New strawberry strain is sweet news to gardeners

It's a significant breakthrough, another triumph for the plant breeder - sweet strawberries from seed in just one year. The seed a gardener sows in January will now put strawberries (true strawberries, as distinct from the Alpine variety) on his ice cream by July 4.

This has never been done before, though it has been dreamed of by plant scientist for a hundred years or more. But such thoughts remained just that - dreams - until 1975, when Pan American Seed Company's Dr. Jagan Sharma began collecting strawberry species ranging from Alaska to Patagonia with this one goal in mind.

The result is Sweetheart, a strawberry that in wide-ranging tests last year produced as many or more berries than Ozark Beauty, one of the best-known everbearers for home gardens. They are reportedly very sweet and flavor filled when ripe.

Sweetheart appears to grow equally well on both sides of the Mason-Dixon line. Trials at the Geo. W. Park Seed Company in South Carolina showed that the berry produced fruit throughout the summer, though at a reduced rate when the sun shone most fiercely.

When starting plants indoors, bear in mind that it takes roughly 12 weeks from sowing to setting out the plants in their final beds and another 6 after that before flowering commences. What the gardener in, say, northern Maine does not want is frozen soil when the time comes to set out the new plants or continuing frost at flowering time. In contrast, an intense Georgia sun could prove equally hard on the young plants.

Using Chicago and Tampa Bay climates for their calculations, Pan America Seeds recommends Sweetheart be sown indoors in January or early February in the North and from July through early September in the South.

Strawberry seeds germinate best between 60 and 65 degrees F. Sprinkle the fine seed over moist, well-drained growing mix (water the mix thoroughly two hours before sowing to see that it is evenly moist throughout). Cover the seeds very lightly with more of the growing mix or with vermiculite.

Place the flat in a clear plastic bag to maintain the moisture level. If the mix starts to dry out, sprinkle water over the flat very gently. Within 3 or 4 weeks the seedlings will start sprouting, often in spurts.

Remove the plastic bag and place the young seedlings in a well-lighted area (under lights or in a south-facing window). Transplanting into separate containers can begin after three to four ''true'' leaves have appeared. The new plants will grow best in temperatures that do not rise above the high 60s during the day or below 50 degrees F. at night.

Enrich the planting bed with compost and well-rotted manure. Lacking these substances, mix in milled peat moss and work in one pound of 10-10-10 fertilizer for every 100 square feet of bed. Do not overfertilize. Excess nitrogen will produce good-looking plants with few berries.

Set out the plants 6 inches apart in rows 2 feet apart - or for broad-bed planting, 12 inches apart in every direction.

Sweetheart sends out runners like every conventional strawberry, but yields a much heavier crop when the runners are removed. This becomes most important if the berries are to be treated as an annual crop. It should be noted, however, that some 25 percent of the plants may not flower the first year.

So, how many plants should you grow? Ted Marston, a spokesman for Pan American Seeds and an avid gardener himself, suggests, ''One for fun, 12 for periodic cereal-topping treats, or 100 to satisfy the strawberry cravings of a family of four.''