Romaine Lettuce in Zone 6A — Mountain West

Romaine lettuce is a winner in our Mountain gardens. Its crisp texture and mild flavor make it perfect for salads, wraps, and even grilling. Plus, the satisfaction of harvesting your own fresh greens when the growing season gets going is hard to beat.

Our high altitude, intense sun, and short growing season can present challenges, but with careful timing, you can absolutely grow beautiful Romaine. The 153-day growing season gives you plenty of opportunity to get a great harvest.

Starting Romaine indoors gives you a head start, but it's not always necessary in our climate. If you want to get the very earliest harvest possible, or if you're dealing with a particularly late spring, it can be a good option.

Start your seeds indoors from mid-March through early April, about four weeks before you intend to transplant them. Use seed trays with a good seed-starting mix, provide warmth (around 70°F), and ensure they get plenty of light, either from a grow light or a very sunny window. Bottom watering helps prevent damping off, which can be a problem in cooler spring conditions.

Transplanting Romaine seedlings outdoors should happen from mid-April through mid-May. Before you put them in the ground, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense sun and cooler nights common in our Mountain spring.

Space your Romaine plants 8-10 inches apart in well-drained soil. Keep an eye on the weather forecast – a late frost or unexpected hail can damage young transplants.

Direct sowing Romaine is a simple and effective way to grow lettuce in our region. You can start sowing seeds directly into your garden beds from early April through mid-July. This gives you succession planting options for a continuous harvest.

Make sure the soil is loose and well-prepared, and that the soil temperature is at least 40°F. Sow seeds about ¼ inch deep and space them 8-10 inches apart. You'll need to thin the seedlings once they emerge. Direct sowing avoids the risk of transplant shock, but the seeds are more vulnerable to pests and weather.

Romaine lettuce needs consistent moisture to produce crisp, tender leaves. Because of our dry climate and low humidity, watering correctly is crucial. Unlike more humid regions, we don't have much natural help.

In the spring and fall, when temperatures are cooler, water deeply about 1-2 times per week, providing about 1 inch of water each time. During the hotter summer months, especially when daytime temps consistently reach the 80s, you may need to water every other day or even daily. Use the "finger test" – if the top 2 inches of soil feel dry, it's time to water.

Water at the base of the plants to avoid wetting the leaves, which can encourage fungal diseases, even in our dry climate. Signs of underwatering include wilting and stunted growth. Overwatering can lead to root rot, so make sure your soil drains well. A layer of mulch, like straw or wood chips, can help retain moisture and keep the soil cool, which is essential during our brief but intense summer heat.

You can expect your first Romaine harvest starting in early June and continuing through early October. Romaine is ready to harvest when the outer leaves are 6-8 inches long, or when the whole head feels firm and full-sized.

To harvest, you can either cut off the outer leaves as needed, allowing the inner leaves to continue growing, or cut the entire head at the base. Be sure to harvest before the center stem starts to elongate, as this indicates bolting, which will make the leaves bitter.

To extend your harvest, plant new seeds every few weeks. As the first frost approaches in early October, consider covering your plants with a frost blanket to protect them from the cold and extend the harvest a bit longer.

Here are some common problems you might encounter growing Romaine lettuce in the Mountain West:

Bolting in Heat

• What it looks like: A tall flower stalk emerges from the center of the lettuce head. The leaves become tough and bitter.
• What causes it: Extended warm temperatures, especially when days consistently exceed 80°F. Our intense sun can exacerbate this.
• How to fix/prevent it: Time your plantings for cooler weather in spring and fall. Choose bolt-resistant varieties. Provide shade during the hottest part of the day with shade cloth. Mulch to keep the soil cool. Harvest promptly once it begins.

Aphids

• What it looks like: Clusters of tiny green, black, or white insects on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Rapidly reproducing sap-sucking insects. Populations boom in warm weather.
• How to fix/prevent it: A strong spray of water can knock them off. Encourage beneficial insects like ladybugs. Insecticidal soap can be effective. Neem oil is another option. Avoid over-fertilizing with nitrogen.

Slugs

• What it looks like: Irregular holes in leaves, especially on lower leaves. Silvery slime trails on leaves and soil. Damage is worse at night.
• What causes it: Slugs are gastropods that feed at night in moist conditions.
• How to fix/prevent it: Beer traps can be effective. Iron phosphate bait is a pet-safe option. Remove hiding spots like mulch and debris. Water in the morning so the soil surface dries out by evening. Diatomaceous earth can create a barrier.

Tip Burn

• What it looks like: Brown, dried edges on inner lettuce leaves. Can affect outer leaves in severe cases.
• What causes it: Calcium uptake problems caused by inconsistent watering, similar to blossom end rot in tomatoes. Worsened by 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. Choose resistant varieties. Provide shade in warm weather.

Mountain West Specific Challenges: Our moderate-at-altitude summer heat, combined with low humidity and dry rainfall, means consistent watering and shade during the hottest part of the day are essential for preventing bolting and tip burn. The intense UV radiation can also stress plants, so providing some protection is beneficial.

Companion planting can really boost your Romaine lettuce in our Mountain gardens. Carrots are great companions because they deter lettuce root aphids. Radishes are also beneficial, acting as a trap crop for flea beetles and other pests that might otherwise target your lettuce. Strawberries make good ground cover, helping to retain moisture and suppress weeds. Chives and onions deter aphids and other pests with their strong scent.

Avoid planting celery and parsley near your Romaine. Celery attracts whiteflies, which can also infest your lettuce. Parsley can compete for nutrients and attract swallowtail butterfly caterpillars, which might munch on your lettuce.