Early-season vegetable gardeners don't have to wait until the danger of frost has passed to begin sowing seed. Frost-tolerant greens and such root crops as carrots, beets and turnips can be planted during March with great success, provided gardeners pay attention to a couple of basic rules.
With any of these vegetables, the most important task is to make sure conditions are right for germination. Seeds must be planted in damp soil, and the soil should remain uniformly moist until the seedlings form their roots. Be wary of supplying two much water, which will wash seeds away, or too little, which will impede germination and, ultimately, may kill the seedlings.
A soaker hose turned on for 30 minutes to an hour a day is the perfect way to deliver water continually without flooding the planting bed. Small, light-weight seeds, such as those of lettuce and carrot, are easily swept away. I've seen an early spring rain erase a planting of carrots only to have "volunteers" pop up several feet away. Overwatering from a hose can create the same problem.
Seeds with hard covers, such as those of spinach and beets, may benefit from soaking for a few hours before being placed into moist soil.
Conditions also should be right for germination to take place as quickly as possible. This means that the soil into which these seeds are sown must be sufficiently warm. Turnip seed can germinate in soil that has warmed to just 40 degrees, and beets aren't far behind.
A general rule is that when the daytime temperatures consistently hit the upper 50s, it's safe to sow beets, turnips, carrots, spinach and most varieties of lettuce. Even though the overnight temperatures may dip into the 30s, anything shy of a hard freeze probably will not kill your seeds. Some kinds of lettuce may be less tolerant of frost, so check the seed package for planting guidelines.
Because a speedy germination increases the chances that your seeds will take, you can do a couple of things to nudge them along. Carrots, which can take weeks to germinate, are particularly problematic.
Many years ago I picked up a tip for incubating carrot seed, which I have used repeatedly. This came from Joe Thomasson's "Growing Vegetables in the Great Plains," which is an outstanding University Press of Kansas book that speaks directly to gardeners in this climate.
Thomasson recommended planting carrot seed into a furrow about 1/2 an inch deep, covering the row with plastic wrap from the kitchen and securing the edges of the plastic with weight, such as bricks or boards. You'll need to be sure that the plastic is anchored all along the edges or the wind can whip it away.
This method does indeed speed up germination. The plastic cover also keeps moisture in the soil, so you don't have to water. The only failure I had occurred after a sudden temperature spike into the high 70s, which amplified the greenhouse effect and baked the seeds.
Other kinds of seeds germinate quickly enough in moist soil that you don't have to resort to such measures. However, all of the seedlings, even those for root crops, should have some basic wind protection. If your garden is buffeted all day long, your new seedlings won't survive. The trick is to provide a wind barrier that also won't block sunlight. This rules out bales of hay, but you might consider stacking pieces of scrap lumber along the windward side of a row to erect a barrier a couple of inches high.
Every garden spot is different, so experiment with different solutions, and don't be afraid to be creative.