Navigating snowy, ice-covered roads on the way to market is among the challenges facing a new type of raspberry grower. A few innovative producers are harvesting up to 60 flats (720 half-pints) of fresh raspberries from a 20 X 30 ft. house between February and May, and selling them for $2,000.

Greenhouses have been used for many years to produce tomatoes and cucumbers during winter, but these vegetables require relatively warm temperatures and high levels of light, making their production expensive. Raspberries, however, are uniquely suited for greenhouse production during the off-season. They grow best at a relatively cool temperature (70F) and do not require supplemental light to produce a crop, especially if production is targeted for May and June. In northern states, many greenhouses are empty during the winter months, but these could be used to grow raspberries with only moderate inputs, providing greenhouse owners with an opportunity to produce an extremely high value crop during a time of the year when they are realizing no return on their capital investment and when no domestic raspberries are available.
The vast majority of winter raspberries currently on the market are flown in from the Southern Hemisphere. Quality is generally poor because raspberries have an extremely short post-harvest life and bruise easily during shipping. As a result, consumers are willing to pay between $3.00 and $6.00 per half-pint for fresh fruit of superior quality, and restaurant chefs seem willing to pay even more.

Local raspberry production is now possible because of two accomplishments in the area of entomology. First, bumble bees have now been domesticated and are available in small hives for pollinating greenhouse crops. Bumble bees perform better than honey bees in greenhouses, especially under the cooler temperatures used for growing raspberries. Second, predatory mites are now available that feed on phytophagous mites, and these can keep populations of damaging mites at low levels.

Compared to field production, greenhouse-produced berries are larger, firmer and much less prone to fruit rot. Fruit tends to be slightly less sweet and more acid in the greenhouse, but well within the limits of acceptability. Varieties differ in performance and flavor, and what might do well in the field will not necessarily perform well in the greenhouse. According to our research and work done in Belgium, the variety Tulameen from British Columbia is the best floricane-fruiting raspberry in the greenhouse, although it will not survive most winters outdoors in northern climates. If temperatures drop from above freezing to below 10F, canes can be damaged.

Production basics
Tulameen tissue-cultured raspberry plugs are planted into 3 to 5 gallon pots after the danger of frost is over. Nursery matured plants can be used as well, but these will require the larger pots because of their greater root mass. Prior to planting, pots are filled with equal parts sand:peat:perlite:vermiculite. The proportion of sand can be reduced in the larger pots to reduce the weight. Plants are grown outdoors on a gravel bed with irrigation through the summer. While outdoors, plants are fertilized while irrigating with a complete soluble fertilizer solution containing 100 ppm N, and pest outbreaks are managed using conventional practices. Rows of pots need to be spaced at least 8 ft. apart so that sufficient light will be intercepted by the lower leaves on the canes. If the plants are too close together, the lower buds will not produce fruiting laterals once they are moved into the greenhouse. In October after leaf drop, pots are moved closer together and bales of straw are placed around the group of pots to help protect them from cold temperatures. Root systems are more sensitive to cold temperatures than canes, so temperatures near 10F for an extended period of time will kill both roots and canes in pots that are setting on top of the ground. In late December, plants are moved into the greenhouse. (Plants may have to be moved earlier if late December temperatures are forecast to fall below 10F. In this case, move plants into an unheated greenhouse or garage/barn until the end of December.)

Late December is the normal time (for us) when plants have received a sufficient amount of chilling to break dormancy (between 800 and 1,000 hours). To ensure that a sufficient number of chilling hours has been achieved, plants can be moved into a cooler (that does not contain other fruit) for 8 weeks prior to moving them into a warm greenhouse. In our experience, buds on thicker canes and buds lower on the cane have a longer chilling requirement than buds on thin canes or near the top of canes.

Once in a warm greenhouse, canes are spaced pot-to-pot with 5.5 ft. between rows, trellised, and watered with a 100 ppm N complete fertilizer solution. Short or broken canes are removed. Canes should be supported by an I-trellis; spreading canes into a V-trellis is not an efficient use of greenhouse space. We have found that an application of horticultural oil to canes after several days in the greenhouse, but before bud break, can reduce mite problems later on. Household fans are used to circulate air down the rows to reduce pockets of high humidity (ideal is 65 – 75%) and reduce the subsequent risk of fungal infection. Temperatures are maintained at 65 – 70F during the day and can be as low as 50 – 55F at night – ideal for raspberries but too cold for most other plants. Supplemental light can accelerate development by 2 to 3 weeks and increase yield by 20 – 30%, but may not be economical, depending on markets. Night temperatures also can be maintained at 70F, if heating costs are not excessive.

Six weeks after moving plants into a lighted greenhouse, they flower. Bumble bees are used to pollinate the flowers, and fruiting can begin as early as late-February, about 10 weeks after moving plants indoors. Once flowering begins, the nutrient solution is reduced to 50 ppm nitrogen. With one-year-old plants in small pots, double rows (with row centers 5.5 ft. apart) and a pot-to-pot spacing can be used so that approx. 20 plants are contained in each 3 m (10 ft.) length of row. Each “baby” plant produces about two half-pints (350g) of fruit.

During fruiting, primocanes for next year’s crop will be growing. These will grow extremely tall if they are not managed properly. Our procedure for managing primocanes is to remove all but 4 new canes (primocanes). These 4 will bear next year’s crop. When these primocanes reach about 3.5 ft. in height, the top 4 inches is removed. Pimocanes temporarily stop growth, allowing fruit to be harvested without interference from primocanes.

After the first harvest is over (in April, May or June), floricanes are removed. If plants were first planted in pots less than 3 gal in size, they should be replanted into 5 gallon pots with primocanes intact and placed outside in full sun for the second growing season. Raspberry plants must be supported outdoors to prevent the wind from blowing them over. We use bamboo stakes to hold canes upright in the pots. When primocanes reach 6 ft. (1.8 m) they are topped again to retard growth. Plants are returned to the greenhouse in mid-December – after the chilling requirement had been fulfilled. Rapidly satisfying the chilling requirement is one advantage that northern growers have over more southern producers.

In the second production cycle indoors, plants are spaced 22 in. (0.6m) apart in single rows, with 5.5 ft. (1.7m) between rows, and canes are trellised upright to a single wire. In the second and third fruiting years, both fruiting laterals and primocanes will be growing and competing for limited light. In order to regulate self-shading yet have large primocanes for next year’s crop, we tip the largest 4 primocanes per pot at a 3 ft. (1 m) height; the remaining primocanes are removed at ground level. This allows the fruit to be harvested without significant interference from the primocanes. Primocanes will begin regrowth after several weeks, and will continue growing once they are moved outdoors again. Raspberry plants will fruit for three years before beginning a decline.

Digging canes
If producers do not want to take the time to grow their own plants, full-grown raspberry canes dug directly from the field in early winter can be used to produce fruit that same spring. These plants will be slower to flower and fruit than full-grown potted plants, but are nearly as productive. Such nursery stock is called “long cane.” Sakuma Brothers Farms (360-757-6611) in Washington sells long cane raspberries. If using “long-cane” plants, begin with 5 gallon pots and allow them to acclimate in pots in a cold greenhouse for two weeks before gradually warming the house to 70F.

Twospotted spider mites thrive in the warm, dry conditions of a greenhouse. To reduce the number of mites, we treat canes with horticultural oil within a few days of being moved inside the greenhouse. Since no pesticides have been labeled for greenhouse raspberry production, biological controls must be used for twospotted spider mites after plants are moved inside. We release predatory mites once, after leaves emerge from the canes. Cool temperatures, high humidity, a release of predatory mites, and removal of infested leaves help reduce numbers of pests.

Artificial chilling for earlier crop
We have artificially chilled plants by moving them into 34F coolers as earlier as mid-August, with leaves intact. After 8 weeks in coolers and 8 weeks in the greenhouse, plants produced flowers by December. Defoliation of plants prior to chilling had no effect. This finding opens the possibility of significantly lengthening the fruiting season of greenhouse raspberries. Starting production earlier would have several advantages. Raspberry production would be complete before bedding plant season begins in April, allowing more greenhouse space to be available for use. Also, vents are closed when the outdoor temperatures are cold, allowing the grower to supplement the atmosphere with carbon dioxide, enhancing plant growth. Once temperatures warm in spring, houses have to be vented to cool them and the carbon dioxide escapes. Again, an earlier start would improve efficiency of carbon dioxide use.

Coolers for late spring crop
Dormant raspberry canes can be stored successfully for several months in a humid cooler maintained at about 34F. Pots should be watered when needed to maintain soil moisture. Approx. 12 weeks before fruit is desired, plants can be moved to a warm greenhouse. This allows the grower to produce fruit in late spring and early summer, up until the time field-grown fruit is available. The longer plants are stored in the cooler, the shorter will be the time between bud break and fruiting. This is because plants respond to warmer temperatures faster with more accumulated chilling, and because the day lengths are longer later in the spring when plants are growing.

Fall-fruiting varieties
Primocane-fruiting varieties will continue fruiting in the fall, well past the time that production would normally cease outdoors due to cold weather, if they are moved inside the greenhouse in early September. Pinching primocanes when they are about 3 ft. tall will delay flowering and fruiting, shifting the crop to late in the fall. This practice has been successful under high plastic tunnels, and well as in greenhouses. Bumble bees are used for pollination, and the fertility program is similar – 100 -150 ppm N until fruiting, then 50 ppm N in a complete fertilizer formula. Varieties that have performed well in the fall include Autumn Britten, Hertiage and Caroline.

Summary
It is now possible to produce raspberries close to market during most months of the year.Raspberries are the most perishable of all fruits, so even though they can now be grown close to market, they must still be handled with the utmost of care. Raspberries must be cooled quickly after harvest, and delivered to the customer as soon as possible. Most markets for winter raspberries are small, so a producer will need to line up and supply a larger number of smaller markets.

Despite these challenges, the opportunities for producers are great. At this point, there exist only a few winter raspberry producers, so the market is wide open. Furthermore, the quality that can be produced is very high. Consumers and restaurant chefs are willing to pay very high prices for high quality berries in winter. Several growers are already producing winter raspberries in northern states.¶

For more information on greenhouse raspberry production, visit the website: http://www.hort.cornell.edu/department/faculty/pritts/greenhouse/Frontpage.htm