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Strawberries (greenhouse)

Image: Opachevsky Irina/Shutterstock.com

A member of the rose family, the strawberry (Fragaria x ananassa) can be found in various native forms across the globe. It is believed the fruit was first cultivated in ancient Rome and much later in France in the 1300s.

By 1825, commercial production was on the rise in the United States. In 1838, fruit grower and plant breeder Charles Hovey introduced the “Hovey,” which quickly became one of the first popular cultivars. Since then, the red berries have grown larger, sweeter, and juicier thanks to modern breeding practices.

In botanical terms, this fruit is not technically a berry. Because the plant includes flowers with more than one ovary and its small seeds appear on the outside (not the inside), they are classified as an aggregate fruit.

Today, strawberries are grown in every U.S. state and Canadian province. For most of the year, California leads in domestic production. However, in the winter months, Florida takes over as the nation’s top producer.

As demand for the wildly popular fruit continues to climb, greenhouses are filling supply gaps from field production. Japan and Europe have been producing the berries in greenhouses for many years.

More recently, strawberry greenhouses have been popping up in Mexico and Ontario, Canada as well as a handful across the United States. Given the delicacy of strawberries, greenhouse growing not only extends the growing season but limits damage from pests, disease, and weather.

Types & Varieties
Traditional field strawberries are categorized into June bearing, everbearing, and day neutral. June bearing crops produce fruit over a two to three-week period in June, hence the name. These are the most popular with commercial growers because they generally produce larger berries.

Ever bearing varieties do not actually produce crops year-round. Depending on the growing region, these varieties produce crops twice a year: in the spring and again in the late summer or early fall.

SEASONAL AVAILABILITY
As a more recent commercial endeavor, greenhouse strawberries are not widely available, nor in substantial year-round supply—but they can be sourced from growers in the United States (Arizona), Canada, Mexico, the Netherlands, and Japan.

Day neutral crops produce fruit all season long when the temperature is between 35 and 85°F. In milder years, these plants can continue producing berries as late as October. However, day neutral varieties produce smaller berries as compared to June bearing and everbearing.

All three types can be successfully grown in greenhouses. To promote flowering in the greenhouse, June bearing crops require short days, while ever bearing varieties prefer longer days with supplemental lighting.

A wide range of varieties can be grown in the greenhouse, including Tristar, Albion, Selva, Jewel, Honeoye, Evie-2, Sweet Charlie, Seascape, Cabrillo, Monterey, and Strawberry Festival. New greenhouse varieties are released every year.

CULTIVATION
Greenhouse strawberries do best when daytime temperatures range between 65 and 77°F. If daytime temperatures rise above 77°F, it can negatively impact plant growth. At night, the plants require temperatures between 50 and 54°F.

Nighttime temperatures greater than 59°F can cause respiration, leading to lower quality fruit due to less sugar and higher acidity. Fruit may also lose moisture and be mealy. To prevent disease, the plants should be grown in low humidity, less than 85% is optimal.

Because of temperature restrictions, greenhouse strawberries are primarily grown from the fall through the spring within the United States. Light level is crucial to produce thriving plants: crops should receive a minimum daily light integral of between 12 and 20 mole units per square meter each day. In sunnier regions, when light levels reach beyond 30 moles, greenhouses should be shaded to protect plants.

Plants are typically grown in soilless systems, which come in a variety of container sizes and shapes or hanging baskets suspended from rails or placed on top of gutters.

Alternatively, greenhouse crops can be grown in long troughs filled with various types of material or substrate.

Strawberries do best when irrigated frequently, in small amounts, to prevent waterlogging roots. Yellowing leaves should be pruned to promote air flow and encourage growth, and misshapen or distorted fruits should be removed.

Like field varieties, greenhouse strawberries are picked by hand. The fruit should be harvested at least once a day or even twice a day during warmer, sunny periods.

When picked, the berries should be at least 50% ripe but no more than 75% ripe. During harvesting, the calyx should remain attached to the fruit. Overly ripe fruit will not ship well.

Once harvested, berries should be stored at temperatures between 36 and 41°F in 90 to 95% relative humidity.

Packaging by the pint or quart can be cardboard or plastic clamshells, though newer packaging using recycled and recyclable materials is gaining favor.


Pests & Diseases
While pests and diseases are greatly reduced in greenhouses, plants are still susceptible to threats. Spider mites can damage foliage, which reduces photosynthesis and negatively impacts growth. Thrips can harm plants and fruit, and also transmit disease.

Strawberry aphids leave behind waste known as honeydew, which can cause sooty molds to form on the berries, making the fruit unmarketable.

Greenhouse strawberries also can be impacted by whiteflies, which transmit viruses and reduce yields by feeding on leaves.

The most common disease that impacts greenhouse strawberries is botrytis, or grey mold. The mold will start to grow in humid and cool conditions when temperatures drop to 32°F or below. Mold can infect plants during flowering and remain hidden until the fruit begins to grow, leading to unmarketable berries.

References: Berry Growers of Ontario, California Strawberry Commission, Cornell Cooperative Extension, Iowa State University Department of Horticulture, University of Arizona College of Agriculture and Life Sciences, University of California Integrated Pest Management, University of Missouri Integrated Pest Management, University of Nebraska – Lincoln Agronomy & Horticulture Department, University of Vermont Extension, USDA.

GRADES & GOOD ARRIVAL
The following information is for strawberries as a category and there are no guidelines or tolerances specific to
greenhouse-produced fruit.

Strawberries are classified as U.S. No. 1, U.S. Combination, or U.S. No. 2 based on characteristics such as size, firmness, ripeness, development, and coloring, as well as being free from damage caused by mold, decay, dirt, moisture, disease, pests, and mechanical harm.


Generally speaking, the percentage of defects shown on a timely government inspection certificate should not exceed the percentage of allowable defects, provided: (1) transportation conditions were normal; (2) the USDA or CFIA inspection was timely; and (3) the entire lot was inspected.

U.S. Grade Standards Days Since Shipment % of Defects Allowed Optimum Transit Temp. (°F)
10-5-2 5
4
3
2
1
15-8-3
14-8-4
13-7-3
12-6-2
10-5-2
32°

Canadian good arrival guidelines (unless otherwise noted) are broken down into five parts as follows: maximum percentage of defects, maximum percentage of permanent defects, maximum percentage for any single permanent defect, maximum percentage for any single condition defect, and maximum for decay. Canadian destination guidelines are 15-10-5-10-3.

References: DRC, PACA, USDA.


This information is for your personal, noncommercial use only.