Sep 24, 2018
Techniques, varieties fuel success with hydroponic strawberries

Consumers want fresh strawberries year-round – and they’re willing to pay for them. Those peak prices from October through March – capped by strong demand through the Thanksgiving, Christmas, New Year and Valentine’s Day holidays – has fueled a 70 percent increase in the number of greenhouses producing strawberries since 2009.

Strawberries can be a challenging crop, said Mark Kroggel, lecturer in controlled environment agriculture in Ohio State University’s Department of Horticulture and Crop Science. He was a speaker on strawberry production at a recent Hydroponic Greenhouse Workshop at the Ohio Agricultural and Research Development Center at Wooster, Ohio.

“Strawberry plants are easy to grow, but getting quality fruit to sell is the tough part,” Kroggel said.

Choosing and planting

Short-day (June bearing) and ever-bearing (day-neutral) are the two main types of strawberry available.

Flowering is initiated on short-day cultivars under cool temperatures and short-day lengths. Day length is crucial. Flower initiation is promoted by day lengths of eight to 13 hours. As day lengths exceed that, flower initiation stops.

Ever-bearing cultivars also have an important temperature-day length interaction, but temperatures are the most crucial. Flowering is initiated under all day lengths, but the number of flowers increases with longer days. However, flower initiation is suppressed at higher temperatures of more than 86º F.

For greenhouse growers new to strawberries, ever-bearing cultivars may be the ones to consider.

“Short-day cultivars have such a critical photo period. If day length exceeds that critical threshold, it will prevent flower initiation,” Kroggel said. “Using ever-bearing cultivars would ensure the plants had growth and development and ensure flower and fruit establishment.”

Winter production in the greenhouse starts in late August. “Put your plants into your production system to get some growth and maybe even a crown or two before flowering,” Kroggel said.

Ever-bearing cultivars flower continuously so you need to remove flowers for about the first four weeks depending on the starting maturity. When plants have grown six new leaves in the greenhouse, it’s OK to leave the flowers on.

“Short-day plants set out in August are not going to develop any flowers until October,” Kroggel said. “For ever-bearing, flowering in September is going to mean fruit in October.”

Strawberries grow well in a variety of production scenarios, including trough or bucket systems. Photo: Gary Pullano

Light and heat

Strawberries like cool weather. They are not a hot weather plant like tomatoes or cucumbers. Optimal temperatures are 70 to 75º F, with nighttime temperatures of about 50º F or a little more inside the greenhouse.

“You want it cool at night when fruit is present, so you don’t lose fruit quality,” Kroggel said. “Forty-five degrees at night is not a problem for strawberries. Strawberries like it cool.”

The amount of light coming into the greenhouse is typically measured by daily light integral (DLI). The DLI measures the units of photons (light particles) received per area per day on the plant. DLI is typically measured in moles (mol) of photons per square meter per day.

Strawberries prefer 15 to 25 mols of DLI with a minimum of 12, measured at the canopy level, in the greenhouse. The 12-mol minimum is pretty standard for most greenhouse crops.

Ultraviolet light (UV) helps produce good fruit color so greenhouse glazings that allow some UV transmission are better than those that permit none.

Extending the day length in the greenhouse during the short days of winter is important to keep some cultivars active. Some cultivars will exhibit dormancy responses during the shortest days of winter. Extending the day length to 12 hours or more will help keep the plants active. Extending the day length can be accomplished with very low light levels (two to three micromoles per square meter per second) and can be done by adding light at the end of the day or by night break lighting (middle of the night).

Managing day length may also be required to get the strawberries to enter dormancy. Do this by reducing day length, if and or when dormancy is desired.

Mark Kroggel

Humidity and CO2

Keep relative humidity at 60 to 70 percent during the day and much higher at night. Relative humidity of 90 to 95 percent inside the canopy for at least three hours during the night is needed to slow transpiration and prevent tip burn. After the high-humidity period, the humidity can be lowered to dry out plants and help prevent foliar diseases.

Tip burn is a calcium deficiency in the leaves. It’s caused when plants are transpiring heavily and the calcium in the xylem stream is going mostly to the expanded leaves and not to the developing tip. The problem can’t be solved with calcium fertilizer because sufficient nutrients are available. They’re just not getting to the right place because of excessive transpiration. About three to three and a half hours of very high humidity (95 percent) at night pretty much shuts down transpiration and allows the calcium to reach the growing tip.

Adding CO2 to the air in the greenhouse is another tool. “We know strawberries respond to CO2 enrichment,” Kroggel said. Ventilation or air leakage from the greenhouse sometimes makes CO2 enrichment hard to accomplish.

“In the winter, when you’re not exhausting air, you may want to look at CO2 enrichment,” he said. A challenge is distributing the CO2 through a low growing crop like strawberries. You may want to deliver the CO2 directly into the canopy with a perforated tube.

It may be best to try more than one strawberry cultivar if you’re new to the crop. “Start with as many varieties as you reasonably can,” Kroggel said. Strawberries are very site-specific and this is the best way to find what works well in your greenhouse.

Picking and packing

Fruit color and how a cultivar ripens are also important traits to consider. Albion, an industry standard, ripens to a bright, ruby red. “Sometimes it may have a little whiteness on the shoulders,” Kroggel said. “That means you probably have another day to ripen. Sometimes you may never get the berry 100 percent red, but the fruit is still good.”

Portola, another popular variety, has an orange color. The fruit reddens, but you’re basically waiting for the orange color to fade.

“Some varieties let you have another day or two on the plant after coloring to get higher sugar,” Kroggel said. “Some cultivars you have to pick right now.”

Picking time may also depend on marketing strategy. “We’re often pushing berries to the maximum ripeness,” Kroggel said. “If we’re going to pick and sell them right away, we can’t be too rough on the fruit.”

If you plan to pick today to sell tomorrow, overnight refrigeration is essential. “Strawberries will take temperatures down to freezing, but 37 to 40 degrees – standard refrigeration temperature – is fine,” Kroggel said.

Proper sanitation is the most important practice for managing diseases and insect pests. It removes inoculum and sources of infestation.

“If you grow strawberries, you’ll have spider mites,” Kroggel said. Predator mites are a common solution.

“We’ll use Safer soap every three days as a spot treatment if we have a big outbreak,” he said. Be sure to wash it off before releasing predator mites. The predator mites don’t like the Safer soap and may avoid the treated leaves.

Botrytis can be a problem as the fruit ripens and sugar content increases. Remove the diseased fruit as soon as you see it.

If you move beyond biological controls, know the pesticide residuals and their effects on beneficials. Don’t use products that taste bad because you can’t wash berries.

Strawberries grow well in a variety of production systems. Trough or bucket systems can be used successfully. “Start with where you’re at,” Kroggel said. “Start with the system you’re using and build on that. Strawberries are always a learning experience.”

Proactive pollination

Misshapen fruit is the telltale sign of incomplete pollination.

“If the fruit is incompletely pollinated, it’ll grow on one side and not the other,” Kroggel said. “If you want good, marketable fruit, you want good pollination.”

However, pollination in greenhouse strawberries takes some work – the lack of air movement and insects are more challenging. Indoors, “we don’t have the benefit of wind pollination,” he said. Natural pollinators, such as wild bees are also unavailable.

In larger greenhouses, bumblebees are the preferred pollinator because they’ll visit the plants several times a day. But in smaller greenhouses, artificial air movement is adequate for spreading pollen from plant to plant. Pollination can be as simple as someone walking down the aisles with a small leaf blower.

In early morning, there may be some moisture from humidity on the strawberry flowers that could prevent even pollination. Avoid the early evening, too, because flowers may be past the prime time for pollination. “Middle of the day is best,” Kroggel said.

– Dean Peterson, VGN Correspondent

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