Lettuce, the crunchy cold-loving crop!
Jess Nguyen, Master Gardener Intern
What leafy vegetable could be a more fitting staple of summer than the humble lettuce, which makes up the backbone of a refreshing salad and adds a fresh crunch to any picnic sandwich? Cultivation of lettuce dates back to the ancient Egyptians, who used the plant as an important food crop and seed oil source. Today, there are hundreds of varieties of lettuce grown across the globe. Keep reading to find out how you can grow this quick-growing crop in your own garden.
What leafy vegetable could be a more fitting staple of summer than the humble lettuce, which makes up the backbone of a refreshing salad and adds a fresh crunch to any picnic sandwich? Lettuce (Lactuca sativa) is an annual cool-season crop that grows well in the spring and fall. Lettuce is a fantastic candidate for succession planting in your garden, as your lettuce harvest will be ready once the summer starts to warm up and can then be replaced by heat-loving plants.
Lactuca sativa has hundreds of varieties (wow!) that can be categorized into different types. Some commonly-grown categories are as follows:
Non-heading lettuce, which includes loose-leaf lettuce varieties;
Soft-headed lettuce, which includes varieties such as butterhead lettuce;
Ruffled-headed lettuce, which includes French crisp/summer crisp/Batavia lettuce;
Tall and compact head lettuce such as Romaine lettuce;
Dense, solid head lettuce, which includes iceberg lettuce.
For beginner gardeners, loose-leaf lettuce tends to be the easiest type of lettuce to grow!
Lettuce seeds are very small and require loose, well-draining, and well-tilled/non-clumpy soil to effectively germinate. Adding compost to your soil before sowing lettuce seeds will help the lettuce to produce large and well-shaped heads. Lettuce prefers cooler temperatures; a soil temperature range between 45°F and 65°F (7°C and 18°C) is ideal. Sow lettuce seeds on the surface of the soil, 1/4 to 1/2 inches deep. When choosing a spot to grow lettuce, pick an area that gets five to six hours of sunlight. In hotter temperatures, lettuce benefits from an area that gets shade in the afternoon.
An easy method to control the quality of your starting soil and the growing environment would be to start lettuce plants indoors. You can start seeds when you would otherwise not be able to: when the temperature is still too cold in spring to plant and when the temperature is too hot in the summer for lettuce to thrive! Sow seeds about four weeks before you intend to transplant them. Thin seedlings to encourage the largest plants to thrive. Harden your seedlings before transplanting. In the spring, transplant lettuce about a week after the last frost, when freezing temperatures no longer persist. In the fall, transplant lettuce about two months before the first frost date.
For direct sowing, sow seeds in the spring as soon as the soil is workable; this should be two to four weeks before your last frost. For fall harvest, sow lettuce seeds about three months before the first frost date, which for Minnesota tends to occur around late August. Soil in late summer can be cooled to suitable temperatures by covering with damp hay to shield the soil from the sun before planting. Rows of lettuce should be 18 to 30 inches apart. Thin seedlings to the appropriate spacing based on the variety that you’ve planted. For example, most loose-leaf varieties recommend seedlings spaced four inches apart, while Romaine lettuce should be planted eight inches apart.
Lettuce has very shallow roots, so frequent watering is important for growth. Soil should remain moist but not be overly-wet, which can lead to disease. Thankfully, it is easy to see when your lettuce needs water–look for the obvious signs of wilting that tell you to water the leaves to cool down your lettuce plants.
Some common pests for lettuce plants include cutworms, which can cut seedlings and lettuce plants off at the soil line, causing seedlings to die and growing plants to wilt. Cutworms should be hand-picked off of the plants, and collars that extend two inches into the soil surrounding the plants can help deter the worms from further damage. Slugs, which cause holes in lettuce leaves, should also be removed from the plants by hand. Aphids are a pest that can stunt the growth of the lettuce causing yellowed and misshapen leaves and can also cause disease. Remove aphids by blasting them with a water spray, or by introducing biological controls such as ladybugs into your garden.
One common problem when growing lettuce is bolting, which is caused when lettuce plants grow in temperatures that are consistently above 75°F. Bolting is when lettuce plants produce a central stalk to flower, which causes the leaves of the lettuce plants to become very bitter and unsuitable for eating. Long days and intense sunlight can also cause bolting. Therefore, for most areas in Minnesota, the months of June, July, and August have conditions that are too hot to grow lettuce effectively. Use these months for heat-loving plants instead!
Harvest lettuce when the leaves are full-sized but still young, as mature leaves tend to go bitter. In the spring, leaves will have the best flavor before the weather becomes hot and dry. In the fall when cooler temperatures benefit lettuce, beware of prolonged freezes, which can damage your harvest! You can harvest lettuce by removing outer leaves (which can also be done as the plant is still growing) or by cutting the plant at or slightly above the soil line. Store your lettuce in a loose plastic bag in the refrigerator. When you are ready to use your lettuce, rinse the lettuce thoroughly in cool water, then dry the leaves using a salad spinner or a towel. Wilted lettuce can be revived by a soak in an ice-water bath to maintain crispy, crunchy leaves all summer long.
For more information about growing lettuce, see this article by the University of Minnesota’s Marissa Schuh and Jill MacKenzie.
Did you know that you can grow lettuce even during the coldest winters, using indoor hydroponics systems? Learn more about hydroponic lettuce in this article by the University of Minnesota’s Natalie Hoidal, Amanda Reardon, Leah Worth, and Mary Rogers.
“Growing lettuce, endive and radicchio in home gardens” https://extension.umn.edu/vegetables/growing-lettuce-endive-and-radicchio
“Small-scale hydroponics” https://extension.umn.edu/how/small-scale-hydroponics#lighting-2644462
Photo credits: Wikimedia https://upload.wikimedia.org/wikipedia/commons/b/b6/Red_leaf_lettuce_J1.JPG (1), pxhere https://c.pxhere.com/photos/c6/a2/lettuce_vegetables_food_eat_vitamins-1018166.jpg!d (2), flickr https://www.flickr.com/photos/joi/506693397 (3)