Originally published in Issue 9
Greenhouse growers looking to diversify into edible crops may want to consider strawberries, which can be adapted to production systems they are currently using for other crops.
Greenhouse growers looking to diversify their product mix with a fall to spring edible crop might want to consider strawberries.
“There is still a pretty big hole in the strawberry supply chain for November, December and January,” said University of Arizona research specialist Mark Kroggel. “In Arizona, we can produce good quality strawberries in greenhouses from October through April. The best greenhouse strawberry yields occur during March and April.
“Off-season greenhouse strawberry production is trying to accomplish two things: Fill a void in the local supply. And more importantly, produce a premium product. Greenhouse strawberries are going to be better tasting than the field-grown strawberries consumers find in grocery stores at this time of year. Consumers should be willing to pay a premium price for these highly flavored greenhouse berries.”
Use existing production systems
Kroggel said one of the advantages of growing greenhouse strawberries is they can be adapted to existing production systems.
“Growers should use their existing production systems and try to make them work,” he said. “They are familiar with how their systems work. This also helps to minimize investment costs.
“In most cases, strawberries are going to be different than anything else that growers have produced before. But strawberries are adaptable to all types of growing systems. Growers need to start with what they have so that they can learn as much as they can about the plants. They need to become familiar with how strawberries grow. That’s going to take a couple of years. Then if growers want to switch to a different, more expensive production system designed for strawberries, they can.”
Temperature control is critical
Strawberries grown in greenhouses prefer day temperatures below 77ºF (25ºC), which Kroggel said is a temperature that works for many food crops. The ideal temperature range for strawberries is 65ºF-77ºF (18ºC -24ºC).
“The temperature shouldn’t go much above 77ºF because higher temperatures can negatively affect growth,” he said. “Night temperature is much more important for strawberries. We try to maintain night temperatures between 50ºF-54ºF (10ºC-12ºC). Being able to maintain the temperature below 59ºF (15ºC) at night is critical for strawberries because higher night temperatures result in lower quality due to respiration in the fruit.
“If greenhouses cannot be cooled to 59ºF or lower at night, fruit quality is going to be drastically affected. Primarily the acidity will be too high, the Brix (sugar content) will be too low and the surface of the fruit will be off. The strawberry starts to get mealy or soft. The texture, sweetness and acidity are all affected by the temperature.”
Kroggel said greenhouse strawberries are typically grown in the United States from the fall through the spring. He said most growers wouldn’t be producing strawberries in a greenhouse during the summer because of the competition from field-grown crops.
“There are greenhouse growers in Europe who produce strawberries year-round, but they have a climate that is more amenable to that type of production,” he said. “Day temperatures are less important than night temperatures in regards to fruit quality.
“We can grow a crop later in the spring when the outside day temperature can reach 95ºF-100ºF, but because of the low humidity in Arizona, we can still cool the greenhouse temperature to 75ºF during the day and 59ºF or cooler at night. We can maintain the fruit quality. In most U.S. locations, growers should be able to maintain the required cooler night temperature during the fall to spring period.”
Ensuring adequate light levels
Kroggel said growers interested in producing greenhouse strawberries should be able to provide a minimum daily light integral (DLI) of 12 moles per square meter per day inside the greenhouse.
“Light levels below 12 moles are most likely too low for strawberries,” he said. “The big difference between a fruiting crop and an ornamental flowering crop is that fruit is expensive for the plant to produce. It takes a lot of energy to produce a strawberry or tomato. Ornamental plants can produce leaves and flowers under lower light conditions. Growers in areas that don’t receive 12 moles of light from November through February are going to produce a minimal yield of fruit. With reduced light and photosynthetic activity, the plants cannot support as many fruit.”
Growers in high light areas like Arizona also need to be concerned about too much light. Kroggel said growers should try to keep light levels below 25 moles per square meter per day.
“We have seen some plant stress when the light level starts to reach 30 moles per day in March,” he said. “That’s when we start to shade the greenhouses.”
Humidity control to prevent disease, tipburn
Kroggel said if the greenhouse humidity level is 85 percent or higher during the day and night, foliar diseases including powdery mildew and Botrytis on the fruit, can occur.
“In those parts of the country where the greenhouses are closed at night there can be a problem of high humidity,” he said. “If it’s too cold, heating can lower the humidity. Venting during the day to allow outside air to enter the greenhouse helps to lower the humidity. That’s standard practice and normal day time humidity is usually manageable.”
If the greenhouse humidity is high, the plant canopy can remain wet if there is not adequate air flow. Kroggel said the horizontal airflow fans in the university’s strawberry greenhouses run continuously and help keep the canopy dry. Because of Arizona’s low humidity levels, he said fog has to be used in the greenhouses at night during certain times of the year to raise the humidity in order to prevent tipburn on strawberries.
“We prevent leaf and calyx tipburn by humidifying at night,” he said. “We try to maintain 95 percent humidity inside the plant canopy for three hours at night. Typically that creates a high enough night humidity to prevent tipburn, but is not a long enough time to promote disease.”
Tipburn in strawberry is caused by calcium deficiency just like in poinsettias and lettuce. Kroggel said strawberry tipburn occurs very early in the leaf and calyx development stage.
“When a leaf is developing, if there isn’t sufficient calcium then leaf tipburn has already occurred before the leaf emerges,” he said. “During the day when transpiration in the plant is high, calcium is moving into the mature leaves and not into the growing tip.”
For plants with a mild case of tipburn, Kroggel said there is probably not going to be much effect on photosynthesis. In severe cases, tipburn could impact the fruit.
“If the tipburn is mild and is not on all the leaves, it is probably not affecting photosynthesis that much,” he said. “If the tipburn is severe, then the area of photosynthetic activity is being impaired. Over half the leaves may not be working properly.
“If tipburn occurs on the calyx of the fruit, then most consumers are not going to want to purchase the fruit. The fruit itself might be beautiful, but most consumers won’t buy it because of the calyx burn. Similar to ornamental plants, a grower is impairing his ability to market the fruit if there is calyx burn.”
Kroggel said like poinsettias and chrysanthemums, strawberries respond to short day conditions. These plants, called June-bearing types, require short days in order to initiate flowering.
“We are growing both June-bearing and ever-bearing strawberry varieties,” he said. “Ever-bearing varieties prefer longer days and lighting helps to promote flowering.”
Kroggel said 12-13 hours of day light are likely short enough to initiate flowering in most U.S. June-bearing varieties.
“For winter production, if a grower is planting June-bearing varieties in August, by the time the plants start growing and developing there will be 12 hours of light and the plants will begin to initiate flowers,” he said. “Then the plants start to fruit naturally because the days are getting shorter. From the time flower initiation occurs until flowers appear takes about a month. From flowers to fruit is about a month. If plants initiate flowers in late September, the fruit should be ready to harvest in December.”
Kroggel said in some parts of the world greenhouse strawberry growers provide short day treatments to ensure the plants initiate flowering and produce fruit. Some strawberry growers can pull black cloth just like ornamental plant growers do with poinsettias and mums as long as the temperature under the cloth doesn’t get too high. Other growers use temperature-controlled growth chambers to provide short days.
“We do use photoperiodic lighting on ever-bearing varieties to promote flowering because winter days are too short,” he said. “Ever-bearing varieties prefer longer days and the lighting helps promote flowering. We use T5 fluorescent lights to do photoperiodic lighting, but we don’t do any supplemental lighting. About 3 micromoles per square meter per second at the canopy level is a sufficient amount of light.”
For more: Mark Kroggel, University of Arizona, College of Agriculture and Life Sciences, The School of Plant Sciences, Tucson, Ariz.; (520) 626-3928; email@example.com. For more information on greenhouse strawberry production: Hydroponic Strawberry Information Website, http://www.cals.arizona.edu/strawberry; Sustainable Hydroponic and Soilless Strawberry Production Systems, https://www.youtube.com/user/sustainablehydro.
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.