Gearing up to Grow: Head lettuce

Authors: Natalie Hoidal, Marissa Schuh, Annalisa Hultberg Extension Educators, University of Minnesota Extension.

We’re kicking off our crop deep dive series again this year, with deep-dive articles on a variety of crops to help you prepare for the growing season. We’re starting the year with head lettuce and Brassicas, two sets of early season crops to jump start the growing season.

Photo: UMN Extension

Favorite varieties for different times of year

We love head lettuce. It’s cold hardy and quick to grow, but harvest is so much quicker and easier than baby leaf lettuce or other leafy greens like spinach. There’s been a lot of breeding focus on head lettuce over the last few years, with a special focus on mini head lettuce. Here are a few categories to consider (This is not an exhaustive list of every type of lettuce, but these are the basics):

• Iceberg, Romaine, and butterhead are the most common old-school lettuce varieties. They produce full-size heads with different characteristics. Iceberg types may be the least appealing to your customers, but they have the longest storage life of the three types (up to 21 days with proper storage). Butterhead lettuce has the softest leaves and may be the most appealing, but has the shortest shelf life. Romaine is a happy medium for many growers.
• Salanova lettuce was developed by Dutch plant breeding company Rijk Zwaan, and released in 2005. It has since become a staple on vegetable farms. What makes it unique is that it has many small, tender leaves compared to fewer larger leaves on other more standard varieties. This allows growers to harvest it like a head lettuce, but then chop the base off, allowing the leaves to separate. The resulting product is soft, tender leaves that resemble baby leaf lettuce.
• Batavia and summer crisp lettuce: Batavia and summer crisp type lettuce are bred for warm summer temperatures and slow bolting, making them a good fit for summer successions (they also tend to handle cold weather fairly well).
• Mini lettuce: mini lettuce types have become more popular in recent years, In part, they are harvested earlier, making them a good fit for early spring or late fall successions. Restaurants often like to buy mini lettuce heads, and they can be less intimidating to customers than giant heads of lettuce. There are mini versions of most lettuce types including mini icebergs, mini romaines, and mini butterheads.

Characteristics to look for

• Early spring plantings: earliness (or shorter days to maturity)
• Summer plantings: heat tolerance, bolting resistance, tipburn resistance
• Fall plantings: cold tolerance, downy mildew resistance, bottom rot resistance, earliness (or shorter days to maturity)

Succession planting

If your goal is to harvest lettuce all season long, you should be planting a new succession every 10 days or so. As a general rule of thumb, you should be choosing a new variety every 3-4 weeks to fit the conditions of the season in which it will be growing (e.g. cold to hot in spring, hot to hot in summer, hot to cold in fall).

Johnny’s Selected Seeds maintains a nice chart showing which of their varieties are best suited to different times of the year, review it here.

How early can you plant?

The conventional wisdom around planting lettuce is that you can plant as soon as the soil is workable (i.e. not frozen, and not too wet). The ideal temperature for lettuce is around 60 - 65 degrees F. A pretty reliable planting window for high tunnels is early April, and for fields, around mid to late April. Like all crops, head lettuce is susceptible to damping off pathogens when the soil is cold, so it’s best to start seeds indoors about 3-4 weeks before transplanting, even in a high tunnel. This gives you a jump-start on the season, and healthier plants overall. 

Pests

Most of the pest issues we see in lettuce are linked to moisture. Moisture plays a key part of diseases like damping off and bottom rot, and drives slug populations.

Damping off & bottom rot

Damping off is caused by many pathogens that are present in soils across the state. Prevention is our main strategy for dealing with it.

• Keep greenhouse equipment, tools, surfaces, and tray clean.
• Monitor irrigation to avoid over-watering transplants.
• Transplant or direct seed when soils are warm. This will speed growth, allowing for speedy germination and establishment of plants, getting them out of the life stage where damping off pathogens can kill the plant.

One of the pathogens responsible for damping off Rhizoctonia solani, can also cause issues for older lettuce plants, bottom rot. Bottom rot starts on the oldest leaves, causing rusty brown patches on the oldest leaves, often around the time the plants reach the heading stage. It will then move up the leaves and midribs, and finally the stems as the entire plant collapse.

Many of the same preventative measures work on plants of all ages. If struggling with bottom rot in older plants…

• Look for varieties that grow more vertically, which will make that hop from soil to leaf harder for the pathogen.
• Plant on beds to help deal with moisture.
• Weed to help with airflow and to reduce other plants where the pathogen can multiply.
• There are conventional products that allow for greenhouse (thus high tunnel) use, but nothing with an OMRI label. See the Midwest Vegetable Production Guide for more info.

White mold

Another lettuce disease we see, especially in high tunnel lettuce, is white mold. Like R. solani above, white mold is one of the few plant pathogens whose host range crosses vegetable families, as it can infect beans, cabbage, tomato, and 300 other plant species. This makes it possible to build up in areas with limited rotation (AKA high tunnels).

Once this disease is spotted in your tunnel, it will be there for a long time. White mold produces a specialized resting structure, which waits in the soil until conditions are right and a host plant is present (kind of like a weed seed in some ways). We can see these resting structures on infected plants, they look like mouse droppings.

Lettuce with white mold will eventually wilt entirely. Photo: Ben Phillips, Michigan State University, Bugwood.org

• As this one hangs around so long, our goal is to prevent it from getting established, primarily through rotation and cleaning of tools/machinery.
• We can also try to make high tunnel conditions less conducive – this disease is likes relatively cool temps (59-70𝆩F). Spores are produced when things are moist (16+ hours of wetness) and/or the relative humidity is high (90% or more).
• There are some preventive fungicides available that can slow down infections (though many of these are conventional, and make sure to check if the label allows for greenhouse use). Contans is a biological product with mixed efficacy, read the label carefully and pay close attention to soil moisture if using this product.
• When infected plants are found, remove the disease plant (carefully, to not throw dirt/spores around), and remove the soil 4-6 inches around the plant base where the resting structures may have fallen. Clean out the tunnel thoroughly at the end of the season.
• In cases where the disease is entrenched in a high tunnel, more intense measures, such as fumigation or anaerobic soil disinfestation may be considered.

Slugs

Slugs like moist, dark environments with lots of decaying stuff. Modify your growing space to reduce the presence of these types of spaces. Removing hiding places, like stones, weedy patches, and dense mulches. Using raised bed can help with overall soil moisture, which makes a space less attractive to slugs.

There are effective slug baits with the active ingredient of iron phosphate, Sluggo is a common brand (which is also OMRI listed). These products can be used preventatively around the perimeter of an area with a history of slug issues, or after slugs show up by placing it in bands between the rows. Apply the product in the evening after it rains or you irrigate.

Postharvest handling, washing and shelf life

Depending on how the greens were grown, they may need a light rinse to remove sand or soil. Use a clean sink or stock tank filled with potable water to submerge the greens. Agitate gently to remove the visible soil. Consider using a food-grade sanitizer in the water to clean and water and prevent cross contamination between the leaves in the water. After rinsing the greens, allow them to dry slightly on a clean spray table or with a large commercial salad spinner.

Maintaining some water on the leaves during storage is actually beneficial, as this will allow this humidity to keep the leaves fresh for longer. Store the leaves in a clean plastic bag that is loosely closed to retain moisture.

Storage Temperatures

According to the USDA, lettuces do best when maintained in a refrigeration temperature from 32-36 degrees. They also stay freshest when there is high humidity, and when they are stored away from high ethylene producing crops such as apples and tomatoes. It is very important that you get the greens as cold as possible after harvest so that they last longer for your customers.

Salad mix is rinsed gently in a clean tank with potable water
Photo: Joan Olson, Prairie Drifter Farm