Yes, planting lettuce in August is achievable, but it requires specific strategies to overcome the intense summer heat for a successful late fall or early winter harvest. Although lettuce is a cool-season crop, sowing seeds now takes advantage of the long days for fast growth once temperatures begin to moderate. The goal of this mid-summer planting is to establish young plants that will thrive as the weather naturally cools, producing sweet, crisp leaves later in the season. Success hinges entirely on managing the heat stress during the initial weeks of germination and early growth.
The Timing Challenge of August Planting
The primary obstacle to August planting is the high soil temperature, which directly causes thermal dormancy in lettuce seeds. Lettuce seeds struggle to sprout when soil temperatures consistently rise above 75°F, with many varieties failing entirely if the soil exceeds 80°F. This inability to germinate, known as thermo-inhibition, is a natural survival mechanism preventing the seedling from emerging into conditions where it cannot survive.
If seeds manage to germinate, high temperatures stress the young plants, forcing them to prematurely “bolt.” Bolting is the plant’s rapid shift from leafy growth to producing a flower stalk and seeds. This switch correlates with the buildup of bitter-tasting compounds in the leaves, which renders the harvest inedible.
Prolonged exposure to warm temperatures accelerates the bolting process and diminishes leaf quality. The resulting harvest can be small, tough, and intensely bitter. Therefore, managing the immediate environment around the seed and seedling is paramount for a successful crop.
Selecting Heat-Tolerant Varieties
Selecting cultivars specifically bred for slow bolting and heat resistance is essential. Loose-leaf and Romaine types generally possess greater heat tolerance compared to Crisphead varieties. Loose-leaf varieties, such as ‘Black Seeded Simpson’ and ‘Salad Bowl’, are excellent choices because harvesting individual leaves naturally helps delay bolting.
Romaine lettuces, known for their upright growth, handle warmer conditions well. The cultivar ‘Parris Island Cos’ is a widely recommended Romaine variety known for its resilience and ability to maintain good flavor in heat. For a butterhead type, ‘Buttercrunch’ offers improved slow-bolting characteristics suitable for August sowing.
Essential Techniques for Mid-Summer Sowing
To bypass thermal dormancy, focus on cooling the soil during the germination phase. Planting seeds in the late evening allows the soil to cool down overnight, providing favorable temperatures. A highly effective technique is to pre-chill the planting area by watering thoroughly and covering the soil with a board or burlap for a day or two before sowing.
After sowing, use chilled water for the initial watering to immediately drop the soil temperature around the seed. Maintaining consistent moisture is crucial, as evaporation provides a cooling effect. Watering must be shallow and frequent to prevent a hard surface crust that emerging seedlings cannot penetrate.
Providing temporary shade is non-negotiable for young plants to survive the intense afternoon sun. Positioning the planting bed to utilize existing shade from a fence or taller crops, like tomatoes or corn, is an easy solution. Alternatively, a 30 to 40 percent woven shade cloth suspended several feet above the bed will reduce the sun’s intensity without blocking all the necessary light.
Heat-stressed plants benefit from aggressive and early thinning of the seedlings. Reducing competition ensures the remaining plants have sufficient air circulation and access to moisture and nutrients, encouraging rapid, healthy growth. This focused care ensures the plants are robust enough to produce a full, sweet harvest as cooler weather arrives.