Romaine lettuce, also known as cos lettuce, is a popular garden crop valued for its crisp texture and robust flavor. Successfully cultivating this leafy green requires balancing its need for sunlight with its sensitivity to heat. Understanding the interplay between light, temperature, and moisture is essential for producing sweet, tender heads. This article clarifies the light requirements for Romaine and details the conditions needed for a successful harvest.
Optimal Light and Temperature Conditions
Romaine lettuce requires a minimum of six hours of sunlight daily for healthy growth. While some resources suggest “full sun,” this must be qualified by temperature, as Romaine is a cool-season crop. The ideal air temperature range for growth is between 60°F and 70°F. Growth slows dramatically once temperatures consistently rise above 75°F, and high temperatures intensify the negative effects of direct sunlight.
In cooler spring and fall weather, a location receiving six to eight hours of direct sun is appropriate. However, in regions with hot summers, true “full sun” can be detrimental. The best approach is to provide morning sun, which is less intense, and then offer shade during the hottest afternoon hours. This protects the plant from heat while ensuring it receives the necessary light energy.
Preventing Bolting and Bitterness
When Romaine lettuce is exposed to prolonged periods of high heat or intense light, it shifts into a survival mode known as bolting. Bolting is the rapid elongation of the central stem as the plant diverts energy from leaf production to forming a flower stalk and seeds. This reproductive response marks the end of the edible harvest, as the leaves become tough and develop a bitter flavor.
The bitterness is caused by an increased production of sesquiterpene lactones, a milky white sap known as lactucarium, which concentrates in the leaves and stem. To mitigate this, growers should time planting to avoid the peak of summer heat, typically planting in early spring and late summer for a fall harvest. Providing afternoon shade with taller companion plants or using a shade cloth reduces the ambient temperature and delays bolting. Protecting the plants from heat stress extends the harvest window and maintains the sweet flavor.
Essential Soil and Water Requirements
Consistent moisture works alongside light and temperature to prevent stress-induced bolting and bitterness. Romaine lettuce has a shallow root system, meaning it cannot access deep water sources and is highly susceptible to drying out. Water stress, even independent of high heat, can trigger the plant’s survival mechanism and lead to bitterness.
The lettuce thrives in well-draining, loamy soil that is rich in organic matter and has a slightly acidic pH, ideally between 6.0 and 6.7. Water should be applied consistently and deeply enough to moisten the top few inches of soil, aiming for one to two inches of water per week. Applying a layer of organic mulch around the base of the plants helps regulate soil temperature, conserves moisture, and prevents weeds from competing for water.