My last frost date is not until early May, but by that time my harvest of lettuce, rocket and other salad greens is in full swing. There is good reason to get a head start with salad greens, because they grow best in cool weather. I use several season-stretching techniques to grow extra-early salad crops, which come and go quickly. They are often out of the garden by the time the soil is warm enough to plant tomatoes.
Start Seeds Indoors
Lettuce, spinach and other salad greens transplant easily, so I start my first planting indoors under lights. Starting from seed gives you the opportunity to try unusual varieties you will never find as seedlings, from red-speckled butterheads to baby romaines. Fresh lettuce seeds have high germination rates, and sprout within a few days when barely covered with moist seed-starting mix. The seedlings need plenty of light, and enjoy spending sunny days in bright windowsills.
Pre-Warm the Soil
While the seedlings are growing indoors, I prepare a planting bed in the garden, frame it with old boards held in place with stakes, and top it off with windows saved from a home renovation project. Over the years I have tried various plastic-covered frames for pre-warming the soil and protecting tender transplants, but none worked as well as old windows, which never shift in the wind and hold up to ice, snow, and hail. The one precaution is to avoid old windows that may have been painted with lead paint.
The soil inside the glass-topped frame warms and dries after a week or two. By the time I am ready to use the bed for planting, the soil feels almost warm to the touch.
Harden Off Seedlings with Care
There are many ways to harden off seedlings, which involves gradually exposing them to outdoor conditions. For the last few springs, I have used a clear plastic storage bin, weighted with a couple of bricks in the bottom, as a hardening-off chamber, and I highly recommend it. During the day with the lid removed, seedlings are exposed to full light while the sides of the bin buffer winds. Spending a couple of nights in the closed bin gets the plants accustomed to cool night conditions.
Growing and Sowing Under Cover
Growing extra-early salad crops under glass requires frequent watering because the soil dries out quickly on sunny days, even when the glass is well vented. I have found the same to be true in plastic-covered cold frames, but dry soil between plants has the advantage of suppressing weeds. I use a squirt bottle to water individual plants, which results in far fewer weeds.
At the same time I set out my seedlings, I often direct-sow more seeds into the bed, separating rows of greens with coriander and little radishes. To help keep the soil moist until the seeds germinate, I cover the seeded area with a double thickness of horticultural fleece or other lightweight cloth, and remove it as soon as the seeds sprout.
As winter turns to spring, I gather up my windows and replace them with a tunnel of lightweight horticultural fleece, held aloft with hoops. Fleece ventilates much better than glass, while still protecting the crop from hail and predators. By this time I start harvesting plants that need thinning, because maturing lettuce plants can get big fast!