Pasturization preferred to kill organisms in soil

Reader questions have come to me via e-mail and regular mail, and this week I share some of them with you. If you have gardening questions don’t hesitate to send or e-mail them to one of the addresses below. Please be advised that, regrettably, I am unable to provide individual responses, so please don’t send addressed, stamped envelopes. If you don’t see a response to your questions right away, be patient because eventually most get answered through a regular or Q&A column. Thanks for writing. I very much enjoy your letters.

Q. Will pouring boiling water through potting soil sterilize it? – A Palermo reader

A. Technically speaking, sterilization involves the destruction of all living organisms in the growing medium. Unfortunately, to successfully sterilize potting soil, one must expose it to prolonged high temperatures. The temperature requirements for killing most harmful organisms are not met by exposing the soil to boiling water.

To kill off pathogenic diseases, insects, nematodes and weed seeds, one must heat the medium anywhere from about 120-212 degrees F.

A number of harmful organisms, including destructive fungi, bacteria, slugs, worms, centipedes and water molds are killed in the lower temperature range after about 30 minutes of exposure. Most soil insects are killed in the range of 140-160 degrees F after 30 minutes. Weed seeds are killed between 160-180 degrees F. The hardiest weed seeds and resistant plant viruses are killed at above 200 degrees F.

On the home gardening level, we usually attempt pasteurization rather than sterilization. Pasteurization uses the heat of steam to kill off most soil-borne organisms. You can pasteurize small amounts of growing mediums right in your own oven, but placing moist potting soil in an oven-proof pan and “baking” it at a minimum temperature of 180 degrees for 30 minutes.

Q. I’d like to plant some fruit trees in spring, but most of my property gets shade throughout the day. Can you recommend any fruit-bearing trees that do well in moist shady areas? – M.D., Bradley

A. Most fruit trees and berry bushes prefer to be exposed to full sun. Some varieties will grow in partial shade, but the fruit set will be light. In addition, the fruits harvested from trees and bushes grown in partial shade often are not sweet and tasty.

Generally speaking, sour cherries prefer less sun than other fruit trees, so if you are determined not to clear a larger area for growing fruit trees, thus allowing a greater penetration of sunlight, you might try some sour cherry trees under the current growing conditions. Also, members of the raspberry family may produce fruits in your location.

Q. I don’t have much space to start seeds indoors and I was thinking that I could start some seedlings in hanging baskets by the windows. Do you foresee any problems with this? – E.D., Addison

A. There is really no problem with starting seedlings in a hanging basket, as long as the pot and the medium are sterilized and you thin out the seedlings after germination so they have room to mature. However, there may be another problem with the location you’ve pondered: A window space may provide satisfactory sunlight, but the temperatures will probably wildly fluctuate between the daytime and nighttime hours.

Temperatures near a single-paned window can reach in excess of 90 degrees F on a bright sunny day, and can dip down below 50 degrees at night.

This fluctuation is intolerable for many germinating seeds, which require maintenance of an adequate temperature, and is more than many young seedlings can bear.

If your particular window is insulated by double panes, and if the area receives acceptable nighttime heat, than you may be able to start and grow your seedlings with success. Before you start seeding, monitor the daytime high and nighttime low temperatures to get a feel for what you’re dealing with. Check seed packets for growing information – most will advise you of the ideal germination and growing temperature for various cultivars. If your temperatures by the window are acceptable, sow away!

Q. Last fall, when visiting relatives out of state I saved the curly pod from a honey locust tree. Do honey locusts grow easily in our climate? – T.H., Veazie

A. Honey locust trees thrive in our climate. They grow easily from seed (though before you attempt germination, you’ll need to expose to low temperatures the ones you have saved) and they are also very easily transplanted as saplings. With the proper growing conditions, at maturity this plant self-sows readily from seed and can become weedy – so beware!

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.