Originally published in Issue 10
Which are the best strawberry varieties for greenhouse production? Combining June-bearing and everbearing varieties can help ensure fruit is available during periods of premium pricing.
Trying to decide which strawberry varieties to produce in a controlled environment production system can be a challenge for growers using field-bred varieties. Mark Kroggel, research specialist at the University of Arizona’s Controlled Environment Agriculture Center in Tucson, said it is possible for growers to produce strawberries nearly year-round by combining greenhouse and field production.
June bearing short day strawberry varieties
Kroggel said the traditional strawberries grown for field production are often referred to as June-bearing varieties and are actually short day plants.
“These plants require certain photoperiods to begin flower initiation just like poinsettias,” he said. “The strawberry plants are transplanted into the field and become established during late summer. Depending on the critical photoperiod, which varies between varieties, many of the short day varieties start to initiate flowers when there are 12-13 hours of day light. Flower initiation occurs on the crowns the plants have or produce in the fall.”
During the winter the plants go dormant. In spring, the flowers, which have already been initiated, open and bear fruit. During the early spring as additional growth occurs, the plants continue to initiate and produce flowers until that critical photoperiod is exceeded. Once the critical photoperiod is reached, the plants won’t initiate any more flowers.
Short day strawberry varieties in the greenhouse
Kroggel said when short day strawberry varieties are grown in a greenhouse, the winter dormancy period is eliminated.
“In the fall instead of letting the plants go dormant, they are being kept actively growing in the greenhouse through temperature and nutrition,” he said. “A June-bearing, short day variety that’s planted in August in the greenhouse is going to grow vegetatively until some point in the fall when the days are short enough to initiate flowers. Then it is usually 30 days from flower initiation until flowers appear. It takes another 30 days from the time of flowering until fruit is produced. So, from the point of flower initiation it takes 60 days before fruit is ready to harvest.”
Kroggel said a grower could plant strawberries in the greenhouse during late summer and early fall to produce a crop by Christmas using the natural photoperiod.
“Starting in late September the plants are going to receive 12 hours of natural day light,” he said. “It takes several weeks for the plants to initiate flowers. Then in October there is about four weeks of flower development. In November, from flower to fruit takes another four weeks. Then in December the fruit develops.”
Kroggel said short day varieties produce fruit fairly consistently in the greenhouse.
“What we have seen with our own trials with the June-bearing or short day varieties, once the days start to exceed 12 hours of day light in April, the plants stop initiating flowers. We’ve found that six months is our production limit for any strawberry variety in the greenhouse. The substrate starts to break down and the plants start to lose their vigor. This applies to both short day and everbearing varieties. We also run into problems when we can’t cool the greenhouse causing the fruit quality to suffer. I expect the typical crop life for winter production of greenhouse strawberries for most growers is going to be about six months.”
Inducing strawberries to flower and fruit
Because of the lack of commercially available actively growing starter plants during the summer, Kroggel said he is producing his own tip runners in 38-cell plug trays or 2-inch tree bands (with permission to propagate from patent holders when varieties are protected).
“Since we want to start growing the strawberries during the summer, this is a time of year that there usually are no starter plant material available from most commercial propagators, and so we need to produce our own,” he said. “Usually the very latest dormant runners are available is in June.”
These strawberry plantlets are stuck in a substrate and placed on a mist bench until they are rooted in. Rooting and acclimation takes two to three weeks and the plants need several more weeks of greenhouse growth to be established enough to transplant into a growing system. The plants are ready to be transplanted when they can be removed from the plug cells or pots with roots and substrate intact.
“To initiate flowers in short day plants in this high density propagation when the natural day length is still longer than the flower-inducing day length, we provide the rooted and acclimated plants a short day treatment,” Kroggel said. “This treatment consists of eight hours of daylight in the greenhouse and then the plants are moved into a dark cooler for 16 hours at 59ºF (15ºC). This is the most ideal temperature for flower induction.
“The plants are moved back and forth on carts between the cooler and the greenhouse. It is labor intensive, but they can be moved relatively easily because they are in a dense planting situation. They come out of the cooler at 8 a.m., receive eight hours of light in the greenhouse, and then at 4 p.m. they are moved back into the cooler. A grower who has rolling benches could move the plants back and forth between a cooler and the greenhouse. For the short day varieties that we have grown in the greenhouse it takes three to four weeks before flower initiation occurs. Once the initiation of flower buds is confirmed under a microscope, the short day treatment is no longer needed.”
Kroggel said the 24-hour average temperature for strawberries should not exceed 77ºF (25ºC).
“If growers lived in regions where the temperatures were cool enough to stay below the 77ºF daily average, they could probably pull black cloth to provide the required short day conditions for flower initiation as an alternative to moving the plants into a dark cooler,” he said. “As long as the plants are kept in that temperature range of 59ºF-77ºF pulling black cloth won’t be a problem. Growers need to monitor the temperature under the cloth to be sure not to overheat the plants.”
Everbearing strawberry varieties in the greenhouse
Kroggel said one of the issues with everbearing strawberry varieties is the terminology used to describe them.
“Everbearing varieties are often referred to as being day neutral. We don’t know of any day neutral everbearing strawberry plants,” he said. “These varieties tend to be facultative long day plants. They will flower all the time, but if they’re provided with a longer photoperiod, they will produce more flowers.”
Kroggel said one of the ways to keep everbearing varieties flowering during the short days of winter is to provide them with photoperiodic lighting.
“During short days the everbearing varieties benefit from an extended photoperiod,” he said. “The plants need 2-3 micromoles, which is about 20 footcandles. We provide the plants with 12-14 hours of light using fluorescent or incandescent lights.”
Timing fruit production
For growers using dormant runners or propagating their own tip runners of everbearing varieties, flowers must be removed in order to allow the plants to become established before producing fruit.
“These varieties naturally produce flowers as soon as they can,” Kroggel said. “For a period of four to six weeks after planting into the production system, the flowers should be removed to allow the runners to develop roots and leaves. The plants need to have a good initial vegetative establishment period so they have well-established roots and leaves in order to support the fruit. By removing these flowers some of the fruit is lost, but this establishment period is necessary.”
Strawberry plants produce their first flush of fruit about one month after the flowers appear (short day varieties) or the flowers are left on the plants (everbearing varieties).
“At some point after the first flush, the everbearing variety plants tend to temporarily stop producing flowers,” Kroggel said. “There will be anywhere from a six-week to a two-month period when no fruit is produced. That is a real issue with producing everbearing varieties. Then there is a massive second flush of flowers and fruit.”
Kroggel said this cyclical production of flowers and fruit can be accommodated by staggering planting dates and using different varieties that have varying production schedules.
“Really high yielding everbearing varieties have less cyclical production because they produce more crowns more often. Unfortunately we haven’t found a really high yielding everbearing variety yet with really good flavor. The June-bearing or short day varieties have a more linear production cycle.”
Kroggel said that he recommends that growers producing greenhouse strawberries plant both June-bearing and everbearing varieties.
“With the short day varieties, they begin flowering at some point either naturally or by being induced,” he said “Their weekly yields are fairly consistent and their cumulative yields are linear. The flower and fruit production of the everbearing varieties tend to be cyclical over the season. I recommend growers produce both June-bearing and everbearing varieties to ensure they are always producing fruit. But growers need to know how to manage both types. Being able to produce fruit during November, December and January is critical. This is the period when premium pricing occurs.”
For more information on greenhouse strawberry production, check out: Hydroponic Strawberry Information Website, http://cals.arizona.edu/strawberry; Sustainable Hydroponic and Soilless Strawberry Production Systems, https://www.youtube.com/user/sustainablehydro.
Mark Kroggel provided information on greenhouse strawberry production in Urban Ag News Issue 9, “Strawberries can be adapted to greenhouse production systems” (https://urbanagnews.com/emag/issue-9-2).
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.