Growing lettuce in containers is easy and fun, and you can be harvesting a beautiful, nutritious salad in weeks. There are 6 cultivar groups of lettuce, but the best types for containers are loose leaf and romaine types. These are also the most nutritious types of lettuce, and the easiest to grow. Both come in a variety of colors and textures.
For areas that start freezing in October, choose cold tolerant varieties. You can greatly increase your lettuce growing season by moving your lettuce containers inside during freezing weather and keeping them outside when it is warm. Container soil gets cold faster than ground soil, so it does not provide as much protection against freezing temperatures.
How to Grow Lettuce in Containers: Cultural Considerations
Lettuce thrives in the cool temperatures of spring and fall, but there are a few varieties that do well in the heat of summer. Lettuce seeds do not germinate well when the soil is too warm, generally over 80°F. Seed germinates best when the soil is at 75°F, and plants grow best at 60-65°F.
Lettuce likes nutrient rich, moist soil, so choose an organic soilless potting mix and amend it with compost, and greensand and/or Azomite®. Azomite® is a clay product that is harvested in the Southwest United States that was formed when volcanoes overflowed into the seabed millions of years ago. Unlike many clays, it has the best properties of both lava and the sea, and has over 70 minerals and trace minerals.
If you will be growing lettuce in containers outside, choose a light-colored container that is at least 6 inches deep, but preferably 12 inches deep. Dark colored containers absorb heat, but lettuce prefers cool roots. If this is not possible, keep just the container shaded from the sun by placing it behind other plants or objects. Make sure it has adequate drainage holes, and cover the holes with a coffee filters to allow drainage while retaining soil.
Fill the container to within 1 inch of the top to allow space for watering and then thoroughly moisten the soil. It might be helpful to mix the tiny lettuce seeds with sand and sprinkle the mixture out of a salt shaker to evenly distribute seeds. Top with ¼ inch potting soil and gently water in seeds.
When seeds sprout, fertilize with a fish emulsion or sea kelp fertilizer diluted to half the strength recommended on the bottle. Feed every two weeks at full strength. If you transplant, feed lettuce plants at planting time. Thin plants when necessary by cutting off the weakest plants at the base with scissors. Thinning allows adequate air circulation, plus you can eat the little leafs.
Depending on how much lettuce you want and how you plan to harvest, you can plant a new crop every one to three weeks. You can use lettuce as micro-greens, in which case you can sow heavily once per week. If you plan on harvesting outer leaves for miniature salad mixes, a.k.a. spring greens, sow seeds every three weeks. Start harvesting outer leaves when they get 3 or 4 inches long and leave at least four of the inner leaves to grow. Or you can harvested any stage by cutting off the entire plant at the base. Plant once a week to keep a steady supply of plants that are harvested all at once.
How to Grow Lettuce in Containers Outside
You can either start seeds inside to transplant into containers, or start them by sowing directly into the containers. For transplants started inside, harden them off for a week by putting them out to expose them to full sunlight for longer durations of time and then bringing them back inside for nights.
Especially for during the heat of summer, choose heat tolerant, slow bolting varieties of lettuce. When lettuce bolts, it is forming a flower head, and it gets tough and bitter. Looseleaf types are the most heat tolerant, as a rule.
Good Varieties of Lettuce to Grow in Containers Outside
In spring and fall, almost any type of looseleaf or romaine lettuce can be grown in containers. During summer, it is best to go with more heat tolerant varieties. Choose a variety of colors, textures, and flavors to make a beautiful, nutritious salad. Romaine lettuce is the type to grow for making Caesar salads.
Marvel of Four Seasons or Merveille Quartre Lettuce is a French heirloom lettuce dating back to at least 1885. It forms bibb-type rosettes with red tips. It has a good flavor and crisp texture and withstands heat. (1)
Cimmaron Lettuce is an heirloom lettuce variety from the 18th century that matures in 68 days. The romaine-style leaves are a beautiful reddish-green and loosely folded. This variety is one of the most bolt resistant varieties on the market, being tolerant of both heat and cold. Its 10 to 12 inch savoyed leaves are flavorful for salads or a beautiful garnish. (3)
Red Velvet is a striking, maroon looseleaf variety that is slow to bolt. (1)
Grand Rapids Lettuce is an old favorite loose-leaf lettuce that matures in 45 days. The crisp, tender leaves are light green and frilly. It is extremely resist bolting and tip burn, so is a good choice for hotter areas. (3)
Great Lakes heirloom lettuce was an AAS winner in 1944, and one of the first, if not the first, iceberg-type lettuces to be introduced. It grows into medium-sized heads and matures in 90 days. Great Lakes lettuce is very crisp and flavorful in salads and sandwiches, This variety bolts easily, so start it inside early in spring, or 10 weeks before the first fall frost for a fall crop. In zone 8 or warmer, it is a good winter lettuce. In containers, give it plenty of room to mature, preferably out of hot sunlight. (3)
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