Last Sunday I built my first cold frame.

"Build" is too big a word; "throw together" fits better.

I have a raft of stray lumber in the garage: leftovers from finishing the basement and putting up shelves, survivors of remodeling projects, relics I can't bring myself to throw out. "The Wood Collection" is my wife's sardonic term for it.

First I picked out the largest window, 34 by 42, which was in fine shape, superseded some years ago by a picture window, and then I looked for wood to frame it in. I found a thick, 12-inch wide oak board that was once used as part of a box in which concrete was mixed, and an old door frame protected by six coats of paint. They would stand up to the weather for two years at least, I figured.

I cut to size the 12-inch-wide board for the northern side, and an 8-inch-wide piece for the southern side. I nailed the eastern and western sides together temporarily, then ripped them at an angle with my circular saw. I nailed the frame together, placed the window on top so it slants to the south, and attached it with two sturdy hinges on the northern side.

I chose a sunny spot in my front yard, dug up and raked the soil, and covered it with a half-inch layer of compost. I transplanted my parsley and roquette, and planted lettuce and spinach seeds. My hope is that the cold frame will protect the plants from wind and cold, and we'll have fresh green salad in the dead of winter.

Looking at the cold frame, a passerby might think that it is an opening to a cellar, an entrance to the underworld. "It looks weird," my sons decided.

The project took me two hours, most of which I spent picking the right lumber.

My cold frame is primitive. It is a tentative first step toward solar gardening, which is an ambitious, year-round enterprise.

"Solar cold frames are versatile, efficient structures for use where larger greenhouses are inappropriate or not desired," says the booklet "Food Gardening," published recently by Longwood Gardens in Kennett Square, Pa. "Longwood's solar cold frame transmits sunlight, collects and stores heat in its walls and soil, and radiates warmth back to plants at night. Forming the solar box are south-facing glass doors framed by solid, insulated walls."

The solar cold frame on exhibit at Longwood Gardens has a surface area of 5 1/2 by 9 1/2 feet, and the concrete block foundation sinks 40 inches into the ground. The wooden walls are carefully insulated with fiberglass and tightly constructed. The four glass panels are 7 1/2 feet long. The two center panels are 15 inches wider than the end panels. The narrower panels are equipped with thermal-powered ventilators that open and close automatically to prevent the cold frame from overheating. And the thermal-power hinges contain a compound which expands as the sun heats the surrounding air, forcing open the ventilation sash. "They open very gently to prevent blasts of cold air from striking plants," the booklet says, "and they hold the glass doors at any intermediate position up to 12 inches to maintain the desired temperature inside."

Thermal-powered hinges sound enticing. But I am not ready for the technology yet. I plan to build my second cold frame this weekend, just like the first. Actually, I am just throwing one together.

Q: Should I plant onions now? Isn't there a risk of their getting killed by a cold winter?

A: If you plant onions and garlic now for early spring harvest, do not neglect to provide a heavy mulch -- a half-foot of straw or spoiled hay is not an exaggeration. And mound some debris -- leafmold, for instance -- on top of that. That way the risk of winterkill is much, much lower.

Paradoxically, the more you have improved your soil, the more vulnerable it is to freeze damage. Heavy clay absorbs less water, and consequently there is less chance for frost heaves. But a fine, loose, properly aerated soil absorbs a lot of water, which means that freezing temperatures produce a great deal of soil movement and can heave your bulbs right out of the ground.

Q: Can you prescribe a remedy for a terrible scale infestation that attacked my gardenia? It has survived two winters and two summers in the patio; I want to bring it indoors.

A: Gardenias are very sensitive. If the scale infestation is light, an application of an insecticide soap will do. But if the infestation is severe, a mild concentration of an insecticide spray is necessary. But do not overdo the spraying because the leaves might drop.

When indoors, gardenias need a lot of humidity. A pebble tray is a necessity; using an atomizer to provide a fine mist of water is a good idea.