Looseleaf lettuces don't form a head but grow into clusters of frilly, smooth, or lobed leaves in a range of colors. They mature in about 50 days but can be harvested in as little as 30 (if you pick them at the baby stage). Looseleafs generally fare best in cooler weather, so plan to grow them in spring or fall. They hold up to hot summer weather much better as baby greens than as mature plants.
Butterhead, also known as Bibb and Boston lettuce, forms crumpled-looking heads with tender, dark green outer leaves and crunchy, light-colored hearts. Harvest butterheads when the inner leaves form a loose head, about 60 days after planting. Some butterheads, notably 'Butter-crunch' and 'Optima', hold well in hot weather, but this group typically does best in spring and fall.
Romaine varieties form upright heads with dark green or red-tinged outer leaves and crisp, whitish hearts. Green romaines, particularly 'Jericho' and 'Green Towers', handle heat with aplomb, but red romaines tend to produce open heads and bolt in summer. Romaines take between 55 and 65 days to mature, but baby romaine leaves can be harvested in less than a month.
Batavian lettuce's shiny leaves mature into loose, whorled heads with crisp hearts. At Colorado State University, researchers evaluated the bolting resistance of dozens of lettuce varieties and reported that Batavians stayed crisp and tasty in heat and resisted going to seed longer than any other type of lettuce--in some cases holding for more than 100 days during the height of summer.
Crisphead lettuces, also known as iceberg, grow slowly—maturing in about 75 days--and produce very round, crunchy, pale green heads. While crispheads are known for their resistance to bolting in hot weather, they are prone to internal tip burn during summer and produce the best-quality heads in spring and fall.
All types of lettuce have tiny seeds and shallow root systems that do not compete well with weeds. Market gardeners use a technique called stale bed cultivation to create fine, weed-free soil for lettuce, and this system will work well in your home garden, too.
Start by weeding your lettuce bed thoroughly three weeks before transplanting or direct-sowing. Spread compost 1 inch thick over the soil and dig it in. Rake the bed until the soil is smooth, crumbly, and free of clods, rocks, and crop debris. Then water the bed well. "Let the first flush of weeds come up, and then cultivate lightly," says Frank Stonaker, the lead researcher for the lettuce Bolting Resistance Project at Colorado State University and the coordinator of the university's specialty-crops program. "A stirrup hoe is a nice tool for cultivating, because it cuts about 1/4 inch deep in the soil and gets rid of small weeds without disturbing the soil enough to bring up new weed seeds."
Begin sowing lettuce seed in spring as soon as the ground thaws and the soil is dry enough to rake. Lettuce needs light to germinate, so sprinkle the seeds on top of finely prepared, moist soil and cover them with an extremely light layer of screened compost or a little seed-starting mix. Sow lettuce in rows 10 to 12 inches apart if you plan to harvest mature plants. Thin the stand when the seedlings have three or four leaves, using the tender thinnings in salads. For baby lettuces, sow seed in 3-inch-wide bands, spacing the bands 8 to 10 inches apart. Hold off on thinning baby lettuces, because you want them to grow into a thick strip of greens for harvesting.
Now, here's one of the tricks to successfully extending your lettuce-eating season: About two to three weeks before sowing lettuce seeds directly in your garden, start some in trays inside. Then transplant the seedlings outside at the same time you direct-sow seed. "With transplants, you get more crops from one growing season, because they are in the ground for less time," says Pam Dawling, an organic gardener who manages the 3½ acre food garden at the Twin Oaks intentional community in Louisa, Virginia.
When planting seedlings, water them well 24 hours before setting them out and again one hour before planting. "The goal is to have the plants absorb the maximum amount of water," Dawling explains. Watering well before planting protects the plants from wilting at transplant, she notes.
Transplant the seedlings when they have four to six true leaves. Be sure to harden off both purchased and homegrown seedlings before transplanting by gradually exposing them to outdoor conditions over a period of five to seven days. "I prefer to plant in the late afternoon or evening," Stonaker adds. "That way the transplants don't bake all day in the sun."
Give the transplants a kick start after planting by watering them in with diluted fish emulsion. When thinning directly sown plants or transplanting seedlings, leave 8 to 12 inches between Batavians, romaines, and crispheads, 6 to 8 inches between butterheads, and 4 to 6 inches for full-grown leaf lettuces.
The secret to growing succulent, tasty leaves and preventing premature bolting is to keep lettuce growing fast. Bitterness naturally occurs in older plants. It also develops in drought-stressed young plants because a lack of water causes lettuce to slow its growth and signals the plant to produce more latex, a bitter milky sap. "If you can keep your soil at a moist sponge level, the lettuce will grow continuously to harvest," Stonaker says. "Lettuce that wilts is much more likely to bolt and taste bitter." Conserve water by using soaker hoses or drip tape to irrigate lettuce.
Mulching your lettuce bed does help keep the soil moist, but it attracts slugs and snails, and straw and grass mulch tend to blow into lettuce heads, making postharvest cleaning a chore. Rather than mulching, plant your lettuce a bit closer together, Stonaker suggests. "Lettuce makes a little microclimate for itself when planted together," he explains. "In densely planted beds, the lettuce keeps the soil cool and moist."
Lettuce grows quickly and typically does not need additional fertilizer during the growing season. In fact, fertilizing too much causes the plants to produce very lush leaves that attract aphids. Gardeners can avoid most lettuce problems, including tip burn, bitterness, and fungal disease, by planting lettuce in a site that features full sun and well-drained soil with balanced fertility and by keeping the soil consistently moist but never soggy.
Harvest lettuce in the morning, when the plants' cells are fully hydrated. For head and whole-leaf lettuce plants, slice the plant off at the soil line with a sharp knife. Pinch off individual leaves of leaf lettuce as needed and use scissors to cut baby-sized lettuce. "We aim to cut baby lettuces when they are about 4 inches high," Dawling says. "We cut off about 3 inches and leave an inch." After harvesting baby lettuce, pour diluted fish emulsion on the row to encourage the greens to regrow quickly.
Hold off on washing lettuce until just before eating, because damp greens deteriorate much faster in storage than dry ones. Lettuce stays fresh for two weeks or longer if wrapped in plastic and placed in the crisper drawer of the refrigerator, but it tastes best, and is most satisfying, when eaten fresh.
Three-Season Lettuce Plan
Growing lettuce continuously from spring through fall requires planning, because lettuce matures at different rates depending on the season and variety. If your family eats salad every day, plan on growing two heads of lettuce or a 2-foot-long band of baby greens per person each week.
Plant romaine, butterhead, and leaf lettuce seedlings (purchase some from a nursery if you didn't start your own) and direct-seed a week or two before your average last-frost date. In early spring, plant lettuce every two weeks and install a row cover over your plants to moderate air temperature, protect plants from variable weather, and keep out insect pests. As the weather warms, remove the row cover, begin planting every 7 to 10 days, and switch to heat-tolerant varieties.
Lettuce germinates in just three days when the soil is 68 degrees, but soil temperatures over 85 degrees signal the seed to go dormant. Overcome this summertime problem by planting seedlings of heat-tolerant varieties, especially Batavians. If you direct-sow, store your lettuce seed in the refrigerator and cool down the soil for a few days before planting. "Water the soil and put down burlap bags to keep the soil moist and dark and cool," advises Pam Dawling, a market gardener in Virginia. In her climate, she also recommends laying ice over rows immediately after sowing.
Transplant and direct-sow lettuce to areas that get shade, such as underneath tomato plants or corn. Be sure to keep up on watering, and harvest lettuce at the baby stage before it has a chance to develop bitter-tasting latex. Make successive plantings every 7 to 10 days in summer.
Gardeners in climates where summer temperatures stay consistently about 85 degrees can greatly improve yields and quality of summer lettuce by covering it with shade cloth. A study funded by the Organic Farming Research Foundation found that lettuce grown under 40 percent shade cloth from June through August in Kansas had higher yields, grew larger and faster, and produced reliably higher quality greens than lettuce grown in unshaded test plots.
Lettuce takes longer to mature as fall progresses, in response to cooler weather and shorter days. "In the fall, you need to plant more often, because one day's difference in sowing can make a difference of a week in harvesting," Dawling says. Switch to cold-tolerant varieties in late August and sow or plant seedlings at least once a week through the first half of September for harvests that will mature in late November. "Lettuce is very frost-tolerant," says Frank Stonaker, a researcher at Colorado State University. "It can tolerate temperatures down into the 20s." Install row covers over lettuce when nighttime temperatures dip below 45 degrees to protect the plants from wind and the elements and to keep your garden full of lettuce well into fall.
Best Lettuces for Heat and Cold
Researchers at Colorado State University set out to find which lettuce varieties handle summer heat the best. The Bolting Resistance Project used organic growing practices and tested more than 50 varieties. Several clear winners emerged over the course of the project, and they are listed in this table, along with two varieties that withstand chilly spring and fall temperatures.
New Red Fire 29 baby size, 55 full size. Dense plant with frilly red leaves that fade to green at the base. Slower to bolt than other red lettuces.
Salad Bowl 28 baby size, 49 full size. A lime green, cold-hardy oak leaf. Perennial favorite for spring and fall plantings. Frost-tolerant, but bolts during the heat of summer.
Simpson Elite 53. Pale green, frilly leaves with long harvest period. Very slow bolting for leaf lettuce.
Optima 55. Dark green leaves form a very large head. Resists downy mildew, bottom rot, and tip burn.
Winter Density 28 baby size, 54 full size. Upright, densely packed head with dark green leaves and a pale heart. Grows well in spring, summer, and fall. Frost-tolerant.
Green Towers 60. Heads grow 8 to 12 inches high and wide with dark green, crinkled leaves and a pale, crunchy heart. Holds for up to three months in summer heat.
Magenta 28 baby size, 48 full size. Reddish green, shiny leaves. Practically boltproof. Resists tip burn and bottom rot.
Nevada 28 baby size, 48 full size. Green, crisp, mild-flavored leaves. Excellent performance in spring, summer, and fall. Resists tip burn and bottom rot.