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Lettuce is a cool-season crop best grown when temperatures are below 68°F/20°C, i.e., in spring or fall, and higher temperatures cause this biennial to bolt.
Of all the vegetables consumed, lettuce has the highest per capita consumption rate in the United States, a staggering 23.4 pounds in 2021. Leaf lettuce is the most popular (54%). The thermal dormancy of lettuce will prevent it from germinating if the soil is too warm or cold.
Table of Contents
- Table of Contents
- Growing Summer Lettuce
- The Fundamental Temperature Needs of Lettuce Plants
- How to Grow Lettuce in Summer
- FAQs on The Best 7 Lettuce Types to Grow During Summer Months
- In Closing
Growing Summer Lettuce
Lettuce is a high-value crop, and in a world where annual temperatures are ever-increasing, developing heat-tolerant lettuce is a growing priority. Unsurprisingly, the University of California is the leader in summer lettuce research.
Lettuce is California’s fifth-largest commodity, with a market value of over US$3 billion. Lettuce is a widely grown crop worldwide, with different regions favoring different varieties.
The average annual lettuce consumption in the United States has dropped since prices have rocketed to $3.58 per pound (Average Romaine lettuce price in Dec 2022). The United States exports 3.6 million tons of lettuce annually.
China grows more lettuce than any other country, about 13.5 million tons annually, mainly for local consumption, and Spain exports just under a million tons annually.
Used as a garnish in most fast food products, lettuce is in demand, and finding solutions for growing lettuce without the risk of early bolting is essential.
In 2015/16, Sacromento’s Fair Oaks Horticulture Center did some trials on lettuce bolting times and temperatures. More recently, the University of Delaware and the Practical Farmers of Iowa (PFI) did similar tests.
We want to beat the heat and find the most heat-resistant or heat-tolerant lettuce plants. My efforts are informal, and I hope you also push the boundaries, establishing what works in your area.
The Fundamental Temperature Needs of Lettuce Plants
Lettuce is a cool-season vegetable, and seeds will germinate at 35°F/1.6°C, but the optimum soil temperature is 70 to 75°F/21 – 24°C. The maximum temperature at which lettuce seeds will germinate is 85°F/29.4°C.
Extended periods of temperatures above 85°F/29.4°C will cause lettuce to bolt, a common challenge to growing lettuce in summer. If growing lettuce in summer, ensure afternoon shade, as too much sun is a bolting factor.
Lettuce seed needs light to germinate, but plants do well with three to five hours of sunlight, avoiding the midday sun. Taller plants can be grown as companions to cast some shade, though this is less likely during the middle of the day when the lettuce plants need it most.
Several garden vegetables a biennial plans grown as annuals. Biennial plants grow roots and strengthen their foliage in year one to produce potent, resilient, healthy seeds in year two.
The plants rush the seed production process when the environment changes (more light and higher temperatures). Gardeners call it bolting, and it generally refers to vegetable plants that begin flowering.
Unfortunately, bolting can diminish the quality of your harvest. Lettuce plants develop a bitter taste as they devote more resources to flowering and seed production.
If you want to grow lettuce in summer, look for “slow bolting” seeds. Grow loose-leaf varieties that allow you to harvest outer leaves while the plant is still young.
Remove spent plants to avoid volunteer lettuce plants. Replace spent plants with fresh lettuce plants to keep your summer garden vibrant.
Sowing Lettuce Seeds
Several bloggers suggest direct seeding; while possible, the risks outweigh any benefits. Lettuce plant seeds are disease prone, and I advise gardeners not to produce seeds from previous crops.
Instead, purchase your seeds from a reputable producer that certifies their stock.
How to Grow Lettuce in Summer
Summer Site Selection
Please remember that growing lettuce in summer is reserved for cooler climates or controlled environments. There are several summer crops, but lettuce is not one of them. Avoid growing lettuce in summer in hot climates.
That said, cooler climates allow year-round lettuce production, and I imagine it won’t be long before Washington state, and other northern states become significant producers. Washington state is already the second biggest vegetable crop producer (after California).
Lettuce may bolt in reaction to high-temperature stress, and the leaves may become bitter when the plant transitions to flowering (see photo at right). If the plant becomes water stressed in hot weather, the leaves wilt, toughen, and become bitter.
Managing Summer Temperatures
To successfully grow lettuce in warm weather, consider the following strategies:
- Germinate seeds indoors, managing light, soil moisture, and soil temperatures well.
- Use shade cloth or grow taller plants as companion plantings to cast shade on your lettuce plants, especially in the midday sun. Taller vegetables include sweet corn, stalk beans, and other vining crops.
- Water must absorb heat from the surrounding environment to evaporate, causing it to cool. Water adequately in hot weather, keeping the soil moist but not wet. Lettuce plants need damp soil.
- Mulch lightly with organic mulch to retain moisture and insulate the soil from the sun, keeping temperatures constantly lower.
- Harvest outer leaves with sterile tools to promote fresh regrowth.
- Plant lettuce seeds and transplants in succession to reduce the number of aged plants in a crop.
- Keep nutrition levels up to boost foliar growth. Happy plants don become bitter and bolt, and happy plants have some shade, enough water, and nutrition.
- Examine the plants for insects and illnesses. Destructive insects must be hand-picked and destroyed. Rouge any unhealthy leaves or plants.
- A light layer of organic mulch works wonders.
Using Shade Cloth
Please do not cool your growing lettuce by wetting leaves. Summer stress vectors abound, and wet leaves will cause rapid pathogen spreads and diseases. Keep the soil moist and relaxed, the leaves dry, and the lettuce under a shade cloth in the midday sun.
Lettuce: 7 Heat Tolerant Lettuce Varieties
All lettuce bolts and those that don’t may become bitter if exposed to high temperatures and long days of bright light. Heat tolerance is relative, but some varieties are slower to bolt in the summer heat.
Leaf lettuce allows early harvesting, and romaine lettuce has a reputation for being more heat resilient. My table below lists 21 varieties, of which 10 are leaf lettuces and no Iceberg lettuce.
That’s because loose-leaf lettuces have better airflow and heat dissipation mechanisms than head lettuces. That’s a generalization because Romaine lettuce forms heads but is much less compact than iceberg, and loose-leaf lettuce is one of the best warm-season varieties.
Below is a table of 21 heat-tolerant lettuce varieties, with my selection of the top seven listed first and in order of preference. Several heat-tolerant options are hybrids developed by Johnny’s Select Seeds.
|#||Variety||Type||Days to Maturity||Comment|
|1||Muir||Batavian||50||Extremely heat tolerant|
|3||Salanova® Green Butter||Butterhead Lettuce||55||Excellent bolt tolerance.|
|4||Salanova® Red Butter||Butterhead Lettuce||56||Excellent bolt tolerance.|
|5||Sunland||Romaine Lettuce||56||Great summer performance|
|6||Sparx||Romaine Lettuce||58||Ideal for summer harvests.|
|7||Casey||Butterhead Lettuce||55||Heat tolerant in controlled environments|
|8||Cimmaron||Romaine Lettuce||35||Excellent bolt resistance|
|9||Black-Seeded Simpson||Leaf Lettuces||45||Drought and heat tolerant|
|10||Grand Rapids||Leaf Lettuces||45||Heat tolerant|
|11||Green Star||Leaf Lettuces||45||Heat tolerant|
|12||Slobolt||Leaf Lettuces||45||Good warm-weather lettuce|
|13||Bronze Oakleaf||Leaf||50||Good resistance to hot spells|
|14||Red Salad Bowl||Leaf||55||Good heat tolerance.|
|15||Salad Bowl||Leaf||55||Good resistance to bolting.|
|16||Ruby||Leaf||55||Good bolt and heat resistance|
|17||White Boston||Boston Lettuce||57||Performs well in warm, humid conditions|
|18||Buttercrunch||Butterhead Lettuce||68||Heat tolerant|
|19||Ithaca||Crisphead Lettuce||72||Does well in summer.|
|20||Summer Bibb||Bibb Lettuce||77||Slow to bolt|
|21||Great Lakes||Leaf Lettuce||85||Slow to bolt|
Muir Lettuce is a variety that is slightly sweeter and more flavorful than other types of lettuce. It is a hardy variety of lettuce capable of growing in many climates.
It has large, juicy leaves with a light green color and a hint of bronze. The leaves are also very crisp and have a mild, slightly sweet flavor. Muir lettuce is ideal for salads, sandwiches, wraps, and other recipes.
It is also a great dietary fiber, vitamin, and mineral source.
Muir lettuce is exceptionally tolerant of hot days, and trials have repeatedly been the slowest to bolt. Only buy seeds with certified resistance to downy mildew and bushy tomato stunt virus (lettuce dieback complex).
Skyphos lettuce is a loose-leaf lettuce with a unique, crinkled texture and a sweet and mild flavor. It is famous for salad greens, sandwiches, wraps, and garnishing plates.
Skyphos lettuce is relatively easy to grow and can be harvested in as little as four weeks. The plant is deep green, and the leaves are thick and tender.
It is also very resistant to bolting and can grow in various temperatures, making it an excellent choice for multiple climates.
3. Salanova® Green Butter
Salanova® Green Butter lettuce was developed by the Dutch company Bejo Seeds in 2005. Salanova® Green Butter lettuce is a large, loose-leaf with a mild, buttery flavor, and it is an excellent choice for salads, sandwiches, and wraps.
The leaves are divided into sections, giving the lettuce its distinctive look. Salanova® Green Butter lettuce is also known for its high content of vitamins A and C and other essential vitamins and minerals. This lettuce variety is easy to grow and maintain in the home garden and is resistant to bolting.
4. Salanova® Red Butter
Salanova® Red Butter lettuce is a variety of lettuce developed by the Dutch seed company Rijk Zwaan and has a dark red color, a sweet flavor, and a loose head shape.
The leaves are slightly crunchy and have a mild, buttery taste. Salanova® Red Butter lettuce is famous for salads, sandwiches, wraps, and other dishes, and it is also a great source of vitamins and minerals.
Sunland lettuce is romaine grown in the San Joaquin Valley of California. It is a high-quality variety well-suited for salads, sandwiches, and wraps. Sunland lettuce is produced in a unique environment that combines the ideal soil, climate, and irrigation.
This combination produces a unique flavor and texture unmatched by other lettuces. Sunland lettuce is also known for its high nutritional value, packed with essential vitamins, minerals, and antioxidants.
Its high nutritional content is why it is a popular choice for health-conscious consumers.
Sparx lettuce is a unique variety known for its high levels of nutrition and flavor. It is a chicory family member and is a cross between iceberg and romaine lettuce.
It has a bright, crisp texture and a sweet, nutty flavor. It is rich in vitamins A, C, and K, folate, iron, and calcium. It is also a good source of dietary fiber. Sparx lettuce is a popular choice for salads, sandwiches, and wraps.
Casey Lettuce is a hybrid variety of lettuce developed in Australia. It has a medium-green leaf with a firm, crunchy texture and a sweet, mild flavor.
Casey Lettuce is one of Australia’s most popular lettuce varieties and is also grown in New Zealand, the United States, and several other countries.
It is commonly used as a salad green but can also be eaten cooked or as a topping for sandwiches. Casey Lettuce is a cool-season crop, best planted in the early spring or late summer.
FAQs on The Best 7 Lettuce Types to Grow During Summer Months
If you plant lettuce in early spring, you can beat the heat and get a crop mid-summer. If your local climate is warm, you must use every resource to keep the lettuce plants cool enough not to bolt.
Shade cloth, mulching, persistently moist soil, and a close eye for pests are some tasks that are less important in cooler weather.
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