At Cornell University in Ithaca, New York, researchers have been working on a production system for producing high quality, premium raspberries for the off-season market. In the Northeast, greenhouse growers often have crops that will take them from the late spring season with bedding plants, all the way through the early winter months with poinsettias. After the greenhouses are emptied in December, these greenhouses may sit empty until spring when bedding plants are once again started.
            With winter raspberry production, a grower is able to keep their greenhouse in production, thus capitalizing on a time when fresh raspberry fruit is lacking locally, and also providing further profit for themselves and their families during a period that otherwise would be a downtime for the greenhouses.
            With the use of fall bearing raspberries, different cultivars and storage coolers, year around production of raspberries is possible in the greenhouse as well. Over the past several years, many growers in locations all across the country have started a greenhouse raspberry operation.
            Field production of raspberries has certain considerations for management of plants and pests that differ from winter production, but many aspects of the production methods are similar. Both production approaches will be discussed in this production guide.
            Fresh, high quality raspberries are not readily available in the United States in winter because no significant domestic production system has been developed. Raspberries available in grocery stores during the winter months are mostly grown in Mexico, Central and Southern America.
            The raspberries that finally arrive are very expensive and often of poor quality due to very delicate fruit that is shipped long distances. Fruit quality begins to degrade quickly after harvest and is often damaged in the shipping process. The fruit available in the grocery stores are often well into the process of decomposition and may be moldy.
            The fruit industry has tried to avoid this problem by harvesting the raspberries before they are completely ripe. Even though this will help to preserve the physical appearance of the fruit for display, the fruit is often lacking in flavor and the sugar content is extremely low because the ripening process is brought to a halt when the fruit is picked. The flavor quality of the fruit will not improve once the fruit has been picked.

Characteristics of greenhouse production of raspberries:
• The grower is able to provide the consumer with a high quality fruit year around that will both look good and offer excellent flavor.
• Compared to field production, greenhouse-produced berries are larger, firmer and much less prone to fruit rot.
• The grower is able to achieve a high level of quality because the fruit never becomes wet from rain or irrigation and thus greatly reduces the instance of fruit rotting infections, and the fruit can be harvested at the peak of ripeness for optimum flavor.
• The storage and shelf life of greenhouse raspberries under refrigeration is greatly increased because the fruit has been kept dry, and therefore, fruit rotting infections are much reduced.
• 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; varieties that do well in the field will not necessarily perform well in the greenhouse.
• 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.
            This production guide is intended as a comprehensive resource for both the novice and experienced grower to attain a successful and profitable enterprise of growing fresh, high quality raspberries for the off-season market. Included is information on greenhouse preparation, plant selection, planting, insect and disease control, nutrient and moisture management, harvest, economics and marketing. A glossary, a list of supplementary materials, and general references are located at the end of the book.

Biology of the Cultivated Raspberry
            Cultivated raspberries are of two basic types: primocane and floricane fruiting. The primocane fruiting types produce fruit on the cane tips on the first year’s growth, and a smaller crop lower down the cane in the second year.
            After the second year’s crop, the canes will die and can be removed, as new canes will continue to grow from the perennial root system. Floricane fruiting types produce fruit on the lateral branches that emerge from axillary buds on a second year cane.
            The cane will grow the first season (primocane), go through a winter dormancy period, and after the dormancy requirement has been fulfilled, the lateral buds will break and produce flowers that when pollinated will produce fruit. Similar to the primocane fruiting types, after the floricane fruiting types have produced their crop, the canes will die and can be removed to make room for the new canes arising from the root system.
            The canes typically exhibit an S-shaped growth curve, but the environment can modify this. First year canes grow rapidly after dormancy, but during hot conditions, extension growth slows. If adequate moisture is supplied, elongation can increase. Nodes on first-year canes form at a constant rate over time, and the variation in the growth rate of the cane leads to variation in internode length along the cane.
            The cane typically has short internodes at the base and tip, with long internodes in its midregion. Flower-bud initiation usually occurs under short days and cool temperatures, but with some varieties, initiation will occur once the cane reaches a certain height regardless of day length. If initiation occurs before the first year canes stop growing, the primocane fruiting trait occurs.
            Fruiting lateral branches (laterals) elongate rapidly after bud break, and continue to extend until the terminal fruits begin to form. The flower buds on the laterals may or may not develop into fruit depending on cane vigor and weather conditions in the fall during flower bud initiation.
            A vigorous cane and a mild fall result in more flowers per bud (node) and more nodes with flowers. Thirty or more flowers may be produced on a single flowering lateral. Most buds that reach a diameter of 2 mm continue to develop, set and mature fruit.
            Insect pollination is essential for good fruit set. In the absence of pollinators, 80% lower drupelet set can occur. Raspberry flowers produce copious quantities of nectar, which attract pollinators. The stigmas in the flower buds remain receptive for only about six days, so it is important to have the pollinators available in time to set the fruit.
            The structure of the fully developed fruiting lateral varies depending on its position on the cane. On untipped fruiting canes (canes that have not been pinched or trimmed back to a given height), the laterals at the tip are short, have few nodes and bear a low yield of small fruits. They become progressively more vigorous with higher yields over the middle two-thirds of the cane, although yield of the bottom laterals on the cane is decreased.
            Yields may be greatest in the mid-section or in the top section of a tipped cane, depending on the cane height. Short canes with many nodes tend to have high yields on the laterals in the mid-region, whereas tall canes with few laterals and long internodes tend to produce high yields at the top of the cane.
            Increased cane diameter has been associated with increased yield. However, fruit numbers differ only slightly between thin and thick canes after they have been tipped. Thicker canes generally have more berries per lateral but fewer laterals per cane than thinner canes. Thicker canes tend to be taller, and tend to take longer to break dormancy than thinner canes.
            Fruits take between 30 and 45 days to develop from flowering, depending on the cultivar and the environment. Most of the increase in fresh weight takes place in the last 7-10 days of development. When the fruit becomes overripe, some of the weight is lost.
            The yield of fruit increases rapidly during the first few harvests to a peak, and then declines slowly although the size of the peak and rate of decline varies with cultivar. Individual fruit weight remains fairly constant through the major part of the season and then drops towards the end.
            Next month we will provide you with more details about winter raspberry production.