Since lettuce and leafy greens have short production cycles, greenhouse growers need to stay focused if they want to be successful growing these crops year round.
The increasing demand for locally-grown vegetables is causing more field vegetable growers, ornamental plant growers and new growers to look at trying to satisfy this market. Cornell University horticulture professor Neil Mattson said he works with all three types of growers.
“I see both vegetable field growers and ornamental greenhouse growers trying to produce lettuce and leafy greens year round,” he said. “Both are quite common. Field vegetable growers are looking for a crop that can generate year-round cash flow. Ornamental growers are looking to fill their greenhouses in the off-season. A lot of ornamental growers no longer produce poinsettias in the fall or spring bulb crops and spring plant propagation that they would normally do in the winter. Growers could have as much as a six-month window when their facilities are not being used.”
Mattson said ornamental growers tend to better understand what it takes to grow a year-round crop.
“Ornamental growers tend to be aware of differences in crops and the problems that can arise,” he said. “They also understand the concept that there is much less light in the winter so they may have to consider using supplemental light. Ornamental growers are usually aware of the high cost of heating a greenhouse year round, especially during the winter.
“In general, field vegetable growers who have greenhouses, may only be using those structures in the spring to produce their transplants. That means they may be used to heating a couple months each year.”
Controlling environmental parameters
During this year’s Cultivate’16 conference and trade show in Columbus, Ohio, Mattson did a presentation on the year-round production of leafy greens using controlled environment agriculture (CEA). The main environmental conditions growers need to monitor and control include light, temperature and relative humidity.
Mattson said in the northern half of the United States light availability during the winter months is the most difficult environmental issue to deal with when growing plants year round.
“The target light level for lettuce production is 17 moles of light per square meter per day (daily light integral, mol/m2/d) for optimal growth,” he said. “In most parts of the country achieving that target usually isn’t an issue during the summer. In the winter, in the northern part of the country, light levels can be 5 mol/m2/d on average and even 1-3 mol/m2/d is common. That is three to five times less light than what is needed for optimum lettuce growth during the winter.”
Mattson said supplemental light, using LEDs or high pressure sodium lamps (See the Lamps Needed Calculator, can be provided to deliver higher light levels to increase lettuce biomass. But going above 17 mol/m2/d can cause growers to have issues with tip burn.
“In the case of head lettuce, a grower can go from seed to harvested head (5-6 ounces) in 35 days if there is 17 mol/m2/d. If there is only 8.5 mol/m2/d, it takes the plants twice as long to produce that same biomass.
“For baby leaf greens, if they are seeded and transplanted and grown on for two weeks before harvesting and only receive 8.5 moles of light, the plants will only produce half the yield. If a grower normally harvests 10 ounces per square foot and there is only half the light, the plants will produce only 5 ounces per square foot.”
Mattson said a rule of thumb for New York is a grower can light a 1-acre greenhouse and produce the same yields as not lighting a 3-acre greenhouse to produce the same yields during the winter months.
Lettuce and many other leafy greens are cold tolerant. Mattson said a lot of growers want to grow them cold.
“The Cornell CEA research group proposes that these crops be grown at fairly warm temperatures so that they have the faster 35-day crop cycle,” he said. “This is based on having 17 mol/m2/d and a daily average temperature of 70ºF-75ºF during the day and 65ºF at night.
“During the winter the issue with temperature is paying to heat the greenhouse. It’s easy to control the temperature, growers just have to be willing to crank up the thermostat.”
Mattson said temperature can be really hard to control in southern climates in the U.S.
“Under hot conditions, temperatures in the 80s and 90s, head lettuce is going to bolt prematurely,” he said. “It can be difficult to drop the day temperature low enough to avoid early bolting.”
Beyond trying to reduce the air temperature, research done by the Cornell CEA group has shown that lowering the root zone temperature to 74ºF or less using chilled water can help prevent premature bolting.
“Chilling the root zone temperature allows growers to grow using warmer air temperatures by having the cooler water temperature,” Mattson said. “This is an effective way to chill the plants even when the air temperatures reach the 80s and 90s.”
Mattson said the biggest issue with warmer air temperatures occurs with lettuce and spinach. Warmer temperature and long day conditions can both cause spinach to experience premature bolting. Mattson said warmer temperatures can also promote Pythium root rot on spinach. Spinach is much more susceptible to this water mold than lettuce or other leafy greens.
“Chilling the root zone temperature can help to prevent the disease organism from developing as quickly as at warmer temperatures,” Mattson said. “If the root zone temperature can be kept cool, it won’t completely avoid the Pythium issue, but it will help control it.”
The Cornell CEA group found that the optimum root zone temperature for spinach was 64ºF-65ºF. At this temperature Pythium was deterred, but there was no crop delay. If the root zone temperature is lowered to 62ºF, plant growth is slowed.
Mattson said the relative humidity for lettuce and leafy greens should be between 50-70 percent. He said the lower humidity helps to limit pathogen issues.
“High humidity favors powdery mildew and Botrytis,” he said. “High humidity also favors the physiological disorder tip burn. Tip burn is caused by a calcium deficiency. The higher the relative humidity the less transpiration occurs in the plant resulting in the plant not taking up an adequate amount of calcium.”
Mattson said a relative humidity lower than 50 percent can cause an outer leaf edge burn, which is a physiological disorder.
“This is a different disorder than tip burn caused by calcium deficiency,” Mattson said. “The outer leaves develop lesions where the veins end on the edge of the leaves. The lesions occur where the sap exudes out of the veins and then is reabsorbed by the plant and there is a kind of salt buildup.”
Fertilization, water quality
Although fertilization and water quality are not environmental parameters, growers can have issues with both if they don’t monitor them. Lettuce and leafy greens are not particularly heavy feeders compared to other greenhouse vegetables like tomatoes and other vine crops. Mattson said lettuce and leafy greens are relatively forgiving crops when it comes to fertilization.
“Growers need to monitor the nutrient solution every day in regards to pH and electrical conductivity (EC),” he said. “The reason for testing the nutrient solution at least daily is because in hydroponics the nutrient solution pH can change by one or two units in a day.”
Mattson said for container crops like petunia and geranium, pH does not usually change by more than one unit in a week. He said container growers may check the pH every week, and some may only do it every two weeks. But for hydroponics a grower needs to stay on top of changes in pH.
In addition to daily pH and EC monitoring, Mattson said a detailed elemental analysis of the nutrient solution is important.
“Periodically, about every four weeks, growers should send a sample of their nutrient solution to a testing lab to determine if the plants are absorbing nutrients in the proportions the growers expect,” he said. “Certain elements in the solution may decline over time and a grower may have to add more of these elements and less of others.”
Mattson said some systems, like CropKing’s Fertroller, have automated sensors which measure pH and EC in line so it’s a real time measurement. The controller makes the necessary adjustments.
“Typically if a grower is putting high alkalinity water into the system, the pH tends to creep up over time, so the controller automatically adds acid to reach a target pH,” he said. “Likewise, the machine does that with EC too. If the EC is going down because the plants are taking up nutrients, the controller adds fertilizer stock solution to reach a target EC.”
Mattson said iron deficiency due to high pH is the most common nutrient disorder he sees on lettuce and leafy greens. Occasionally magnesium deficiency occurs because the water source contains enough calcium, but not enough magnesium.
“Many fertilizers don’t include calcium and magnesium, so growers can run into issues with magnesium deficiency,” he said. “Basil tends to have a high need for magnesium. We usually recommend basil be provided twice as much magnesium as lettuce.”
Mattson said growers should test their water more frequently to determine if there have been any changes in the alkalinity of the water, including calcium, magnesium and sodium concentrations.
“In the Northeast this summer, we are experiencing a drought,” he said. “I’ve heard from growers who say the EC of their water is going up, which implies that some salt levels are going up. But we don’t know specifically which salts and that would be useful to know what specifically is changing.
“It really depends on their water quality. In particular, EC, alkalinity and whether there are any nutrients in high concentrations, like sodium, can be an issue. It really comes down to how long they are trying to capture and reuse the water. In our Cornell system we have traditionally grown in floating ponds. We use that same water cycle after cycle for several years. We can continually use the same water because we start with deionized water. However, even if the water has fairly low salt levels, using the same water will result in the accumulation of sodium to harmful levels over time.”
Neil Mattson, Cornell University
School of Integrative Plant Science, Horticulture Section
49D Plant Science
Ithaca, NY 14853
Cornell Controlled Environment Agriculture “Hydroponic Lettuce Handbook”
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.