Lettuce is a cool-season crop that thrives in mild temperatures and is one of the first vegetables gardeners can plant each spring. Growing lettuce in Colorado presents a unique timing challenge due to the state’s highly variable climate and intense seasonal shifts. Successfully harvesting crisp, sweet lettuce requires careful planning to avoid late spring freezes and the stress of early summer heat. This guide provides a practical schedule for when to plant lettuce across Colorado’s diverse growing regions.
Understanding Colorado’s Climate Challenges for Lettuce
Colorado’s mountainous terrain and high elevation create distinct microclimates, making timing the lettuce planting season challenging. The two primary hurdles are late spring frost events and the sudden onset of summer heat. Lettuce is frost-tolerant and can survive temperatures down to 32°F, but a hard freeze can damage or kill young seedlings.
The intense Colorado sun and high summer temperatures cause premature bolting, where the plant shifts energy from leaf production to forming a seed stalk. Bolting is triggered when temperatures consistently rise above 75°F, causing the leaves to become bitter and tough. In areas like the Front Range, the window between the last frost and the start of high heat can be quite narrow, requiring careful timing for maturity.
The Spring Planting Schedule
The first step for an early harvest is determining the average last frost date for your specific location, which serves as the benchmark for all cool-season plantings. To get a head start, gardeners should begin sowing seeds indoors approximately four to six weeks before this date. Starting seeds indoors ensures strong, established seedlings ready for the garden.
Lettuce seeds can be direct-sown outdoors as soon as the soil is workable, typically in mid-March to early April in lower-elevation zones. Germination is best when the soil temperature is between 40°F and 75°F. Planting in soil that is too cold will slow germination, while temperatures above 80°F can prevent it entirely.
Indoor-started transplants must be hardened off before moving to the garden permanently. This process involves gradually exposing the seedlings to outdoor conditions—such as wind, direct sun, and cooler temperatures—over five to seven days. Transplants should be moved out near the average last frost date, but using a cloche or a row cover can provide protection from cold snaps, allowing planting up to two to four weeks earlier.
Extending the Harvest Through Summer and Fall
To ensure a continuous supply of fresh lettuce, gardeners should employ succession planting rather than relying on a single large harvest. This technique involves sowing a small batch of new seeds every two to three weeks starting from the initial spring planting. This staggered schedule ensures a younger set of plants is ready as the older ones mature and near bolting.
The intense heat of mid-summer often forces a break in lettuce production, especially in lower-elevation areas. During this period, planting bolt-resistant varieties, such as ‘Jericho’ or ‘Nevada,’ is advisable. Providing afternoon shade with a shade cloth or planting in the shadow of taller crops helps keep temperatures cooler, delaying bolting. Applying organic mulch, like straw, helps maintain consistent soil moisture and keeps the root zone cool.
The second primary planting window opens in late summer for a fall harvest, often considered the most successful time due to naturally cooling temperatures. Fall planting should be timed so that the lettuce reaches maturity before the first hard freeze of the season. A general guideline is to sow the seeds six to eight weeks before the average first expected frost date in your region.
For example, if the first frost is typically in early October, planting should occur from late August to early September. Because daylight hours decrease in the fall, lettuce matures slightly slower than in the spring, making shorter-maturing varieties beneficial. Covering the plants with a cold frame or row cover as temperatures drop can extend the harvest into late autumn and early winter.