It is a common belief that lettuce, a classic cool-weather crop, cannot be successfully grown during the heat of summer. While traditional varieties struggle in high temperatures, it is entirely possible to cultivate fresh, crisp lettuce through the warmer months with the right knowledge and simple adaptations. Lettuce is a fast-growing annual that thrives in mild conditions, but its rapid decline when exposed to heat can be managed. By selecting specific cultivars and implementing practical cooling strategies, you can maintain a continuous supply of salad greens.
Understanding Heat Stress and Bolting
Lettuce plants are highly sensitive to heat, and temperatures consistently above 75°F (24°C) trigger a survival mechanism known as bolting. Bolting is the rapid transition from producing leafy growth to developing a tall, central flower stalk and seeds. This process is the plant’s final effort to reproduce before it succumbs to the harsh conditions.
This reproductive shift causes an undesirable change in the leaf chemistry. As the plant bolts, it produces higher concentrations of a milky, bitter compound called lactucarium. The increased bitterness acts as a natural defense mechanism against pests, but it renders the leaves tough and unappetizing for human consumption. Once bolting begins, the bitterness is irreversible, making understanding the trigger and prevention methods important for a quality harvest.
Choosing the Right Summer Varieties
Selecting a “slow-bolt” or heat-tolerant cultivar is the most effective first step toward a successful summer harvest. These varieties have been bred to resist the heat and day-length signals that trigger premature flowering in traditional types. Loose Leaf and Romaine lettuces generally perform better in heat than the denser Butterhead or Crisphead (Iceberg) types, which tend to trap heat and bolt quickly.
Loose Leaf varieties, such as ‘Black Seeded Simpson’ and ‘Salad Bowl,’ are excellent choices because they are quick-growing and highly resistant to bolting and tipburn. These types offer an easy-to-harvest structure that allows for continuous picking. Romaine, or Cos, lettuce also includes robust summer options, with ‘Parris Island Cos’ being a classic choice known for its sturdy texture and good bolt resistance. For truly exceptional heat performance, varieties like ‘Jericho,’ developed in Israel for desert climates, can thrive for over 70 days before bolting.
Practical Strategies for Cooling Lettuce Plants
Managing the growing environment is crucial for preventing heat stress and delaying bolting. One of the most effective techniques is providing shade during the hottest parts of the day. Place lettuce in an area that receives morning sun but is protected from the intense afternoon sun. This utilizes the shade cast by taller plants or structures.
For areas without natural shade, a shade cloth is a recommended tool. A density of 30% to 47% is appropriate for lettuce, as it filters intense light and heat while still allowing enough light for photosynthesis. The shade cloth can lower the air temperature around the plants by 10 to 15 degrees Fahrenheit, creating a microclimate that prevents the heat-induced stress response.
Consistent and deep watering is another strategy to combat heat, as it helps to cool the root zone. Lettuce has shallow roots, making it highly susceptible to drying out. Drying out is a major stress factor that encourages bitterness and bolting. Watering deeply in the early morning ensures the soil is cool and moist before the daytime temperatures peak, but avoid wetting the leaves in direct sun.
To maximize the harvest from each plant, use the “cut and come again” harvesting method. This involves removing only the outer leaves instead of the entire head. This technique preserves the central growing point of the plant, encouraging new leaf production and extending the harvest period before the plant bolts.
“Succession planting” involves sowing small batches of seeds every one to two weeks. This ensures that even if one batch bolts prematurely due to a sudden heat wave, another younger crop is always on the way.