Lettuce is a leafy annual vegetable that thrives in cool weather. It is very easy to grow and is is considerably tastier than store-bought. For gardeners who enjoy making sandwiches and salads of all types, lettuce is an indispensable part of the garden.
Lettuce (Lactuca sativa) can be grown in nearly all climate zones, but as a cool-season vegetable it does best when growing in the spring and fall—or even winter in southern climates. During hot weather, the plants bolt (send up flower shoots to produce seeds), and at that this point, the leaves become tough and inedible. For this reason, most gardeners remove leafy varieties of lettuce during the hot months of mid-summer in favor of other vegetables; they may return to lettuce when weather cools again.
There are a surprising number of lettuce varieties to choose from, ranging from limp loose-leaf lettuces ideal for sandwiches to crunchy crisphead (iceberg) lettuces that are the mainstay of many salads.
Lettuce is traditionally planted by seed outdoors as soon as soil reached 40 degrees Fahrenheit, in order to ensure that it matures before the weather becomes too warm. However, best germination occurs at 55 to 65 degrees, so in regions with long, cool summers, planting can be delayed somewhat. The seeds typically sprout in seven to 10 days. Leafy varieties of lettuce take about 30 days to mature enough to harvest, while other types may take as much as six to eight weeks.
|Botanical Name||Lactuca sativa|
|Plant Type||Leafy annual vegetable|
|Size||6 to 12 in. tall, similar spread|
|Sun Exposure||Medium to bright indirect light, or partial shade (indoors), direct sun (outdoors)|
|Soil Type||Fertile, well-draining|
|Soil pH||Slightly acidic (6.0 to 7.0)|
|Native Area||Mediterranean, Siberia|
How to Plant Lettuce
How you plant lettuce will depend on what type you are growing:
- Leafy lettuces are typically planted by seeds sown in rows, with harvesting beginning as soon as the leaves are a few inches tall. If trimmed down to the soil line, leaf lettuce will continue to produce and can be harvested several more times. Available in both green and red cultivars, leaf lettuces are the easiest types to grow. You can seed additional lettuce every two weeks or so to ensure a steady supply of lettuce up until mid-summer. Seeds should be planted about 1/4 inch deep; thin the seedlings out to a 4-inch spacing as they sprout.
- Head lettuces include those knowns as crisphead or iceberg lettuce. They can also be planted from seeds or nursery transplants, and should be spaced about 18 inches apart. These varieties are a bit trickier to grow and are more susceptible to a variety of insect pests.
- Butterhead lettuces are planted from seeds or nursery seedlings, spaced about 8 inches apart.
- Romaine lettuce (also known as cos lettuce) can also be planted by seed, but it more often planted from nursery transplants. They are spaced about 8 inches apart.
All lettuces need a relatively weed-free environment since lettuce doesn't like to compete for nutrients and water.
Lettuce grows best in a full-sun location, but it can benefit from some shade in hot climates, or during the hottest months. Some afternoon shade may delay the bolting process.
All types of lettuce will grow best in a relatively fertile, well-drained soil. The ideal soil pH is slightly acidic to neutral—6.0 to 7.0.
Laying down a thick layer of organic mulch will help retain soil moisture, and also will moderate soil temperatures
Temperature and Humidity
Lettuces of all varieties prefer cooler growing conditions and many types go to seed (bolt) as soon as temperatures begin to exceed 80 degrees Fahrenheit. The best growth occurs at 60 to 70 degrees. Lettuce grows best at relatively low humidity. Humid conditions may foster fungal problems, such as powdery mildew or Botrytis (gray mold).
Feed lettuce plants about three weeks after planting seedlings (or about three weeks after they have been thinned to their final spacing). Use a high-nitrogen slow-release fertilizer mixed into the soil.
Lettuce prefers soil that is high in organic material, with plenty of compost and a steady supply of nitrogen to keep if growing fast.
Growing lettuce can be a little bewildering since there are so many subvarieties. Some sources list as many as 15 different types of lettuce, but most authorities list four main classifications:
- Leaf lettuce grows in loose leaves that emerge directly from the soil. These are quite easy to grow. Some good cultivars include 'Red Sails', 'Tango', and 'Slobolt'.
- Crisphead lettuce forms very tight, spherical heads of leaves. The most popular cultivar is 'Iceberg'; others include 'Ithaca', 'Great Lakes', and 'Crispivo'.
- Butterhead lettuce, also called bibb or Boston lettuce, forms loose heads of tightly folded leaves. Its common name comes from the buttery flavor of the leaves. 'Ermosa', 'Esmeralda', and 'Nancy' are some varieties to try.
- Romaine lettuce, sometimes known as Cos lettuce, grows in tall, tight bundles of crisp, sweet leaves. Some recommended varieties include 'Green Towers', 'Valley Heart', and 'Red Eyes Cos'.
Harvesting lettuce varies, depending on the type:
- Leaf lettuces are generally ready to start harvesting about 30 days after the seeds sprout. Cut off the plants just above ground level; this will usually stimulate a second flush of growth. You may be able to harvest three or four times before the plants bolt.
- Crisphead lettuces are ready to harvest when the spherical heads are 6 to 8 inches in diameter and the leaves are tightly packed. This can take as long as eight weeks or even longer. Pull up the entire plant, then cut off the roots with a sharp knife.
- Butterhead lettuces are ready to harvest when the leaves begin to cup inward, beginning to form a loose head. The clump will be 6 to 8 inches across. You can also harvest butterhead lettuce as you would loose-leaf lettuce, trimming young leaves as they appear. Full clumps will be mature in about 55 days. After this, the leaves will begin to turn tough and the plants will prepare to bolt. Once the clumps mature, they should be pulled from the ground entirely.
- Romaine lettuces when the outer leaves have clearly defined middle ribs and form a fairly tight elongated head. The plants will be 6 to 8 inches tall. Pull them from the ground and trim off the roots. Romaine lettuce is ready to harvest in 75 to 85 days after planting.
Common Pests and Diseases
Most lettuce varieties grow so fast that harvesting is complete before insect pests can cause serious damage. You may, however, battle aphids, leafminers, or whiteflies. Use an insecticidal soap or citrus oil to combat these. More troublesome is when snails or slugs decide to feed on the plants. Diatomaceous earth sprinkled around the plants may prevent these creatures from feeding on your greens.
Potential disease problems include downy mildew, powdery mildew, shot hole, bottom rot, septoria leaf spot, Botrytis (gray mold), and wilt. These diseases usually occur in damp, humid conditions. Affected plants should be removed and destroyed. In future growing seasons, rotate the location of crops so that you don't plant lettuce repeatedly in the same location, since pathogens may linger in the soil.