Growing lettuce in the heat is difficult. If you crave lettuce late into summer, read on. Here are a few tips about how to keep lettuce going longer, along with a few heat-tolerant varieties to give your lettuce the edge it needs to battle the heat.
By the time August rolls around, lettuces are bolting left and right. Summer is too hot for many lettuce varieties. Luckily for both small-scale and commercial growers, there are several heat-resistant lettuces hardy enough to keep your lettuce cravings satisfied all summer long.
Recognizing Bolting Lettuce
The key to a delicious summer lettuce is recognizing when that lettuce is bolting. Lettuce is happiest in temperatures that remain in the sixties. After that, most lettuces tuck tail and bolt right out of your garden. Bolting is the term used to describe a lettuce that has gone to seed. In hot temperatures, lettuces put their energy into seed production instead of producing juicy, leafy greens for your table. The plant does not form a tight, harvest-ready head. Instead, the lettuce grows upwards. The center stalk elongates, spacing the leaves out and resembling kale more than lettuce. The plant then forms the flower that develops into the seed stalk. Unfortunately for us, the leaves turn bitter during this process, making the bolted lettuce inedible and very disappointing. Sometimes the lettuce turns bitter before obvious signs of bolting occur. If you have any doubt about a lettuce in your garden, taste the leaves.
Before you choose a heat tolerant lettuce, think about your garden plan. Planting lettuce in full sun in the spring is an excellent idea. It makes less sense in late summer when the days are long and hot. Choose a garden location that receives morning sun and afternoon shade to reduce the heat stress on the plant. Shade cloth is a great alternative if your garden does not have a shady location. This fabric provides between 30 to 40 percent shade, depending on the grade, and is carried by most farm and garden stores. Use wire hoops or the metal hoops from your cold frames to keep the cloth over the plant. Secure the cloth with clothespins.
Lettuce germinates best in the shade and in cool soil. Your hot garden does not provide ideal starting conditions. Start your seeds indoors and transition them outdoors slowly. This transition is the same as the "hardening off" process used to get vegetables accustomed to cooler spring temperatures and prevents the seedlings from bolting.
Container gardening offers another summer lettuce solution. Simply pick up your lettuce and move it into the shade in the afternoon or drape a shade cloth over each container to keep your lettuce cool.
Waiting for a lettuce to fully head in a heatwave is a recipe for disaster. In truly hot weather, cut your lettuce young for baby greens instead of waiting for the lettuce to head and risking a bolted crop. Cut and come again varieties are not a bad plan for summer production. When they start tasting bitter, call it quits and move on to your next lettuce succession.
Heat Resistant Varieties
The lettuce family is composed of Butterhead lettuce, Leaf lettuce, Oakleaf lettuce, and Romaine lettuce. There are varieties within each family that are more heat resistant than others.
Most seed companies provide information about their products either in their catalogs or on their websites. Heat resistant lettuce varieties are almost always labeled “heat resistant.” The following varieties are heat-tolerant lettuces that I have personally had success with or that receive good reviews from the seed companies I tend to use. That does not mean they are the definitive or the only varieties out there. Check with your preferred seed provider to see what they recommend or carry.
Butterheads are moderately heat tolerant as a rule. They hold out longer than most other lettuces, with the exception of loose leaf varieties, and maintain their sweet taste. For summer growing, consider Skyphos, Adriana, Buttercrunch Bibb, Capitan Bibb, or Red Cross.
Crisphead lettuces do not love warm temperatures. They tend to be more difficult to grow, despite being the most popular type of lettuce. No need to sweat. There are a few varieties that stand up well in the heat. Try Nevada, Anuenue Batavian, or Sierra Batavian.
Leaf and Oakleaf
Loose leaf lettuces are nutritious and heat tolerant. Plus, they are more forgiving of poor soil and imperfect growing conditions, making them easy to grow. Green Star, Magenta, Bronze Arrow Lose Leaf, Red Salad Bowl Oakleaf and Oakleaf Looseleaf Lettuce are all heat tolerant varieties that do well in late summer.
Romaine lettuce is sometimes tricky to grow in the heat, although it is relatively heat tolerant. Look for romaine varieties developed in hotter regions, like Jericho, a variety developed in Egypt. Other heat tolerant romaine lettuces include Anuenue, and Coastal Star.
You can have lettuce and tomatoes at the same time. With shade cloth, careful garden planning, and the right variety, you can enjoy crisp, fresh lettuce into the fall.