Now's the time to start seedlings for a summer garden

ON the day of his high school graduation, Bob Thomson was at school in the morning and in business for himself in the afternoon. That's how eager he was to get into horticulture full time.

With a little borrowed money, he bought a used truck and expanded the business he started as a teen-ager, tending to other people's yards: mowing lawns, trimming hedges, and pruning trees. Later he opened a lawn and garden center in Danvers, Mass., and became well known to New England gardeners through his ''Tips From Thomson'' radio series. You probably know him best as the host of the PBS show ''Crockett's Victory Garden.''

Right now this self-taught man, who over the years has learned more than most about his profession, is pointing to the next few weeks on the gardening calendar, specifically to the importance of raising seedlings for this summer's garden. ''Now's the time to begin,'' he says. ''If you're not content with the varieties available as started plants at the garden center, then growing your own seedlings is the only way to go.''

So how do you do this? Mr. Thomson suggests you:

* Read seed catalogs thoroughly. ''You can easily grow $500 to $700 worth of vegetables in a backyard garden each year,'' he says, ''provided you get the best seeds.'' He recommends the hybrids for their increased vigor and fruitfulness.

* Use soilless starter mixes sold at garden centers. These are physically suitable for seedlings (light and fluffy, providing for good aeration and ease of root penetration) and are unlikely to contain harmful damping-off organisms.

* Any container as deep as a tuna-fish can is appropriate for starting seedlings, provided it has a few holes punched in the bottom to allow for drainage. Fill the containers to within half an inch of the rim and sow the seeds.

Another option is to use the Jiffy 7 seed-starter pellets. These compressed pellets expand to seven times their size when soaked in water, becoming, in effect, little seed-starter pots that can eventally be transplanted outdoors as is. Formerly imported from Europe, the Jiffy 7s are now made in New Brunswick. Root penetration through the sides of the Canadian product is said to be easier than through the older European version.

* Sow the seeds from six to 10 weeks before you plan to set the plants out in the garden. The frost-hardy plants (onions, leeks, petunias, impatiens, etc.) go in first, followed by the half-hardy (cabbage family and lettuce), and finally the frost-tender plants such as tomatoes and peppers.

* Water the sown seeds well, allow excess moisture to drain away, and then place the containers or pellets in a plastic bag (used bread bags are good) with two or three small holes punched in to allow a slow air exchange. These mini-greenhouses will prevent the soil surface from drying out too quickly and will also shield the germinating seeds from a sudden cold draft that may blow their way if someone opens a window or door nearby.

* Give the newly sown seeds some heat (70 to 80 degrees F. is best, although they will germinate somewhat more slowly at lower temperatures). If you can use heating cables or heating pads, available from garden centers, to provide bottom heat, so much the better. Otherwise place the seeded flats near some heat source in the kitchen.

* Once the seedlings are up, remove them from this heat source. You want to start the hardening process right away, in Thomson's view.

Place the seedlings in a sunny window where they will get good and warm during the day but cool off at night (although you should pull them away from the window if there is any danger of their freezing).

Seedlings in a window should be rotated every day to prevent them from leaning to one side. If you grow your seedlings under fluorescent lights, keep the tops of them within four inches of the tubes and reduce the heat at night.

* Harden off the plants before setting them out in the garden. Do this by cracking the window a little or placing them in a cold frame that is not tightly sealed. The idea is to expose the seedlings to fluctuating temperatures (but not damaging cold) and gentle air movement.

Tender seedlings are more readily burned by wind at this stage than they are by temperatures, so shield them from winds at all costs.

''As long as there is air movement around the plants, they will readily harden off,'' Mr. Thomson says. After a week to 10 days of this hardening-off process, the plants should be ready to go into the garden.

Meanwhile, Carefree Garden Products has produced a pinup sheet loaded with information on starting seeds indoors. For one of these sheets and some sample Jiffy 7s, send a self-addressed, stamped (two 20-cent stamps) commercial envelope to Monitor Garden, One Norway Street, Boston, Mass. 02115.