St. Patrick's Day is traditionally potato-planting time. Most of us worked on Wednesday, though, so go to it Saturday or Sunday. First, pick up seed potatoes at the hardware store or plant center. They're sold by the pound, in bulk or in bags, and look just like ordinary, grocery- store variety potatoes, only smaller. They are also dusted with a chemical that's supposed to prevent various fungi and diseases which cause rotting, among other things. If you don't like introducing chemicals into the garden, wash the seed potatoes before planting. I've seen some rotting as a result of this practice, but by and large, they seem to grow fine without the dust. When you get home, examine the potatoes and cut them into chunks the size of a walnut, with each chunk bearing an eye, a small indentation or blemish that may be sprouting the tiny beginning of a root. A potato chunk without an eye may not grow, although some eyes are small enough to be undetectable, so it's worth taking a chance and planting even those pieces that you think don't have eyes. A single chunk can have more than one. Once they are cut up, lay the potatoes down and dig a trench about six inches wide and about that deep. You'll need about 60 feet of trench for each pound of potatoes, which sounds like a lot. But if you do three rows of 20 feet each, with trenches about two feet apart, it really doesn't come to that much space -- depending, of course, on the size of your garden. If you can't afford that much space, try to plant a half-pound's worth. Like young, sweet corn, there's nothing like a baby new potato right out of the ground, boiled lightly and served with butter. Plunk your potato chunks into the trench, about two to four inches apart. If your soil is rich, they can be close; if not, keep them apart. Cover the trenches with straw, if you have it, or straw mixed with leaves. If not, cover them with the soil you dug out of the trench. The major advantage of mulch covering is that when you harvest the potatoes in a few months, there'll much less digging: Just push the straw aside to find the new potatoes. The potato pieces will soften and send out from the eyes numerous roots, which will spread and on which will grow new potatoes. All you'll see above ground is the big, dark-green bushy plant, a member of the nightshade family, which bears lavender blooms. It's a very pretty plant to have in your garden, except that eventually it dies out and looks like a dead weed. This happens when the potatoes are ready for harvesting. ONIONS IN A CROWD: Now is a good time to plant onions, too. What I usually do is spread the onion plantings out along with plantings of other vegetables. I drop in onions with the peas -- the climbing pea varieties, which don't crowd the onions. If you grow bush varieties of peas, plant a frame of onions or garlic around the peas. The advantage of planting onions with other vegetables is that they tend to ward off insects that might enjoy your harvest before you do. Use onions and garlic around broccoli, cabbage and other Brassicas. Don't crowd your onions too close to vegetables that will inhibit the onions' green stalks. I decided to be clever one year and mix onions, garlic and broccoli throughout one bed, only to find all my onions stunted within just a few weeks, when the broccoli had grown to shade them. GLAMOROUS GARLIC: Like onions, garlic can go in now, and can be used as an insect repellant. Glamorously called "garlic sets," what you buy from a catalog or at a garden center looks exactly like the supermarket version, and in fact is no differ these will grow quietly into a whole garlic, much like the one it came from. Plan on leaving them in as long as you possibly can. Garlic planted this weekend shouldn't be harvested until the fall, to obtain the largest size, and some people even leave them in for a second year, which is how you'd definitely treat giant elephant garlic, available through catalogs. HERB MOVERS: If you have any small perennial herbs to relocate, such as a clump of oregano or a couple of thyme plants, do it now. Thyme doesn't really die out during the winter, but new growth is barely starting yet. On deciduous perennials, like tarragon, sage, burnet or oregano, you can detect new growth at the very base of the bare, woody stalks. But don't assume that if you see no growth they're dead. It's still too early to tell on some of these perennials. When you move them, take your garden fork or a large shovel and dig wide around the plant, allowing an extra four inches of soil around the crown. Dig very deep, allowing at least a foot to 18 inches of soil underneath the plant. Loosen it gently, trying not to disturb the root system. Dig the hole for the transplant in advance. Allow a large area for the ball of the plant, and don't plant it any deeper than it was in its original home. Also, if you're going to move it from one soil type to another, prepare the hole with a lining of the same soil the perennial is used to: Moving herbs to a less rich soil will not harm them, as long as the change isn't too sudden. Don't transplant parsley or French sorrel. Their root systems are very deep, and, as older plants, don't take to transplanting at all well. GALA HERB WEEKEND happens this Saturday and Sunday, 9 to 6, at Bittersweet Hill Nurseries, Davidsonville, Maryland. The special weekend includes tours through the Display Herb Garden, exhibits of herbal wreaths, potpourris and dried herb arrangements. Demonstrations include herb propagation, the making of live herb wreaths and herbal bonsai. Take U.S. 50 east to the Davidsonville exit (Route 424). Follow state-road signs to Davidsonville. Just short of two miles is the intersection with Governor's Bridge Road. Turn right to the nursery. For more information, call 301/798-0231.