You can successfully plant lettuce in July, but it requires careful attention and specific adjustments to compensate for the summer heat. Lettuce is fundamentally a cool-season crop that thrives in temperatures between 60°F and 70°F, meaning planting during mid-summer presents a significant challenge. By using the right techniques and selecting heat-tolerant varieties, you can bypass the common issues of bitterness and premature flowering. Success lies in understanding the plant’s biological reaction to high temperatures and implementing environmental controls to mitigate heat stress.
Why Lettuce Struggles in Summer
The primary biological response to high temperatures and long daylight hours is bolting. Bolting is the plant’s premature shift from producing leafy growth to reproductive growth, where it quickly sends up a tall flower stalk to produce seeds. This response is triggered when temperatures consistently rise above 75°F. Once the plant begins to bolt, the leaves become tough and develop a bitter flavor. This bitterness is caused by the plant producing high concentrations of natural compounds called sesquiterpene lactones (lactucarium). These compounds act as a defense mechanism but make the leaves unpalatable. The flavor change can begin even before a visible flower stalk appears.
Techniques for Successful July Planting
Successfully growing lettuce in July depends on creating a cooler microclimate to shield the plants from intense heat and direct sun. Providing afternoon shade is the most immediate method, as the heat of the late day sun is the most damaging. You can achieve this by planting lettuce on the east side of a taller crop or structure, or by using a physical barrier like shade cloth. A shade cloth with a 30% to 50% light reduction rating is recommended to reduce heat and light intensity without fully blocking growth.
Consistent and deep watering is critical for preventing heat stress. Lettuce has shallow roots, so the soil needs to be kept evenly moist to prevent wilting and to keep the root zone cool. Watering deeply in the early morning or late evening is preferable to reduce water loss from evaporation. Applying a thick layer of organic mulch, such as straw or well-aged compost, helps to insulate the soil and maintain a lower, more consistent root temperature.
Preparing the soil with plenty of organic matter is also beneficial for summer planting. Organic material improves the soil’s capacity to retain moisture, which is essential for rapid growth and prevents the water stress that triggers bitterness. If starting from seed, note that lettuce seeds have a condition called thermo-inhibition and may fail to germinate in soil temperatures above 80°F. Starting seeds indoors in a cooler environment and transplanting them out as seedlings is a useful strategy for a July start.
Selecting Heat-Tolerant Varieties
Choosing the right type of lettuce is the simplest way to improve your chances of a successful July harvest. Certain varieties have been selected or bred to be “slow-bolting,” meaning they take longer to initiate the reproductive stage when exposed to heat. Loose-leaf varieties, which are harvested by the “cut-and-come-again” method, and Romaine (Cos) types generally handle heat better than the tightly-headed Crisphead (Iceberg) or most Butterhead types. Specific, reliable heat-tolerant cultivars include ‘Black Seeded Simpson’ and ‘Salad Bowl.’ For Romaine, varieties like ‘Parris Island Cos’ and ‘Little Gem’ are known for their ability to maintain flavor and a crisp texture even during high temperatures.