As soon as the soil dries out enough to be worked, gardeners can start sowing seeds for the most cold-tolerant of the early-season vegetables. Gardeners who putter in the dirt this early in spring -- or late in winter, depending on your point of view -- tend to be the diehards who are happiest with hoe in hand, even if it means wearing mittens.
The earliest vegetables that can be seeded directly into the garden are greens, including spinach, early lettuce, carrots and radishes. What will determine the success of your initial sowing of these vegetables is not air temperature but soil temperature. By early March, the soil in a sunny area of the garden will be warm enough to support germination of certain varieties of these vegetables.
For example, Nelson, a blunt-shaped Nantes carrot, and Tyee, a savoy spinach, are capable of germinating in soil as cool as 40 degrees. Nelson and Tyee are so hardy that they can be planted in fall to overwinter under a blanket of mulch.
By comparison, radishes generally will germinate in soil that registers 50 degrees and warmer.
The rate of germination of all vegetables increases as the soil temperature rises, eventually topping out in the 70s. However, cool-weather vegetables will not thrive in the heat of late spring and early summer, so the improved germination really isn't a gain if the seed doesn't go into the ground until May.
While you will lose more seed earlier in the season, you will have a better harvest from the seed that does germinate. Vegetables that prefer cool weather tend to lose flavor or stop growing when the temperature gets too high. In the case of greens, the plants will bolt and leaf production will stop.
To optimize the benefits from steadily rising soil temperatures and to keep the harvest coming, reseed every 10 to 14 days through the first half of April.
Direct-seeded early vegetables will benefit from the proper planting conditions. Spinach, which has a long taproot, and radishes, which do their growing underground, should be seeded in soil that has been dug to a depth of at least one foot. The soil beneath carrot seed should be loose to a depth of at least 18 inches.
In order to germinate properly, the soil in which the seeds are planted must remain uniformly moist. It's particularly important that the seeds not dry out before or during germination. Spinach seeds, which have a hard cover, will be particularly difficult to germinate in soil that is not consistently moist.
Water the furrow and let the water seep into the soil before you put down seeds. Then, water daily from a spray nozzle at the gentlest setting. What you don't want is to flood the soil.
I learned one spring how easily and how far tiny seeds can move in well-tilled soil, when a spring shower erased the furrow into which I had sown carrots. About a week later, little green carrot tops popped up all over that section of the garden. That was the year of the random garden.
If you have trouble getting seeds to start early in the season, you can speed germination by pulling a length of kitchen plastic wrap over the watered and seeded furrow. Secure the edges of the plastic with rocks.
The plastic will trap moisture in the soil, eliminating the need for daily watering. You'll want to remove the plastic as soon as the seeds germinate, however. The plastic also will trap heat, which can trigger molds and fungus. If you leave the plastic on too long, you may ruin the soil beneath it for the season.
This is a trick for cool weather only. Later in the season, the plastic will bake the seed to the point where it won't germinate.