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., they are topped to retard growth. Plants are re- 

 turned to the greenhouse in mid-December - after the chill- 

 ing requirement had been fulfilled. Rapidly satisfying the 

 chilling requirement is one advantage that northern growers 

 have over more southern producers. Plants are watered regu- 

 larly and fertilized once a week with a soluble balanced fer- 

 tilizer ( 1 00 ppm N). In the second production cycle indoors, 

 plants are spaced 22 in. apart in single rows, with 5.5 ft. 

 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 four 

 primocanes per pot at a 3 ft. 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 out- 

 doors again. Raspberry plants will fruit for three years be- 

 fore beginning a decline. 



If producers do not want to take the time to grow their 

 own plants, fiill-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. 

 If using "long-cane" plants, allow them to acclimate in pots 

 in a cold greenhouse for two weeks before warming the house 

 to 65^. 



Twospotted spider mites thrive in the warm, dry condi- 

 tions 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 con- 

 trols must be used for twospotted spider mites after plants 

 are moved inside. Cool temperatures, high humidity, peri- 

 odic releases of predatory mites, and removal of infested 

 leaves help reduce numbers of pests. 



Experiments 



We are quantifying growth and canopy development, and 

 responses to light, temperature and carbon dioxide, in an 

 attempt to develop a model the describes and predicts growth 

 and yield under various envirormiental conditions. This in- 

 formation will be useful for optimizing greenhouse-grow- 

 ing conditions. Cooperators in Minnesota are examining 

 how changing the environmental conditions after flowering 

 might enhance productivity, and cooperators in Alaska are 

 trying to understand the chilling requirement. 



We are also studying the efTect of initial pot size and 



transplanting on plant growth. Is it worth transplanting one- 

 year-old plants after the first fruiting cycle to save space, or 

 should they be planted directly into their "adult" contain- 

 ers? If planting directly into containers, what size should 

 they be? 



Are "long-cane" plants grown in nurseries in California 

 as good as plants from Washington State? Our trials sug- 

 gest that California plants are somewhat better. 



We have also artificially chilled plants prematurely in 

 coolers in mid-August and mid-September to determine if 

 they would flower earlier than normal. After 8 weeks in 

 coolers, plants chilled in both August and September pro- 

 duced flowers in the greenhouse as early as 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 produc- 

 tion earlier would have several advantages. Raspberry pro- 

 duction would be complete before bedding-plant season be- 

 gins in April, allowing more greenhouse space to be avail- 

 able for use. Also, vents are closed when the outdoor tem- 

 peratures 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. 



Researchers in Ontario are extending the season with 

 primocane-fruiting varieties, moving them inside the green- 

 house in early September before the onset of cold weather. 

 These varieties continue to fruit during autumn, and by prun- 

 ing them, it is possible to obtain continuous fruiting during 

 the winter. However, production then is less than with 

 floricane-fruiting varieties. 



Summaiy 



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 cus- 

 tomer as soon as possible. Most markets for winter raspber- 

 ries are small, so a producer will need to line up and supply 

 a larger number of smaller markets than typical. 



Despite these challenges, the opportunities for produc- 

 ers are great. At this point, there exist only a few winter 

 raspberry producers , so the market is open. Furthermore, 

 the quality that can be produced is very high. Consumers 

 and restaurant chefs are willing to pay high prices for high 

 quality berries in winter. Further research will lead to an 

 extended harvest season, and eventually, to year round pro- 

 duction. 



For more information on greenhouse raspberry produc- 

 tion, visit the website: http://www.hort.comell.edu/depart- 

 ment/faculfy/pritts/greenhouse/Frontpage.htm 



i^ it it ^ it 



Fruit Notes, Volume 65, 2000 



21 



