We grow lettuce to harvest year round, here in central Virginia. From the end of April into November, we harvest lettuce heads outdoors, from our raised bed area. From October to early March we harvest outer leaves from leaf lettuces and romaines in our solar greenhouse, and in our solar-heated double-layer hoophouse from October to April. From mid-November we harvest salad mixes containing lettuce from our hoophouse. From 4 December to late May we have baby lettuce mix from the hoophouse. Note that we have overlaps at both spring and fall transitions. We’re approaching the spring transition, and that is what I am going to focus on in this post.
Our Winter Hardiness Zone in central Virginia is 7a, which means our annual minimum temperature averages 0°F to 5°F (-18°C to -15°C). Our average date of the last spring frost over the past 14 years is April 29 (later than 5/13 one year in 10).
More About Growing Lettuce
See my post Lettuce All Year in a Changing Climate, which includes links to my slideshow about growing lettuce year round, and our updated Lettuce Varieties list and Lettuce Log (planting schedule). It also includes keys to succeeding with year-round lettuce (dates for succession planting) and what causes bolting.
I have lots of posts about growing lettuce! Here are some of them:
Early Lettuce Production See this post for details of growing lettuce transplants in flats in our greenhouse.
Cold Tolerant Lettuce – In the 2017-2018 winter we had some extremely cold weather, with outdoor night-time temperatures of -3°F (-19°C) two nights, followed by -8°F (-22°C) 6 and -9°F (-23°C). Our guideline is that if we expect the night-time low to be 8°F (-13°C) or lower outdoors, we use rowcover in the hoophouse. It wasn’t enough to save all the lettuce. See this post for which varieties survived.
And I have a whole year of Lettuce for the Month posts. See here for the overview. These posts are mainly about our favorite varieties for each time of year.
Head Lettuce Transplanted Outdoors
From February to November/December, we grow lettuce outside from transplants. Transplanting gets a head start on weed control, which is important all the way from planting out to a couple of weeks before harvest. Don’t waste time hoeing lettuce you will be harvesting next week. I generally find that if we hoe once, a couple of weeks after transplanting, that is all the weed control we need at the fast-growing time of year.
From mid-January until mid-March we sow in flats in the greenhouse, with heating for the January-March sowings, to get the seeds germinated, then good old solar energy to grow them to transplanting size. We spot the seedlings out to give the plants more space, and harden off the flats of transplants in our cold frame for two weeks before planting them in the garden. The first few plantings will get rowcover after transplanting, to protect the young plants from cold and speed up the growth rate. March 9 is our goal for transplanting our first sowing (January 17) outdoors. January 17 to March 9 is 52 days, a very long time for the seedlings to grow to transplant size! We hope to harvest that first lettuce as heads in late April.
During April we put the seed flats in the greenhouse without extra heating, as temperatures in the greenhouse are warm enough to germinate the seeds. Growth speeds up, and in warm weather we transplant three weeks after sowing. The intervals between one transplanting date and the next decrease from 9 days in spring to 5 or 6 days in summer. We transplant 120 lettuce from each sowing – about one week’s worth for 100 people.
Soil Temperatures for Lettuce Germination
The soil temperature range for germination of lettuce seeds is 35-85°F (2-29°C), with 40-80°F (4-27°C) being the optimum range and 75°F (24°C) the ideal. At 41°F (5°C) lettuce takes 15 days to germinate; at 50°F (10°C) it takes 7 days; at 59°F (15°C) 4 days; at 68°F (20°C) only 2.5 days; at 77°F (25°C) 2.2 days. Then the time to germination increases: 2.6 days at 86°F (30°C); after that it’s too hot. A soil thermometer soon pays for itself and saves lost crops and frustration.
Bare-Root Transplanted Lettuce in Summer
From May to late September we use an outdoor nursery seedbed and do bare-root transplants (with heat-tolerant varieties). The soil temperature does not vary as much as the air temperature, and we no longer worry about cool nights.
If it’s too hot for lettuce seed to germinate, you can find a cooler place (put a seeded flat in a plastic bag in the fridge or on the concrete floor in the basement). Or cool down a small part of the world. From June we put shade-cloth over the lettuce seedbed, and only sow in the evening. In July and August, we water the sowed seedbed with freshly drawn cold water, line up ice cubes along the seed rows, and cover with shade cloth.We make sure to keep the seedbed damp, using cold water each time.
From July to September we sow lettuce every 5 days! See Success with summer lettuce. Of course, transplanting that lettuce in hot weather takes care too. We do that late in the day, and water as we go. We cover the transplants with hoops and shade-cloth, and water daily until they are well established. We stop sowing for outdoors on August 29.
Lettuce After Summer Ends
From September 11-17 we sow lettuce in a nursery seedbed to transplant in our greenhouse, and on September 15 and 24 to transplant in our hoophouse. This is our fall transition and I’ll write about that when the time comes.