Summer Lettuce…. To grow or not to grow?
Lettuce (Lactuca sativa) is not usually a vegetable we think of growing in the middle of summer in central Virginia. It is typically thought of as a cool season crop, sensitive to high temperatures and dry soils. Lettuce tends to bolt in response to the stress of high temperatures and the leaves become bitter and tough as the plant puts its growth into flowering. However, you too can grow fresh lettuce during the summer by choosing the correct varieties, paying attention to the planting location, using shade or seasonal covering, and watering sufficiently. The following techniques will hopefully convince you to meet the heat challenge.
First, some basics:
Lettuce will grow in a wide range of soils, but prefers a slightly acidic, loamy soil, pH of 6.0-6.5, that has been amended with organic matter. Lettuce is an excellent choice for growing in raised beds or in containers.
Seed Facts and Planting
The optimum soil temperature for germination is 60-80 degrees F. According to Clemson University research, lettuce seeds will not germinate if the soil temperature is above 95 degrees F. Lettuce is easily grown by direct seeding but if your soil temperature is over 90 degrees F, you may want to consider one of two methods: 1) precool the soil by watering and then cover the area with a board for a few days, or 2) germinate seeds indoors in a flat away from the heat and then transplant the seedlings outdoors. Either way, be sure to plant seeds no deeper that 1/8-1/4″ since many types of lettuce need light to germinate. Or, just “dust” the seeds with fine soil or compost, and then tamp down with a rake end so that the seeds make good soil contact. Keep the seedbed moist and thin the seedlings as necessary.
Choose a planting site that provides shade because too much sun is a problem during the heat of summer. Here are some techniques to try:
- Plant in a shady area of your garden, or on the north or east side of tall plantings such as corn or caged tomatoes.
- Take a low, hooped structure made of wire or PVC plastic hoops and place over rebar that you’ve driven into the ground. Top with a shade cloth. Cloths can be ordered from landscape suppliers who can provide different-sized cloths to fit your area. The cloths come with grommets and so can be tied down. These cloths should last for years.
- Repurpose lightweight bed sheets as shade cloth
- Use lightweight row covers to provide some shade; I personally think you need more shade than this alone.
- Plant lettuce in containers that you can easily move into the shade.
Lettuce is a fast-growing crop with varieties typically maturing within 30-60 days. Sowing small amounts every 7-14 days is the recommended summer planting schedule that will provide a continuous harvest. This is also called succession planting. Selecting varieties with heat-tolerance and slow-bolting is one of the keys to success.
In general, there are four major categories of lettuce:
- Heading or Crisphead,
- Cos or Romaine,
Heading or crisphead types are in general adapted to northern conditions and require the most care. They are very heat-sensitive and would not be my choice for summer plantings. If you insist, VA Tech lists (VCE Publication 426-480) ‘Summer-Time’ as a heat-tolerant, heading lettuce.
Cos or Romaine lettuce has long, narrow leaves with an upright growth habit forming loose, elongated heads, firmer than loose-leaf lettuces, with a crispy, center rib. Parris Island cos (29-days), and Jericho (60-days) are considered two good heat-tolerant choices in this category.
Butterhead lettuce is a soft, tender, and loose-heading type that is divided into two subgroups: Boston and bibb. Bibb lettuces are smaller and darker green than the Boston type. All have a delicate, sweet, buttery flavor. Bibb lettuce will develop bitterness if temperatures are above 95 degrees F. Three good choices are: Buttercrunch (55-days), a small, fast-maturing, dark green bibb lettuce; Tom Thumb, (35-50-days) an easy, fast-growing miniature variety of butterhead; and Red Cross (48d), a heat- tolerant, red butterhead.
Loose-leaf lettuce, with either green or reddish leaves, is the most commonly grown lettuce. Leaves may be smooth, round, wrinkled, serrated or curled. The Summercrisp/Batavia lettuces are heat-tolerant plants that initially are open like loose-leaf, but then mature to a more compact bunch or head. Good choices in this category: Muir (50-days) is extremely heat-tolerant; Nevada (48-days) has great flavor and stays mild; Sierra (45-days) is a cut-and-come-again type with tall, open heads, thick, bright green-with-red-tinged leaves, and a sweet and nutty flavor; Simpson Elite (48 days) is a loose-leaf type that is also a great summer choice similar to Black Seeded Simpson but is slower-bolting and more heat-tolerant.
Butterhead Lettuce Loose-leaf Lettuce
Fertilizing: Incorporate compost and lots of organic matter into the soil prior to planting. In general, the recommendation is to use 5-10-10 at 3 pounds per 100 square feet before planting. Although lettuce has low fertility needs, it does need some nitrogen for good green leaf growth. Use a balanced starter fertilizer or compost initially, and then side-dress with compost tea or fertilizer at least once during the growing season. Of course, the amount of fertilizer depends on your soil type, with sandy soils needing the most.
Watering: The most critical period for moisture is during germination and plant establishment. Water frequently during the first 2 weeks to keep the seedbed moist but not waterlogged. A light sprinkling of leaf compost, mulch, or straw will help to conserve moisture. Continue to water lettuce once established every 4-5 days to keep the plants healthy and producing. Clemson Extension recommends watering sufficiently enough to moisten the soil to a depth of 4-6 inches. Lettuce grown in containers or raised beds will need to be watered more frequently. Over-watering in clay soils can lead to disease issues, so be careful. Water early in the day so that foliage has time to dry before dark.
Weeding: Keep your bed weed-free, which will avoid water competition and help manage insects. Lettuce is shallow-rooted so be careful to prevent root injury. Tight plant spacing will help plants to quickly shade the soil and help suppress weeds.
Insects and Diseases: The most common pests are aphids, flea beetles, slugs, cutworms and leafhoppers.
The most common diseases include dampening-off, Bottom rot disease, and aster-yellows, which is spread by leafhoppers. Both insects and diseases tend to be controlled by using cultural garden practices. You may consider using insecticidal soaps or horticultural oils, but be sure to read the labels.
In summary, you can grow lettuce in the summertime heat by:
- Choosing varieties that are heat-tolerant and slow-bolting. My 3 favorites are easy to remember: Muir, Sierra, and Nevada.
- Choose a planting site that provides shade.
- Don’t skimp on water.
- Make successive plantings to replace spent plants.
- Use sound cultural practices: inspect plants for insects and diseases, hand pick and destroy destructive insects, and remove diseased leaves or plants.
- Harvest regularly and enjoy!
Thanks for stopping by The Garden Shed. We hope you visit us again next month.
“Lettuce”, University of Georgia Extension, Publication 1312, http://www.clemson.edu/extension/hgic/plants/vegetables/crops/hgic1312.html
“Vegetable Profiles: Lettuce,” University of Maryland Extension, https://extension.umd.edu/growit/vegetable-profiles-lettuce
“Home Garden Lettuce”, University of Georgia Extension, Publication C1018, http://extension.uga.edu/publications/detail.cfm?number=C1018
“Vegetables Recommended for Virginia”, Virginia Cooperative Extension, Publication 426-480, https://www.pubs.ext.vt.edu/426/426-480/426-480.html
The Timber Press Guide to Vegetable Gardening in the Southeast (Regional Vegetable Gardening Series) by Ira Wallace, 2013