Planting and harvesting fresh lettuce in July is possible, but it requires a strategic approach. July presents unique challenges for this cool-season crop, primarily intense heat and prolonged daylight hours. Success hinges on two main factors: selecting heat-tolerant varieties and implementing specific cultural techniques to create a cooler microclimate. With careful planning, you can maintain a steady supply of crisp greens throughout the summer.
Understanding Lettuce’s Sensitivity to Summer Heat
Lettuce is a cool-weather crop, thriving when air temperatures are consistently between 60°F and 70°F. When temperatures rise above 75°F to 80°F, the plant undergoes a physiological change known as “bolting.” Bolting is the plant’s survival mechanism, triggered by environmental stress, where it prematurely shifts energy from producing leafy greens to developing a central flower stalk to set seed.
This process rapidly increases the concentration of a bitter compound called lactucin within the leaves. Once the plant bolts, the leaves become tough, bitter, and unpalatable, ending the harvest. July’s combination of high heat and long daylight hours accelerates this transition.
Selecting the Right Varieties for Mid-Summer Planting
Choosing the correct cultivar is the most significant step toward a successful July harvest, as many modern varieties are bred to be “slow-bolt” or “heat-tolerant.” Loose-leaf lettuces are the most forgiving group, bolting slower than head-forming types like crisphead or traditional romaine.
Recommended Heat-Tolerant Varieties
For mid-summer planting, focus on these types:
- Loose-leaf varieties: ‘Black Seeded Simpson’ and ‘Salad Bowl’ are known for their resilience and ability to maintain good flavor under stress.
- Butterhead (Bibb) varieties: Look for ‘Buttercrunch’ or ‘Marvel of Four Season,’ which form loose rosettes with tender leaves.
- Romaine varieties: The compact ‘Little Gem’ or the robust ‘Jericho’ perform well in warmer conditions.
- Batavian (Summer Crisp) types: Varieties like ‘Nevada’ or ‘Sierra’ consistently outperform others in temperatures exceeding 85°F, making them ideal for extremely hot climates.
Essential Techniques for Mid-Summer Success
Mitigating the summer heat requires creating a cooler microclimate specifically for your lettuce plants. The most effective method is providing shade during the hottest part of the day. Plant in a location that receives morning sun but is protected from intense afternoon rays. Alternatively, suspend a shade cloth rated for 30% to 40% light reduction over the planting area to significantly lower air and soil temperature.
Consistent and deep watering is essential, as lettuce has a shallow root system and is vulnerable to water stress. Dry soil stresses the plant and causes leaves to wilt, toughen, and become bitter. Water early in the morning to ensure plants have access to moisture before the heat of the day and to help keep the roots cool.
Applying a 2-inch layer of organic mulch, such as straw or aged compost, is another technique. Mulch serves a dual purpose: insulating the soil to prevent roots from overheating and maintaining an even soil moisture level. Keeping the seedbed cool is necessary for direct sowing, as lettuce seeds often fail to germinate when soil temperatures climb above 80°F.
Planning Your July Planting Schedule
A successful summer lettuce strategy involves continuous, small-scale planting, known as succession planting. Instead of sowing an entire bed at once, sow a small batch of seeds or transplants every one to two weeks. This method ensures a steady supply of greens and minimizes the risk of losing the entire crop to a sudden heat wave.
Due to the high risk of bolting, summer lettuce is best harvested using the “cut-and-come-again” method. Individual outer leaves are picked rather than waiting for a full head to form. This practice encourages the plant to continue producing new foliage, keeping the leaves tender and flavorful. Sowings made in late July and early August are important, as these plants will mature as the weather naturally cools, providing a substantial harvest for the fall season.