The Berry DisPatch

Newsletter of the Wisconsin Berry Growers Association

April, 2002

Raspberry Ramblings

Brian R. Smith

University of Wisconsin – River Falls

Plant Propagation and Forms of Planting Stock Available

The purchase of disease-free, vigorous planting stock form a reputable nursery greatly improves planting establishment longevity, berry yield and quality and the likelihood of receiving true-to-name plants. The best growing conditions or cultivar will not overcome the effects of planting poor quality, diseased plants. "Disease-free" refers to freedom from viruses, crown gall, verticillium wilt, spurblight, anthracnose and all other major diseases. Viruses are the key component in this list. They are the most insidious group of diseases in that they may remain latent or undetectable for long periods of time. Virus symptoms may be present continuously or they may appear only when the plant is stressed, fruiting or in various stages of development. Once a plant contracts a virus, it can not be cured by spraying or culture. It must be eliminated from the planting or virus will spread to healthy plants. Propagating plants from an existing planting allows viruses and other pests if present, to be spread to the new planting and greatly shorten its lifespan.

Reputable nursery companies have set up virus-indexing programs in order to ensure that the "mother" plants they propagate from are virus-free. While young plant stock is grown up to size for sale, strict cultural guidelines are followed in order to reduce the chance of pest invasions.

The two most typical raspberry planting stock forms available in the nursery trade are bare root cane stock and tissue-cultured stock. Bare root cane stock may or may not have been propagated from virus-indexed or tissue-cultured stock plants. One or more growing seasons have passed since the original mother plants were set in the field. The plants one receives are dormant cane stock dug as “suckers” produced from the original mother plants. This is the cheapest planting stock form that can be purchased but also the most likely to already harbor viruses and other pests even though they have been certified by nursery inspectors.

Tissue-cultured plants are almost always propagated from disease free and virus-indexed mother plants. A very small piece of the growing point (apical meristem) or other tissue from the mother plant is placed in a suitable sterile container containing a gel-like substrate that sustains growth and development into “plantlets” with shoots and roots. These plantlets are then removed from the sterile microenvironment and grown in the greenhouse for a few months in order for them to acclimate. If these small plants are sold directly from the greenhouse environment they are usually just designated usually just designated as "tissue culture" or "TC" plants. Some TC plants however, are grown on further outside in pots or directly in the field and eventually exposed to colder temperatures to induce dormancy. When sold, these plants are typically labeled as “nursery matured” (NM). Both TC and NM forms are more expensive than bare root cane stock but are also much more vigorous and the least likely to harbor viruses and other pests. Even though TC and NM require more special care than bare root stock early in the establishment year, they will typically outgrow bare root stock in many instances by fall and will bear fruit earlier and in greater quantities. Additionally tissue-cultured plantings usually have greater longevity than bare root cane stock. All these advantages will likely lead to an easy recovery of the initial extra costs incurred with tissue-cultured stock.

Planting and Care of Raspberry Stock.

Plant bare root cane stock as early in the spring as possible, but not so early that the soil structure will be compromised by excessively wet conditions or there is still plant injury risk from temperatures below 20°F. Both forms of tissue-cultured stock (TC & NM) require special care the first season if they are to survive and flourish. The tender green, actively growing TC stock must be planted after all risk of frost is past in the spring (normally when you would transplant tomatoes in the garden). The NM dormant form can be planted earlier, but should not be exposed to temperatures below around 25°F (still more sensitive than cane stock).

Both forms of tissue-culture stock require frequent, shallow irrigation, multiple light nitrogen applications ( no more than 10-15 lb actual N/A/application), and strict weed control. Tissue culture stock is very sensitive to herbicides, so lower rates must be used more often than with bare root stock. The safest route with TC & NM stock is to refrain from using herbicides until late summer, using shallow cultivation or mulches up until plants start to sucker.

Mulching

Generally, research has indicated that organic mulches are highly beneficial in the establishment year to encourage fast sucker development but in subsequent years, mulches are considered more detrimental. Temporary mulches may be the answer in the second year and beyond in situations where they are applied in mid summer in order to carry plants through typical drought periods. They should be removed by late August in order to facilitate plant acclimation for winter. If raspberries are grown on heavy soils, organic mulches can be applied in late October to early November in order to reduce the incidence of frost heaving. Mulches should then be promptly removed in early spring. If mulches appear to be contributing to excessively high soil moisture conditions on heavier soils, they should be removed. Raspberries are very susceptible to "wet feet" and will contract diseases such as Phytophthora root rot that can kill plantings quite quickly.

Biennial Bearing System For Summer-Bearing Red Raspberries

This cultural system was devised in order to eliminate the need for time-consuming detailed thinning and pruning procedures normally associated with summer-bearing reds. There are several options with regard to this system, but all are based on alternate year mowing off of half of the planting each year during the dormant season. Since only one-year-old canes fruit in summer bearers, only half of the planting will fruit in any given year. Yields are typically reduced at the most 30% versus an expected 50%, since the primocanes that emerge after dormant mowing will grow unimpeded by floricanes. This presence of only primocanes in the half of the planting that was mowed means less overall crowding, and potentially closer row spacings. These closer row spacings translate into even lower yield loss, rendering Biennial System yields nearly equal to the traditional annual system but, also without the need for the hand/detailed thinning-pruning. Some growers will also practice "primocane suppression" (young primocanes are pinched, mowed or chemically burned back) 2-3 times in the year of fruiting in order to further increase yields by preventing competition with floricanes. Yields can actually end up greater than 200% of traditional plantings in this case.

New plantings should go through at least one complete fruiting cycle before the first mowing (end of second growing season). One half of the planting can then be mowed off in a contiguous section or every other row can be mowed off throughout the planting.