Lettuce varieties come in many colors, flavors, growth patterns and habits, so growing a few colorful varieties can make a beautiful and nutritious salad. When choosing lettuce, consider the temperatures in your area, the nutritional level of the lettuce, and your tastes and needs. Lettuce varieties fall into 6 cultivar groups. “Cultivar” means named, cultivated varieties.
Types of Lettuce
- Romaine lettuce or cos lettuce (L. sativa var. romana) is the most nutritious type of lettuce and it performs best in fertile soil. It is somewhat tolerant of heat and can be grown in partial shade.
- Looseleaf lettuce (L. sativa var. crispa) does not form heads, is the second most nutritious type of lettuce and has the most diversity of colors and textures. It is the easiest to grow since it will tolerate poorer soil and is more heat tolerant than most types.
- Buttercos lettuce is intermediate between butterhead and cos, with loosely formed heads.
- Bibb lettuce, a.k.a. Butterhead lettuce, (L. sativa var. capitata) is generally the best for cooler regions and is more heat tolerant then crisphead lettuce. It forms loose heads with lighter colored interiors.
- Crisphead lettuce is the least tolerant of heat and the most difficult to grow. Also known as Batavian endive or escarole, it requires a long cool season to fully develop. It is the most popular type for its crisp leaves that are tightly folded, and it is the least nutritious. It must be blanched, meaning its outer leaves are tied together to protect the inner leaves from sunlight. The most familiar crisphead cultivars are Iceburg lettuces.
- Chinese lettuce (L. sativa var. asparagina) grows long, dark green, sword-shape leaves which are used for stir frying. Chinese lettuces are more bitter than Western lettuces, and are divided into the types used for leaves and the types used for stems.
How to Grow Lettuce: Cultural Considerations
All lettuce (Lactuca sativa) thrives in the cool temperatures of spring and fall in the temperature range of 60 to 65°F. Many varieties will survive colder temperatures, as low as 20°F, if they have been thoroughly hardened. There are some varieties that will survive much lower temperatures. Lettuce requires moist soil and if the soil dries out, it will cause it to be bitter and to bolt.
The most heat tolerant lettuces are the loose leaf types, followed by romaine, then butterhead, then bibb, then crisphead. During the 2 to 3 leaf stage, if lettuce is exposed to temperatures below 50°F it is more prone to bolting earlier. Starting lettuce inside at least three weeks before transplanting out can reduce bolting.
How to Grow Lettuce from Seed
In early spring, start transplants inside one month before planting out, but by late spring switch to direct seeding into the garden. On seed packets, the date to maturity includes both the time it takes to grow inside as well as the days outside, unlike tomatoes and most other transplanted vegetables.
Fill containers with soil, leaving room to water at the top, and thoroughly moisten. Lettuce seeds are tiny, so it may help to mix them with sand in a re-purposed salt shaker to help distribute seeds more evenly. Cover seeds with 1/4 inch soil and water lightly. To help keep the seedbed moist during germination, you can cover it with a breathable material such as burlap, but remove it as soon as seedlings are up. As seedlings grow, thin to the spacing indicated on the seed packet to maintain good air circulation. If you plan to cut lettuce small instead of letting it mature, it will not need as much space as stated on the packet.
Seeds germinate best in cool, moist soil. Lettuce does not germinate well when the soil gets warm, generally over 80°F. Seeds germinates best between 40° and 70°F. To extend lettuce season, plant more seeds every 2 to 3 weeks and choose some slow bolting lettuce varieties, a.k.a. bolt resistant. Slow bolting means that the lettuce will withstand heat for longer or will wait for longer before it starts to bloom. In some cases, being exposed to cold weather when it is young will cause lettuce to bolt. When lettuce bolts or blooms, it is sending up a flower shoot, and then the lettuce gets bitter and tough. This can be delayed by continually harvesting the outer leaves, but generally once the temperature reaches 85°F for extended periods, is a signal to lettuce to bloom and complete its lifecycle.
If you want to plant lettuce during the late summer and early fall when it is still hot outside, germinate seeds in the refrigerator for 4 to 6 days. Some varieties will grow well in light shade or under shade cloth. Light shade can either be in a location which gets 4-6 hours of direct sunlight per day, or gets at least 8 hours of bright, dappled sunlight such as sunlight filtered through high-branched trees or tall garden plants. Another way to keep the soil cool in the garden is to keep an area covered with burlap or boards, and germinate seeds in this shade. Make sure to promptly remove the cover after the seed germinates, or you will be inviting pests to dine on your seedlings in the cool shade.
How to Grow Lettuce in Shade
Growing lettuce in partial shade during the heat of the summer can help extend your lettuce season. Tall vegetation such as corn, squash, or tomatoes provides partial shade to lettuce. Mulch the root zones of plants before the soil gets hot to keep lettuce roots cool. Romaine lettuce, as a rule, is more tolerant of shade.
Good Lettuce Varieties to Grow in the Heat
The most heat tolerant crisphead lettuces cultivars include Santa Fe lettuce, Sierra lettuce, and Summertime lettuce. Looseleaf varieties are the most heat tolerant, but some cultivars that do best are Red Salad Bowl and Salad Bowl lettuce and Slo-Bolt lettuce . The most heat tolerant buttercos cultivar is Winter Density lettuce. Romaine lettuces as a rule are heat tolerant, especially Jericho lettuce and Larissa lettuce. One of the most heat tolerant butterhead lettuce cultivars is Capitan lettuce.
How to Grow Lettuce in Containers
You can grow lettuce in containers inside or outside. Loose leaf type lettuces or cut-and-come-again lettuces are the most suitable. Harvest the outside leaves to let the middle leaves continue to grow.
Plant seeds in a container that is at least 6 inches deep. Grow lettuce inside a few inches under grow lights which are on for 12-16 hours per day. Shop lights which hold several bulbs with half of the florescent bulbs warm and half of the florescent bulbs cool are the most economical. Grow lights will need to be moved up as the plants grow, so suspending them on chains is handy.
If you want to grow lettuce in containers outside, a location with morning exposure is best. Since the soil in containers gets warm, the best-suited varieties for outside are heat tolerant. It is best to provide shade during the hottest time of the day while still allowing at least 6 hours of full sunlight. Containers will likely need watered every day, and more often during hot and/or windy days.
How to Grow Iceburg Lettuce
Iceburg lettuce is a type of crisphead lettuce which forms tight, light-colored heads, and it is the most difficult type of lettuce to grow. In areas with short growing seasons and harsh winters, it takes two seasons and entails bringing it inside to store in a cool area during the winter. In the warmer areas, starting seed in the late summer to mature in the fall may be beneficial. There are more heat tolerant iceburg-type lettuces such as Sierra lettuce or a Batavian crisphead lettuce.
How to Harvest Lettuce
The method to harvest lettuce depends on the type of lettuce, and how you would like to harvest in the future. If you would like for the same plant to keep producing leaves, cut only the outside leaves, leaving at least 4 center leaves. If you would like to harvest an entire area and use it to replant, cut the entire plant off at the base. For heading type lettuces, cut the entire plant or just the center part. Harvesting before 7 AM preserves the glucose content of lettuce. To store lettuce, wash and dry it, then store in a moist paper towel in an airtight container. Most lettuces will keep in the refrigerator for about a week.
To keep lettuce harvests available from late spring through light frosts, choose varieties that tolerate cold for spring. Crops planted in late summer will be ready in fall, so plant varieties that will tolerate frost. Plant loose leaf and romaine lettuces that are tolerant of heat during the summer, and harvest them while they are young. Covering plants during cold spells with blankets, row covers, or whatever you have available with protect them. Be sure to remove the covers when it warms up. If you use plastic, your plants can bake very quickly in the sun, even on a cool sunny day.
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