Romaine lettuce is a popular cool-weather crop known for its crisp texture and tall, upright heads. Successful cultivation relies on precise timing, as the plant thrives in mild temperatures but is highly sensitive to heat. Planting at the wrong time can lead to poor germination, stunted growth, or a bitter taste. Understanding the specific temperature requirements and seasonal windows ensures a bountiful harvest of tender, sweet leaves.
Optimal Timing for Seasonal Planting
The growing season for romaine lettuce is divided into two primary windows: spring and fall, both defined by the local frost dates. In the spring, the goal is to get the plants established early enough to mature before the summer heat arrives. Seeds can be direct-sown into the garden soil as soon as it is workable, typically two to four weeks before the average last expected spring frost date.
Lettuce seeds have the ability to germinate in soil temperatures as low as 40°F, although the process is slow at that extreme. For faster, more reliable germination, the soil temperature should ideally be between 60°F and 68°F. Once the plants emerge, they will grow best when the ambient air temperature consistently ranges from 60°F to 70°F. Young, properly established plants can tolerate a light frost, surviving temperatures as low as 20°F.
The fall planting window is often preferred by experienced gardeners because the plants mature in gradually cooling conditions, which can produce sweeter leaves. To calculate the fall planting time, count back six to eight weeks from the average date of the first expected fall frost in your area. This schedule allows the heads to fully develop during the mild days of late summer and early autumn. Starting the fall crop in late summer may require some temporary afternoon shade to protect the young seedlings from intense heat until the season shifts.
Starting Seeds Indoors vs. Direct Sowing
Deciding whether to start seeds indoors or sow them directly into the garden affects how early a harvest can be achieved. Starting seeds indoors offers a significant head start on the growing season. Sow seeds indoors approximately four weeks before the planned date of transplanting to the garden.
Transplanting seedlings outdoors can occur as early as three to four weeks before the last spring frost. While germinating indoors, the seeds and trays must be kept cool, as high temperatures above 75°F can cause the seeds to become dormant and fail to sprout. Once seedlings have developed four to six true leaves and have been gradually acclimated to outdoor conditions, they are ready to be moved to the garden.
Direct sowing is a simpler method, particularly well-suited for the fall crop or quick mid-season batches. When the soil is cool and moist, direct sowing eliminates the need for transplanting and the process of hardening off seedlings. Seeds should be planted only about one-eighth to one-quarter of an inch deep, as lettuce seeds require light to successfully germinate.
Strategies for Continuous Harvesting
To ensure a steady supply of fresh romaine, the technique of succession planting is highly effective. Instead of planting the entire seed packet at once, sow a small batch of seeds every 10 to 14 days. This staggered approach ensures that plants continually reach maturity, providing a consistent supply of harvestable heads throughout the spring and fall seasons.
Succession planting also helps mitigate the risk of a plant defect known as bolting. Bolting occurs when the plant prematurely sends up a tall flower stalk to produce seeds, usually triggered by high temperatures above 75°F or 80°F. This process causes the leaves to become tough and develop a distinctly bitter flavor.
By planting in small, regular intervals, the gardener ensures that if one batch of plants bolts due to an unexpected heat wave, a younger, more temperature-tolerant batch is already in the ground. Stopping succession planting about one month before consistent summer temperatures begin prevents the entire crop from bolting. When the weather cools again in late summer, the process can be restarted for the fall season.