October 2002

Raspberries: A New Greenhouse Crop

Raspberries have an exceptionally short shelf life, are
easily damaged when handled and do not transport well. Because of their
perishable nature, the opportunities to market high-quality, fresh raspberries
produced under favorable field conditions in distant locations are limited. In
an effort to extend local availability and the season for fresh raspberries,
greenhouse production should be considered. The targeted local market is for
high-value, top-quality, off-season, fresh raspberries, since greenhouse
production is expected to be more expensive than conventional field systems.

The raspberry plant

The raspberry production cycle differs from most commonly
grown greenhouse crops. Raspberries used in commercial production are perennial
plants with fruit developing on the canes in a two-year cropping sequence. The
first-year growth is referred to as primocanes, which are called floricanes in
the second year. Based on the fruiting characteristics, there are two main
types of raspberries. The summer, or floricane-bearing, type produces flowers
and berries on the floricanes following natural cooling during the winter. The
fall, or primocane-bearing, type produces a limited number of flowers and
Á berries on the upper portion of the primocanes at the end of the
growing season. During the second year, fall-bearing types continue to flower
similar to summer-bearing types following colder temperatures.

The primocane-bearing variety ‘Tulameen’ responds favorably
to a greenhouse environment in respect to yield, berry size and flavor. Other
varieties, such as ‘Chilliwack’, ‘Canby’, ‘Encore’ and ‘Malahat’, may also
perform well depending on the crop cycle, local growing conditions and desired
scheduling. We chose to grow Tulameen because of positive reports and
recommendations for greenhouse production. Raspberry canes of Tulameen are also
readily available for purchase from commercial propagators and nurseries.

Raspberry production

Various techniques and schedules may be implemented to
produce raspberries in a greenhouse environment for marketing at different
times of the year. Bare-root, multi-cane plants can be potted into 3-gallon
containers in May and allowed to grow outdoors during the summer. The
tall-growing raspberry canes in containers easily get top-heavy. To improve pot
and plant stability, sand mixed with a peat-lite medium (Premier Pro-Mix BX,
Premier Brands Inc., Red Hill, Pa.) was initially used as the potting mix. The
sand was later omitted since no advantage could be distinguished for adding
sand to the growing medium.

The plants can be left outside in the fall if local
conditions allow for natural cooling, or placed in a cooler at approximately
40° F. Prior to cooling, plants are pruned, leaving 3-4 of the most
vigorous primocanes. Numerous, high-quality raspberries are more efficiently
produced on a few strong and well-supported canes than on many weaker ones.
Pruning also facilitates handling and improves disease and pest control. Low
light continuously or for a few hours each day is beneficial, and maintaining
even soil moisture is essential during artificial cooling. Under natural
conditions, the chilling requirement is met by mid-December, and the plants are
brought into a greenhouse at 55-65° F. Flowering is expected to initiate
after 6-8 weeks and fruiting after 10-12 weeks. The first ripe raspberries in
this production system may be picked in early March, and harvest is expected to
continue for up to 60 days.

For plants placed in a cooler, pots may be taken out in
sequential intervals to have continuous flowering and fruit production over a
longer time period. In our studies, five weeks at 40° F was sufficient to
fulfill the cooling requirement. As a guideline, we recommend cooling Tulameen
for at least six weeks or approximately 1,000 hours to ensure proper exposure
to low temperatures. On the other hand, raspberry canes left in a cooler for up
to six months grew and developed without any reduction in flowering or berry
production. Holding canes at the low temperature for greenhouse forcing
provides opportunities to schedule fresh raspberry production for various
marketing opportunities.

Although raspberries do not require high light levels,
supplemental lighting during the winter to at least 750 foot-candles will
improve the growth, flowering and, subsequently, the yield in Northern areas.
Extending the daylength to 16 hours is beneficial both in respect to a
photoperiodic long-day effect and ability to provide adequate daily total
amounts of light. At low light levels, the rate of plant and crop development
is expected to be slower.

Spacing and arrangements of the pots in the greenhouse
depend on management techniques, logistics and the size of the plants.
Initially, plants may be kept pot to pot in single or double rows. Spacing with
20-22 inches between larger plants allows for adequate development and
handling. The rows may be 3-5 1/2 feet apart. Trellising the rows to keep canes
upright while fruit develops facilitates management of the plants, pest control
and harvest. A fertilizer regime with a water-soluble fertilizer high in
potassium (Hydrosol 5-11-26) and a general-purpose fertilizer (Peter’s
15-16-17) at alternate irrigations worked well in our studies.

Pollination

To set fruit, raspberry flowers need to be pollinated. Bumblebees
are now commonly used for crops in greenhouse production systems that require
pollination. Bumblebees are easier to manage and remain more active in cloudy
weather and lower temperatures than honeybees. Equally or more efficient
pollination has been reported with bumblebees compared to honeybees in
greenhouses and plastic tunnels. Bumblebee hives are commercially available
year-round, and one colony is expected to provide efficient pollination for 6-8
weeks. On a limited or experimental scale, successful pollination can also be
accomplished by hand using an artist’s brush during periods of slow bumblebee
activity.

Long-cane plants

Another option for greenhouse raspberry production is long-
or single-cane plants. Bare-root long canes (3-4 feet) shipped during late fall
or early spring from the Pacific Northwest already have the required cooling
and can immediately be planted and forced into flower and berry production or
alternatively kept in a cooler for later forcing. The long canes were planted
in 3-gallon pots and evaluated for productivity and yield using similar
management and production techniques as described for the multi-cane plants.
Several single-cane plants in a larger container is another production strategy
that may be evaluated.

At the completion of berry production, each floricane was
pruned to the soil level and removed. The newly developing primocanes were cut
back, leaving approximately four feet of growth immediately prior to returning
the plants to a cooler. The pruning practice appears to stimulate bud formation
on the primocanes. Removing the top portion of the primocanes also facilitates
plant management, pest control, handling and the transfer of plants to and from
the cooler. Care to avoid damaging the developing buds while moving plants is
critical. Yield increased for the single-cane plants during the second
production cycle compared to the initial crop. Following the second crop cycle,
the single-cane plants were discarded. Under our conditions, it was deemed more
efficient and economical to bring in new long-cane plants than to maintain the
old canes for a third cropping cycle.

Yield

Fresh raspberries are sold in units of half-pints (180
grams). Retail prices between $3 and $6 for a half-pint are common. Up to 12
half-pints of high-quality berries have been harvested from individual,
greenhouse-grown, multi-cane plants. Most berries are between three and six
grams in size with some berries as large as eight grams.

Potential pest problems

The main pest problem both with multi-cane and long-cane
raspberries is infestations of two-spotted spider mites (Tetranychus urticae).
Relative humidity is naturally low during Northern winter conditions, providing
an ideal environment for spider mites. Control measures, including beneficials
(Phytoseiulus persimilis), safer soap and sanitation, are used repeatedly to
control and manage outbreaks. Good relative humidity also helps keep spider
mites in check. Although infestations can be sustained to manageable levels
through continuous monitoring and integration of pest control procedures,
spider mite attacks are likely to be a continuous challenge and an obstacle in
greenhouse raspberry production. Thrips and aphids are other potential pest
problems. Although observed on plants in our trials, neither thrip nor aphid
infestations have been severe. Biological control measures are available such
as Amblyseius cucumeris for thrips and lacewing larvae for aphid control.

Conclusions

Under Northern conditions, we recommend using single or long
canes rather than multi-cane plants. The long-cane raspberry plants are easier
to handle and maintain in a greenhouse production system, and a larger number
of plants can be productively grown in an available designated area. In
addition, the long canes are easier to ship than multi-cane plants and may be
planted immediately or stored in a cooler for later forcing.

The raspberries produced in our study were of exceptional
quality, size, color and flavor. Raspberries shipped from more Southern
locations cannot compete with the superior quality of locally grown, fresh
greenhouse raspberries. Raspberries offer a high-value greenhouse crop
alternative that may be worth considering for local marketing to consumers
willing to pay a premium price for high-quality, fresh raspberries.