August 18, 2017

Extend the Raspberry Season with High Tunnels

by Aimee Talbothigh tunnel raspberry

Raspberries are a common and popular fruit for fresh eating. Raspberries are also an ideal crop to produce for fresh markets or value-added products, as they are easy to grow, produce a crop by their second year, and continue producing for up to ten years. They have the potential to be a very lucrative crop in Minnesota. The average revenue from raspberries is $10,500 per acre (Lewis, n.d.) with the potential to earn more by implementing season extension techniques.

Raspberries that bear fruit during the summer are known as floricane-fruiting raspberries while ever-bearing types or fall-bearing types are known as primocane-fruiting raspberries. raspberry plant diagramSummer production is relatively short but season extension techniques, such as the use of high tunnels, can extend the growing season in the spring and into the fall. Some parts of Minnesota, particularly those in the northern half of the state, often pose extra threats to raspberry production due to the risk of early killing frosts in the fall. Furthermore, the newly invasive fruit fly, spotted wing drosophila, is also an economic threat to fall-bearing raspberries with the potential to decimate a crop. Thus, fall-bearing raspberries in Minnesota may significantly benefit from high tunnels. High tunnels can extend the harvest in the fall by 3 to 4 weeks (Hanson et al., 2013; Yao et al., 2011), which would allow growers to sell their fruit during the off-season when the fresh market value is higher.

Studies have shown that primocane raspberries grown in high tunnels grew taller and produced more branches in addition to producing a higher yield and better quality berries (Hanson et al., 2013; Rogers et al., 2015; Yao and Rosen, 2011). Other benefits of high tunnels include wind and rain protection, and additional protection from extreme temperatures during the winter. There is also decreased disease and pest pressure due to the protective nature of the tunnels (Hanson et al., 2013; Hanson et al., 2011; Yao and Rosen, 2011).

High Tunnels

So what exactly are high tunnels? High tunnels are a type of unheated greenhouse. Though, in cold climates supplemental heating from small heaters may be utilized. High tunnels can vary in size, design, and cost. The design is usually simpler than a greenhouse, consisting of large hoops covered with plastic. The sides and ends can be rolled up to regulate the temperature and moisture inside the tunnels. In the summer, sides are often kept rolled up during the day and night, but in the fall, the sides are rolled down at night to keep in heat. In Minnesota, the plastic is often removed before winter due to our heavy snow loads. High tunnel raspberries and blackberries by Heidenreich et al. (2012) has in-depth information about tunnel types, costs, construction, and maintenance.

high tunnel styles

Four sizes and styles of high tunnels. On left is a cathedral style, while the rest are quonset style in various sizes. High tunnels can have soft ends made of plastic (right), or hard ends with plywood panels (left) to provide extra strength.

Raspberry Establishment and Maintenance

The same soil and nutrient requirements apply to high tunnel production as to field production, and site selection is very important. Be sure to test your soil for nutrients and pH before planting your crop. Raspberries require rich, well-drained soils with pH between 6.0 and 6.5, good air circulation, and full sunlight. An obvious additional requirement of high tunnel raspberry production is irrigation since the plastic excludes precipitation. Most high tunnel production utilizes drip irrigation and often fertilizes the plants by fertigation at the same time. 1 to 1.5 inches of water weekly and 100 ppm N twice weekly should be supplied to plants if needed. Additionally, raspberries require pollination to set fruit. This means that you must keep the sides or ends rolled up in order to allow pollinators access during flowering.

Pre-plant preparation includes working compost or mulch into the soil to build organic matter, planting cover crops to suppress weeds and build organic matter, and addition of fertilizer if needed, which can be determined by taking a soil test. Ideally, these should be performed at least one year in advance of raspberry planting. It is important to note that if you plan to use high tunnels for raspberry production, it is much easier to plant raspberries with the anticipation of using a high tunnel rather than retrofitting a high tunnel over pre-existing raspberry rows, because raspberries in high tunnels are closer together. High tunnel raspberries should be planted 2 ft apart within rows and 6 ft between rows.

Additionally, raspberries need a strong trellis support and yearly pruning. A V-shaped trellis is common. Trellising must allow the entire plant canopy access to sunlight and good air circulation to decrease the risk of disease. Pruning for fall-bearing raspberries is done before new growth in the spring, usually in March, by cutting the canes down to the base.

Once established, raspberries can compete with weeds, but extra weed control must be practiced in the first year after planting when raspberries are not yet very competitive. Hand pulling is effective if the weeds don’t get out of control. A layer of mulch around the raspberry bases will also help to suppress weeds. OMRI-approved herbicides may also be used.

Cultivars

Primocane cultivars that are winter hardy enough to grow in Minnesota include ‘Caroline,’ ‘Autumn Britten,’ ‘Autumn Bliss,’ ‘Heritage,’ and ‘Polana.’ See Figure 2 (adopted from Tepe and Hoover, 2015). This is not an exhaustive list but they are cultivars that have been tested and performed well (Hanson et al., 2011; Yao and Rosen, 2011). If you have known diseases in your fields, be sure to select resistant varieties in order to minimize the risk of disease.

Figure 2. Primocane cultivars recommended for high tunnel production in Minnesota.

Cultivar Zone 4 hardiness Zone 3 hardiness Ripening period
Autumn Bliss Good Fair Early
Autumn Britten Very good Good Early
Caroline Good Fair Late
Heritage Very good Good Late
Polana Good Good Early

Pest Management

Pests that may infest raspberries include tarnished plant bugs, sawflies, Japanese beetles, aphids, and spider mites. These pests are usually not severe enough to cause a substantial problem. Many can be controlled with OMRI-approved pesticides or biocontrol. For example, predatory mites can be released to control spider mites.

However, the newly invasive fruit fly, spotted wing drosophila (Drososphila suzukii or SWD), can cause great economic damage. SWD is invasive species from Asia and was found for the first time in Minnesota in 2012. Unlike other fruit flies that lay eggs in overripe fruit, SWD females have a large serrated ovipositor that allows them to lay eggs in ripening fruit. The eggs then develop into the larvae that use the berries as a food source. Raspberries are especially susceptible to SWD infestation due to their soft skin. SWD develop rapidly, so multiple generations occur within one growing season. They generally begin to appear in July and peak when fall-bearing raspberries are ripening, thus posing a significant problem.

Organic growers are limited in SWD management options. Currently there are no known cultivars that are resistant to SWD, and efficacy of OMRI-approved pesticides such as spinosad and pyrethrins is limited (Hanson et al., 2013). There is also no known biological control. Practicing good sanitation (i.e. removing all dropped fruit and stripping plants of all berries) may help to reduce SWD numbers.

Weekly scouting is key to an effective pest management strategy. SWD look very similar to the common fruit fly, but SWD acquired their name because the males have a black spot on the tip of each wing (see Figure 3; photo credit: Isaacs et al., 2010), which common fruit flies do not have. SWD females lack these black spots but are identifiable with a microscope by their prominent ovipositor (see Figure 4; photo credit: Isaacs et al., 2010).

High tunnels can offer additional protection from SWD by physically excluding them. Rogers et al. (2015) found that high tunnels covered in plastic with ends enclosed by 80 gram insect netting and entire tunnels covered with 80 gram insect netting significantly decreased levels of SWD infestation (2% infested fruit and 35% infested fruit, respectively) compared to uncovered control plots (81% infested fruit) and insecticide-treated open plots (60% infested fruit). However, Rogers et al. evaluated mini high tunnels that were naturally vented from the insect netting and did not need to ventilate them. Issues with high tunnel ventilation may arise because the sides often need to be rolled up. It is possible that sides and ends can be open until plants develop green berries (generally in August) after which they should be protected from SWD. More research must be done. High tunnels alter the microclimate and may create an unfavorable environment for SWD development and thus may a viable and complementary management strategy for SWD.

UMExtensionVertMVisit University of Minnesota Extension for more information on fruit production and season extension techniques.


References

Hanson, E., Von Weihe, M, Schilder, A.C., Chanon, A.M., Scheerens, J.C.  2011. High tunnel and open field production of floricane- and primocane-fruiting raspberry cultivars. Hort. Tech. 21: 412-418.

Hanson, E.J., Gluck, B.I., and Schilder, A. 2013. High tunnels for organic raspberry production in the Midwestern US. Acta Hort. 1001: 73-78.

Heidenreich, C., Pritts, M., Demchak, K., Hanson, E., Weber, C., and Kelly, M.J. 2012. High tunnel raspberries and blackberries. Cornell Dept. of Horticulture Publication.

Isaacs, R., Hahn, N., Tritten, B., and Garcia, C. 2010. Spotted wing drosophila: a new invasive pest of Michigan fruit crops. MSU Extension Bulletin E-3140. Accessed on 5 Nov. 2015 at http://www.ipm.msu.edu/uploads/files/E-3140.pdf

Lewis, J. n.d. Keys to successful organic raspberry production. Perennia. Accessed on 4 Nov. 2015 at https://acornconference.files.wordpress.com/2012/12/keys-to-successful-organic-raspberry-production_small.pdf

Rogers, M.A., Burkness, E.C., and Hutchison, W.D. 2015. Evaluation of covered tunnels for management of Drosophila suzukii in fall-bearing red raspberries: potential for reducing insecticide use. Submitted to Journal of Pest Science.

Tepe, E.S. and Hoover, E.E. 2015. Raspberries for the home garden. University of Minnesota Extension. Accessed on 4 Nov. 2015 at www.extension.umn.edu/garden/yard-garden/fruit/raspberries-for-the-home-garden/

Yao, S. and Rosen, C.J. 2011. Primocane-fruiting raspberry production in high tunnels in a cold region of the upper Midwestern United States. Hort. Tech. 21: 429-434.


Written in partial fulfillment of requirements for HORT5031, Fruit Production and Viticulture for Local and Organic Markets.