As a cool-weather crop, lettuce (Lactuca sativa) offers one of the earliest garden harvests in the form of crisp, mild-flavored leaves. Available in head and loose-leaf varieties, this garden staple is grown as an annual plant throughout the United States. A common problem with lettuce is that its leaves get bitter, but knowing when to prepare for the transition from edible to inedible leaves helps gardeners maximize harvests without losing an entire crop to unsavory lettuce.
Causes of Bitter Lettuce
Lettuce gets bitter because it is bolting, which means the plant produces a stalk that blooms and forms seeds. The two reasons lettuce bolts are:
- Heat -- Lettuce thrives when average daytime temperatures are between 60 and 70 degrees Fahrenheit and tends to bolt at higher temperatures. When the plant begins to take a conical shape, rather than its typical round form, it is about to bolt. Harvest the entire head of lettuce, orthe entire plant of a loose-leaf lettuce variety. Once the head has produced a stalk, it is too late to harvest lettuce because it is already tough and bitter.* Drought -- A lack of water can cause lettuce to become stressed, signaling it to produce seeds by bolting. Keep the soil evenly moist to avoid drought stress in lettuce plants.
Prevention of Bitter Lettuce
Get harvests from lettuce before it bolts by starting lettuce seeds indoors under grow lights eight to 10 weeks before your location's last average annual frost date. Transplant the resulting seedlings outside about one month before that last frost date. Starting seeds indoors, rather than sowing them directly in the garden, allows for earlier harvests.
As temperatures reach 70 degrees Fahrenheit, start checking for signs of bolting in the lettuce, and taste its leaves frequently to test for a bitter flavor. If the lettuce starts to get bitter, harvest it immediately, and put it in a refrigerator, which may help reduce the bitterness.
Keep lettuce plants' soil consistently moist by watering any time the surface of the soil is dry. Lettuce has shallow roots, so all of its moisture comes from the top few inches of soil where the roots reside. Mulching around lettuce plants to conserve soil moisture is an option, but mulch attract slugs and other pests. So it is better to avoid using mulch except in extremely dry areas. Don't let the mulch touch the plants.
When shopping for lettuce seeds, select bolt-resistant varieties. Although they will still bolt in extreme heat, they do not bolt as quickly as other options, offering more time for harvests. Bolt-resistant varieties include 'Slobolt' (Lactuca sativa 'Slobolt'), 'Gentilina' (Lactuca sativa 'Gentilina') and 'Oak Leaf' (Lactuca sativa 'Oak Leaf').