The time has come to give seedlings their head start

A flat of onion seedlings – Gold Coin, a cipollini type that I could not purchase as field-grown transplants and must grow from seed – sits in my classroom window, a reminder that a summer in the garden is only two months away. On sunny days the afternoon sun is intense and by the end of the day the seedlings are hugging the glass. I rotate it every workday to keep the seedlings growing as upright as possible. I water them as often as necessary, and this past week I added a half-strength shot of Miracle-Gro to the water. I have ambitions for these seedlings.

Onions are the only plants that I would start from seed this early in the year. They will be planted as early as the garden’s soil can be worked and will tolerate a light frost. It is still too early to start tomato seeds or any of the other warm-season vegetables that should go in the ground as transplants, including peppers, eggplants and cucumbers.

Enthusiasm must be tempered with knowledge that seedlings of these crops should be planted in the garden no later than eight weeks from the day the seed was sown. Older transplants, pot-bound and leggy, will remain stunted after planting in the garden and produce poorly.

Memorial Day is often chosen as planting day in central Maine, as good a guess for last frost date as any. Sow the seeds for garden transplants in early April, no sooner. This will give you six weeks of growth indoors, under lights, and a final week for hardening outside before planting in the garden.

Here are some additional tips for producing strong, healthy transplants:

. Seeds planted in cold, wet soils germinate slowly if at all. Reduce the germination time with bottom heat, maintaining a soil temperature between 65 and 70 degrees. Heating mats and soil thermometers are available from mail-order garden supply houses.

. Use a sterile growing mix with a starter nutrient charge and a wetting agent such as Fafard No. 2 or Pro Mix. Wet the mix ahead of use to the moisture content of a wrung-out sponge.

. Use new or sterilized 2- to 3-inch peat pots or cell packs with similar size cells. Seeds and seedlings stay too wet for too long in larger pots.

. Sow two to three seeds per pot or cell, thinning to the sturdiest seedling after the first set of true leaves develops.

. Sow the seeds on the surface of the growing mix and cover with a thin layer (about twice the diameter of the seed) of a fine-textured germinating mix such as Ready-Earth.

. Cover the pots or cell packs with a loose layer of shrink-wrap to maintain uniform moisture.

. Once the seeds have germinated, remove the cover and the bottom heat. Optimum growing temperatures for air and soil are 65 degrees during the day, 60 degrees at night.

. Provide supplemental lighting as soon as the seedlings emerge; window light alone is too low in both duration and intensity. Standard fluorescent tubes (shop lights) are adequate. Keep the lights on for 14 hours a day, maintaining the tubes two to four inches above the growing seedlings.

. Water the pots gently and thoroughly on sowing day, then as often as necessary to avoid excessive drying. Use a half-strength solution of water-soluble fertilizer to provide essential nutrients, watering from above to leach excess nutrients from the pot.

. Before transplanting your seedlings to the garden, they must be hardened with a slow transition to outdoor conditions. Begin by setting them outside (temperature above 45 degrees) in partial shade for one or two hours a day, gradually increasing the light and the length of exposure over a one-week period.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.