Plant sweet corn outside. Sow cucumber and eggplant seeds in flats indoors. Start acclimatizing tomato and pepper plants by moving them into cold frames--but not yet outdoors. And if your yard is well protected from cold winds, plant beans outside.

A warning: The advice above is only conventional wisdom.

Planting dates were once set by the top astrologers of the land, and pharaohs and caciques presided over the ceremonies launching particularly important crops. Science hasn't completely taken over these functions. In their recommendations, agricultural extension services rely on the safety net of statistical averages--last frost dates being the most important components. And the dates given extend over 10 days or even more.

The experts do not agree. It's all a matter of temperament, and the battle is between the bold and the circumspect.

"Don't set out your basil until the middle of May--and that's the earliest date," says Thomas De Baggio, founder of the Arlington herb nursery Earthworks and a student of herb lore. "Basil is similar to eggplant in its great sensitivity to cold."

De Baggio takes exception to a remark by James Horne, manager of the Washington Cathedral greenhouse, who suggested in this space two weeks ago that all herbs may be safely planted outside by April 20.

"You may set out herb plants such as dill, parsley, chervil and coriander, which achieve their best growth in cool weather and which can even take frost," De Baggio says. "But basil is an exception. It will rot in cool, cloudy weather. It needs a soil with a temperature that is between 60 and 65 degrees. It must have warm nights, and the kind of cool nights we had this April will definitely slow it down.

"I don't set out my basil until the first of June. I pinch it down to four leaves when I transplant it, and then every three weeks I cut it back to two leaves on each stem. It needs side-dressings of fertilizers every two weeks; it needs a fertile, fast-draining soil. It just won't grow in your bog."

De Baggio says that basil is similar to hot-weather vegetables such as eggplant, tomato and pepper in the sense that it requires continuous, steady, unchecked growth, possible only in temperatures always above 60 degrees.

He says that if you put out your basil before the middle of May, the basil will, at best, just sit there; more likely, it will suffer a setback or die.

"Well, there are two schools of thought on planting, and nothing is absolute," says Horne. "I have everything out in the garden by May 1--and that means everything, including tomatoes, peppers and eggplants. And I set out my basil by April 20--or even April l7, which is the last frost date in my area."

But, Horne cautions, you must first harden off your plants by keeping them in an unheated room or garage for two or three days before setting them out.

"I take a bit of a chance," he says. "But last year I set out 12 tomato plants by April 23, and I didn't lose one, and on June 23 I picked my first ripe tomato."

He acknowledges that "freakish cold weather" might occur, but, he says, that happens once in 10 years, and even then, one loses only one-fourth of the plants set out.

As for basil, Horne says he has had success exposing it to the outdoors early for as many as 10 years. "Actually, the basils I lost were on account of the wind," he says. "The wind does more damage than cold weather."

The Bethesda Community Garden Club will hold its annual plant sale Thursday, 8:30 a.m. to 2:30 p.m., at the Montgomery County Farm Womens Market, 7155 Wisconsin Ave., Bethesda. Plants include annuals, perennials, ground covers, azaleas, hollies, wild flowers, herbs and vegetables. Proceeds will benefit community landscaping, garden therapy and a scholarship fund.