Lettuce can't take the heat.
Lettuce is a fairly hardy, cool-weather vegetable that thrives when the average daily temperature is between 60 and 70°F. It should be planted in early spring or late summer. At high temperatures, growth is stunted, the leaves may be bitter and the seedstalk forms and elongates rapidly. Some types and varieties of lettuce withstand heat better than others.
There are five distinct types of lettuce: leaf (also called loose-leaf lettuce), Cos or romaine, crisphead, butterhead and stem (also called asparagus lettuce).
Leaf lettuce, the most widely adapted type, produces crisp leaves loosely arranged on the stalk. Nearly every garden has at least a short row of leaf lettuce, making it the most widely planted salad vegetable. Cos or romaine forms an upright, elongated head and is an excellent addition to salads and sandwiches. The butterhead varieties are generally small, loose-heading types that have tender, soft leaves with a delicate sweet flavor. Stem lettuce forms an enlarged seedstalk that is used mainly in stewed, creamed and Chinese dishes.
Crisphead varieties, the iceberg types common at supermarkets all over the country, are adapted to northern conditions and require the most care. In areas without long, cool seasons, they generally are grown from transplants, started early and moved to the garden as soon as the soil can be worked. They are extremely sensitive to heat and must mature before the first hot spell of summer to achieve high-quality heads. If an unseasonably early heat wave hits before they have matured, they almost certainly fail. In many locations, crisphead lettuce plants started in late summer to mature in the cooler weather of fall have a much better chance of success.
When to Plant
Leaf, Cos and Butterhead lettuce can be planted anytime in the spring when the soil is dry enough to rake the surface.
- Two or more successive plantings at 10 to 14 day intervals provide a continuous supply of lettuce.
- Lettuce does not withstand hot summer days well and spring planting should be completed at least a month before the really hot days of early summer begin.
- Plantings started in late summer mature during cool fall weather.
- Watering is essential for seed germination and establishment of seedlings.
- Some shade may also benefit summer sowings. Heat-tolerant varieties (mainly loose-leaf types) may be grown in the shade of taller crops through most of the summer if extra care is taken about irrigation and soil selection.
Head lettuce must be transplanted in most locations and requires more care than other types of lettuce.
- Start transplants for a spring crop indoors or in a cold frame and set them in the garden as early in the spring as the weather settles.
- Harden transplants outdoors so that they become acclimated to the conditions under which they will be grown, but do not allow growth to stop entirely.
- Cos, butterhead and leaf varieties also can be transplanted for earlier harvest.
- In the heat of summer, lettuce seedlings started in a protected location in the shade can be transplanted later into moderate sites for some limited success.
Spacing & Depth
Plant seeds 1/4 to 1/2 inch deep (10 seeds per foot) in single, double or triple rows 12 to 18 inches apart. Thin seedlings to 4 inches apart for leaf lettuce and 6 to 8 inches apart for Cos or Butterhead. The seedlings removed may be transplanted or eaten. Transplant Crisphead seedlings 10 to 12 inches apart in the row.
Because lettuce has shallow roots, it should be hoed or cultivated carefully. Frequent light watering causes the leaves to develop rapidly, resulting in high-quality lettuce. Overwatering, especially in heavy soils, can lead to disease, soft growth and scalding or burning of the leaf margins. Organic mulches can help moderate soil temperature and the microenvironment to produce quality lettuce in less than ideal weather conditions.
- Aphids: Watch for buildup of colonies of aphids on the undersides of the leaves.
- Tipburn is a physiological condition that causes lettuce to "die back" at the edges of the leaves. It results from a change in the moisture relationship between the soil and the plant. Clip off any brown leaf tissue and use the remainder of the leaf. Frequent light watering helps to prevent tipburn. Some varieties are resistant to this condition.
- Foliage rots can be a problem, especially in hot or wet seasons. Providing good soil and air drainage for the lettuce bed can help to minimize damage in most years.
Leaf lettuce may be cut whenever it is large enough to use. Cutting every other plant at ground level gives the remaining plants more space for growth. Leaf lettuce reaches maximum size (6 to 12 ounces) in 50 to 60 days. Butterhead varieties form small, loose heads that weigh 4 to 8 ounces at harvest (60 to 70 days). The innermost leaves, that tend to blanch themselves, are a delicacy. Cos varieties have an upright growth habit and form a long, medium-dense head.
To store lettuce, wash, drip dry and place in a plastic bag in the refrigerator. Lettuce keeps best at 32°F and high (96%) humidity.
Selection & Storage
Lettuce is a cool weather crop. It can be divided into two categories: head lettuce and leaf lettuce.
Iceberg lettuce is the most popular lettuce in the United States. It is a head lettuce that is also low in nutritional value and flavor. Because of its superior shipping qualities, iceberg has been most available which accounts for its popularity. The most abundant nutrient in iceberg lettuce is water. Dark green lettuce leaves always indicate higher fiber, flavor, and nutritional value.
Growing lettuce in your garden will give you a firsthand opportunity to taste fresh flavorful leaves which (unlike iceberg) need little or no dressing. Leaf lettuce and romaine provide flavor and crunch and are excellent salad and sandwich selections.
Lettuce leaves should be free of wilt, rot and rust. Harvest crisp green leaves. Wrap fresh, unwashed leaves in plastic wrap and store in the refrigerator for a few days if necessary. Cooler temperature will keep lettuce fresh longer. The coolest part of most refrigerators is usually on the first shelf against the rear wall.
Avoid storing lettuce with apples, pears or bananas. These fruits release ethylene gas, a natural ripening agent, that will cause the lettuce to develop brown spots and decay quickly. Toss lettuce that looks slimy or has black spots. The slime is the residue of bacterial decomposition and the black spots are usually mold.
Questions and Answers for Growing Lettuce
Q. Why didn't my lettuce seeds germinate?
A. Failure of seeds to germinate is caused by insufficient moisture or old seed. Lettuce seed does not keep well and it is advisable to obtain new seed each spring. Store seed for fall gardens in a sealed container in the refrigerator. Some lettuce varieties (especially the white-seeded types) have seed that requires light for germination. These types should not be covered with soil but merely pressed into good contact with finely prepared soil. Care then must be taken to keep the seedbed moist, but not soggy, until the seedlings emerge.
Q. Seedstalks have appeared in the center of my lettuce plants. What should I do?
A. The formation of seedstalks is caused by a combination of long days, warm temperatures and age. When seedstalks begin to form, harvest your lettuce immediately and store it in the refrigerator.
Q. My lettuce tastes bitter. What can I do?
A. Lettuce may become bitter during hot weather and when seedstalks begin to form. Wash and store the leaves in the refrigerator for a day or two. Much of the bitterness will disappear.