning more about how to control pests in my garden by using biological organisms. Do you know of any sources for more information and garden supplies related to this topic? – M.B., Bar Harbor

A: A good source is a publication from the California Department of Pesticide Regulation called “Suppliers of Beneficial Organisms in North America.” It lists many companies that sell biological agents and natural enemies to garden pests. Write to the department at 1020 N St., Room 161, Sacramento, CA 95814-5604 or obtain the document on the Web at: www.cdpr.ca.gov/docs/dprdocs/goodbug/organism.htm.

Q: In fall, I dug up some of my begonia tubers, wanting to overwinter them. I had no idea how to treat them, so I put them in my shed, where they have frozen. Should I have prevented them from freezing? Is it too late to take them in? Where should I store them? – J.P., Pittsfield

A: Tuberous begonias must be dug from the soil after the first frost because they aren’t hardy enough to survive the winter cold. After they are dug, the soil should be removed from the tubers. Each tuber should be thoroughly dried before it is set away in storage.

Ideally, tubers should be kept in a cool – not freezing – place that is dark, dry and well-ventilated. An outdoor shed is not the best storage space, and if the tubers have frozen and thawed repeatedly, next year’s blooms aren’t too promising. Hope for the best though. Take them in from the shed and place them in the cellar, or another adequate environment. Pay extra attention to the moisture factor, packing the tubers in fluffy shavings or sand to help wick moisture away from the tubers, thus minimizing the chances for decay.

If the flesh of the tuber is healthy come spring, set them out as you ordinarily would. Perhaps their resilience will surprise you.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, RR1, Box 2120, Montville 04941, or e-mail them to dianagc@ctel.net. Selected questions will be answered in future columns. Include name, address and telephone number.