Crunchy, crisp, bitter, peppery or sweet, the flavors of lettuce are as wide-ranging as the varieties of lettuce. Lettuce is the conduit that serves up other vegetables and refreshes the palate in between bites.
From romaine to butterhead, Boston, Bibb or some combination of several different types, lettuces are the foundation of any salad. With the general public’s growing interest and attention on eating healthy, lettuce is a staple ingredient in many dishes served at restaurants and at home.
Lettuce is a rapidly growing crop that can store well when refrigerated after harvest, and high demand from consumers makes this crop popular among growers.
“There is a lot of competition, especially during the summer season when field production of lettuce is possible,” said A.J. Both, an associate professor/Extension specialist in Controlled Environmental Engineering Bioresource Engineering, the Department of Environmental Sciences at Rutgers University in New Jersey.
Regions like Yuma, Arizona, and California have significant advantages of growing leafy greens with a longer outdoor growing season. As long as fuel prices are relatively low, outdoor growers in California and Arizona will continue to have an advantage in the market.
Yet, there are still opportunities for greenhouse growers to integrate lettuce into their crop production. Growers in states like Texas, Kentucky and other states have found success raising lettuce crops in the greenhouse and marketing it to consumers. “Lettuce can be grown in an inexpensive system,” Both said. “The income generated by sales will largely determine how much money can be invested in the growing system.”
1. Getting started
Seed selection is the first step in getting started with any new crop. Whenever possible, choose varieties developed for greenhouse production. “There are special varieties that are suitable for greenhouse production. The seed supplier should be able to provide information about environmental settings that include temperature, light, humidity, etc.,” Both said.
Seed companies can also provide detailed information about each type of lettuce variety and help growers select one that is compatible with their goals and setup. Ask seed suppliers specific details about all attributes of the variety you select, including details about the harvest window. Both said to ask this question: “How long will it take the variety you chose to reach a marketable size?”
Once seed varieties have been chosen, germinating seedlings is the next step. “Growing lettuce seedlings is just like growing other seedlings,” Both said, noting that having experience growing other crops is helpful.
According to the University of Kentucky Cooperative Extension Service publication “Hydroponic Lettuce,” authored by Cheryl Kaiser and Matt Ernst, “Seeds can be placed into a soilless mix such as peat and perlite in seeding or plug trays. An inert medium such as rockwool or oasis cubes can also be used to produce transplants.”
During the germination process seeds should be misted with overhead irrigation or watered with subirrigation provided on an ebb and flow bench.
Throughout the two to three weeks it takes for seeds to germinate, it’s critical to maintain an environment with high humidity and enough light and heat to encourage quick, healthy growth of seedlings that can be transplanted. Depending on the season seeds are started in, shading may need to be added to the greenhouse to prevent the internal temperature from getting too hot, which can reduce germination rates in some lettuce varieties.
2. Growing medium
Greenhouse growers new to lettuce can choose between three different general types of production systems. Both noted that lettuce can also be grown with soil, in pots using a variety of growing media, in an aeroponic system or hydroponic system.
An aeroponic system grows plants in an air or mist environment without the use of soil or an aggregate medium.
Most commonly, greenhouse lettuce is grown in a hydroponic system, which is the practice of growing plants without soil. A mineral and nutrient mixture feeds the plants. There are several types of hydroponic systems to choose from, but the nutrient film technique (NFT) or the floating raft system, both which are closed systems, are most common.
“A closed system is one in which the surplus nutrient solution is recovered after use and then recycled through the system. This requires monitoring and adjusting the solution so that depleted nutrients can be replenished and the solution sterilized prior to circulating through the system again. In contrast, the nutrient solution in an open system is not recovered and recycled,” according to “Hydroponic Lettuce.”
Hydroponics requires more planning, monitoring and labor. “Unless the lettuce is growing in a highly automated system, the amount of labor can be a concern,” Both cautioned.
But despite the added labor, the process offers benefits. “Hydroponics allow for a high degree of control, including in the root zone,” Both said.
Read more: Year-Round Greenhouse Growing
“Any challenge that can be expected when growing other crops in a greenhouse can cause a problem for lettuce,” Both said, “although when growing lettuce in a hydroponic system, weeds are not typically an issue.”
Sanitation is key to raising a disease- and pest-free crop that will return a premium price. Similar to any other greenhouse crops, aphids, thrips and mites can create the most pest pressure in greenhouse lettuce crops. Daily inspection for signs of pests is crucial to controlling their spread. Few pesticides are labeled for use in greenhouse production, making prevention the vital key to control.
In a hydroponic system, water mold is the most problematic issue. Pythium, phytophthora or other water-borne pathogens can quickly spread and contaminate all of the plants in the system. The “Hydroponic Lettuce” publication noted, “Since there are no fungicides registered for controlling these pathogens on greenhouse lettuce, this type of disease outbreak can result in complete loss of the crop.” (https://www.uky.edu/Ag/CCD/introsheets/hydrolettuce.pdf).
Once an entire crop has been impacted by mold, the tanks holding the nutrient solution must be drained and all equipment disinfected prior to establishing another crop. Greenhouse lettuce is also susceptible to other diseases including Botrytis gray mold, powdery mildew and downy mildew.
Read more: Staying Indoors
Planning for sales begins long before the crop is ready for harvest. “Without a market, or customers, the production will not be profitable,” Both said.
Talk with local restaurants and chefs, small grocers and other markets that prefer local, fresh, year-round availability to lettuce.
While it’s likely that the cost of production for greenhouse lettuce will be higher than traditional field-grown lettuce, a high-quality crop has the potential to draw a premium price in certain markets. Niche markets that pay higher prices do exist, but they take time to develop. If you have the commitment and ability to produce superior quality, off-season and year-round availability, you will have an edge over other producers.
To review the “Hydroponic Lettuce” document referenced in this article, visit https://www.uky.edu/Ag/CCD/introsheets/hydrolettuce.pdf.