Romaine Lettuce in Zone 7B — Desert Southwest

Romaine lettuce is a fantastic addition to your Desert Southwest garden. The crisp, refreshing leaves stand up well in salads and sandwiches, and you'll find the taste far surpasses anything you can buy at the store. Plus, harvesting your own lettuce just steps from your kitchen is incredibly satisfying.

Growing romaine here does come with some challenges, namely our extreme heat and intense sun. But with careful timing, utilizing our long 245-day growing season, and a few smart techniques, you can enjoy homegrown Romaine in both spring and fall.

Starting Romaine lettuce indoors is an option, especially for a very early spring crop. Begin your seeds late January through mid February, about four weeks before you plan to transplant. Use seed trays or small pots filled with a good seed-starting mix, and keep them consistently moist with bottom watering to avoid disturbing the tiny seeds.

Provide warmth and bright light, either from a sunny window or a grow light. Since spring arrives early in the Desert Southwest, starting indoors gives you a head start, but direct sowing often works just as well.

Transplant your Romaine seedlings outdoors early to late March, once they have a few sets of true leaves. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sun and wind.

Space the plants 8-10 inches apart in well-draining soil. Be mindful of late season temperature swings— a sudden cold snap can stress young transplants, so be prepared to cover them if necessary.

Direct sowing Romaine lettuce is easy and often very successful here in the Desert Southwest. Sow seeds mid February through late August, aiming for the cooler parts of that window. Prepare the soil by loosening it and mixing in some compost.

Plant seeds about ¼ inch deep, spacing them 8-10 inches apart. Keep the soil consistently moist until the seeds germinate, which usually takes about a week. Thin seedlings as needed to ensure proper spacing.

Consistent watering is key to growing crisp, delicious Romaine lettuce in our arid climate. During the spring and fall growing seasons, aim to water deeply about 2-3 times per week, providing about 1 inch of water each time. Use the "finger test"—stick your finger about 2 inches into the soil; if it feels dry, it's time to water.

In the extreme heat of summer, you might need to water daily, especially for plants in containers. Focus on watering at the base of the plants to avoid wetting the leaves, which can lead to fungal issues in our monsoon season. Wilting leaves are a sign of underwatering, while yellowing leaves can indicate overwatering.

A layer of mulch around your plants helps to retain moisture and keep the soil cool, which is especially important during our hot summers. Organic mulches like straw or wood chips work well.

You can expect your first Romaine lettuce harvest late April through late November, about 65 days after planting. Look for outer leaves that are 6-8 inches long, or harvest the entire head when it feels firm and full-sized. Cut the leaves or the entire head at the base, leaving the roots intact if you want to try for a second flush of growth.

To encourage continued production, harvest outer leaves regularly, allowing the inner leaves to continue growing. Avoid letting the center stem elongate, as this indicates the plant is bolting and the leaves will become bitter.

As the first frost approaches in mid- November, harvest any remaining heads to enjoy before the cold damages them. You can also cover plants to extend the harvest a little longer.

Here are some common problems you might encounter growing Romaine lettuce in the Desert Southwest:

Bolting in Heat

• What it looks like: A tall flower stalk emerges from the center of the lettuce head, and the leaves become bitter and tough.
• What causes it: Extended periods of warm temperatures, especially when days consistently exceed 80°F, trigger bolting.
• How to fix/prevent it: Time your plantings for cooler weather in spring and fall. Choose bolt-resistant varieties. Provide shade with shade cloth during warm spells. Mulch to keep the soil cool. Harvest lettuce promptly when it reaches maturity, before bolting begins.

Aphids

• What it looks like: Clusters of tiny green, black, or white insects on stems and leaf undersides. Sticky honeydew residue and curled or distorted new growth.
• What causes it: Rapid reproduction in warm weather, and ant colonies that farm them for honeydew.
• How to fix/prevent it: Knock aphids off with a strong spray of water. Encourage natural predators like ladybugs. Use insecticidal soap for heavy infestations. Avoid over-fertilizing with nitrogen.

Slugs

• What it looks like: Irregular holes in leaves, especially on low-growing plants. Silvery slime trails on leaves and soil. Damage is worse at night.
• What causes it: Slugs thrive in moist conditions and feed at night.
• How to fix/prevent it: Set out beer traps (shallow dishes of beer sunk into the soil). Use iron phosphate bait. Remove hiding spots like mulch and debris. Water in the morning so the soil surface dries out by evening.

Tip Burn

• What it looks like: Brown, dried edges on inner lettuce leaves, sometimes spreading to outer leaves.
• What causes it: Calcium uptake problems due to inconsistent watering and high temperatures.
• How to fix/prevent it: Water consistently to maintain even soil moisture. Avoid fertilizers high in ammonium nitrogen. Ensure good soil calcium levels. Provide shade during the hottest parts of the day.

Desert Southwest Specific Challenges: The extreme heat and intense sun can quickly cause bolting and sunscald. Our arid climate demands consistent watering to prevent tip burn. Even monsoon moisture can create favorable conditions for slugs, so vigilance is key.

Companion planting can help your Romaine lettuce thrive in the Desert Southwest. Carrots are good companions because they deter lettuce root aphids. Radishes break up the soil, making it easier for lettuce roots to grow. Strawberries provide ground cover and help retain moisture. Chives and onions deter many pests with their strong scent.

Avoid planting Romaine lettuce near celery and parsley. Celery attracts the same pests as lettuce, increasing the risk of infestation. Parsley can stunt the growth of lettuce.