How to Grow a Winter Garden
Step 1: Determine the size of your cold frame.
Select the glass you will use. I use 2 sliding glass doors a neighbor had replaced and put out for the trash.
Select wood for the frame. I use four 4×6 inch boards cut to form a square under the outside frames of the glass doors when they are lying flat on the ground.
Sink bricks in the dirt at the corners of the square to rest the boards on to form a stable base.
Step 2: Prepare the seed bed.
Select a location that will get good sun in the winter. In August, condition the soil to prepare it for planting. I spread compost on the soil surface and then work it into the dirt. I dig down several inches and turn the compost in, then I moisten and break up the dirt over several days to get a soft bed of soil to receive the seeds.
Step 3: Planting.
On September 1st, I begin planting. I plant spinach according to package directions. Since the soil is quite warm, the germination is terrible, but a few plants will emerge.
Every two weeks throughout the fall, I replant. I plant spinach in any spot that has room for another plant. By the end of the fall, around December 1st, I have a full set of plants. They vary in age from mature to seedlings. I am able to begin harvesting leaves from the mature plants in November, but the harvest is minimal. The days are short and sunlight is minimal, so growth is also slow.
Step 4: Winter growth
As December progresses, snow buries the cold frame and the temperature inside gradually lowers until it is below freezing. The soil freezes to a depth of an inch or so. The plants grow very slowly, and they do not die.
In mid January, when the temperature gets above freezing for several days in a row, dig out the cold frame and expose the plants to sunlight. The glass cover will allow the sunlight to warm the dirt so that it thaws, and the plants resume growing. The mid January days are longer, and the sun is higher in the sky than early December. As long as the outside temperature is moderate, the glass cover will protect the plants from low temperatures and allow the sun to warm the inside to the mid 40's during the day. As long as these conditions exist, the plants will continue to grow, and you can begin harvesting once again. Remember to water the garden. I carry water in a watering can. I also limit opening the cold frame to times when the temperature is above freezing. If the temperature gets very cold, I will add another layer of glass to help protect the plants.
Step 5: Harvest
Whenever leaves on mature plants get large, you can cut them off, wash them, and eat them. The plants are not harmed by taking individual, mature leaves. Be careful not to take young leaves that are just emerging from the center of the plant. Be patient, they will get big.
By late February and early March, the spinach will be growing so fast that you will have a bountiful harvest whenever you want it. I cut leaves every day and put them on my sandwiches for my lunch. We also make salads for dinner.
When a plant becomes unproductive or is old, I pull it up and drop a couple of seeds in the new space. Typically, my plants range in age from seedlings to mature plants. Spinach will keep growing vigorously until it gets too warm and the daylight hours are too long. Then it will go to seed (bolt). I've found nothing to prevent it. The end of the spinach harvest will be around mid June.
When the nighttime temperatures remain above freezing, you can put your glass cover away. You don't need it any more.
Step 6: Fertilizer
Whenever the plants are growing vigorously, they need feeding. I use a balanced garden fertilizer and I apply it once a month, usually in March, April, and May. I also start with a good feeding when I plant the bed on September 1st, but I don't fertilize when the conditions determine slow plant growth.