Mark Gaskell, Ph.D., Small Farms and Specialty Crops Farm Advisor


From our Central Coast Agriculture Highlights newsletter -- February 2000:

PRUNING SOUTHERN HIGHBUSH BLUEBERRIES

New plantings of southern highbush blueberries are continuing in California and timely and careful pruning is important to blueberry productivity. Trials over the past several years indicate that the southern highbush blueberries in mild growing areas of California, in general, act differently from where they have traditionally been grown in the Southeastern U.S. The majority of the southern highbush varieties will stay green over most or all of the winter months depending upon conditions. Most varieties show little tendency to enter into dormancy in milder coastal regions where the temperature rarely drops below freezing. And in mild years, the flowering and fruiting process may continue through the winter months even though the plants may not accumulate enough heat to ripen the fruit during the winter months. Still, the winter months are the best time for an annual pruning to develop overall plant shape and to minimize over-bearing.

Value of Pruning

Pruning is important in blueberry establishment and maintenance for several reasons:

After the blueberry planting comes into production in year three, it is still important to prune once or twice per year. This will open the canopy of the plant to allow light and ventilation to reach the inside of the plant. This will encourage fruiting in the inner part of the plant and reduce pressure from foliar diseases. It is also important to eliminate smaller,horizontal branches, which produce few fruit and are more difficult to pick.

The pruning process generally encourages development of four to six larger erect canes, which form the skeleton of producing branches. The continual process of renewal through pruning assures an adequate number of producing canes to maintain production year after year. Pruning also reduces the number of flowers and developing fruit to improve berry size and hasten fruit development.

Pruning Technique

Pruning with newly established plantings will primarily be directed at shaping the plant, eliminating fruiting, and encouraging new vegetative growth and vigor. With very young plants pruning should be aggressive to attempt to stimulate new growth. The winter months are generally the best time to do a major pruning, but with younger plants it may be done any time. Plants may be fully green and flowering and fruiting during the winter months or they may be semi-dormant or dormant. Varieties which are tending to dormancy will have leaves changing red to purple, and this is normal.

The goal overall, beginning with the earliest pruning, is to develop an upright, open cup shape to the plant. Leave two to four larger, main canes and prune off all branches crossing through the center of the plant and all low-growing branches. Each year the pruning process will attempt to recover this cup shape and eliminate weak and spindly branches. In early years pruning should be more aggressive. You may need to remove one-third to one-half of the plant to achieve the desired form and stimulate vegetative growth. As new, larger canes emerge, these should be encouraged and smaller canes eliminated. Weak and spindly plants require more pruning than more vigorous plants to encourage new vegetative growth. The Misty variety has a reputation for over-producing in the earlier years, and if it is not aggressively pruned, to encourage new vegetative growth, the heavy fruit set will seriously weaken the plant.

A second, lighter pruning following the first production cycle in the early summer is primarily directed at heading back producing branches and eliminating branches that have borne fruit. This will encourage the development of lateral vegetative branches and encourage new flower buds and fruiting. We are still determining if pruning management can be manipulated to adjust the flowering schedule to alter the late season production pattern.

In more mature plantings, pruning will serve to maintain plant shape, open up the central part of the plant, and to thin somewhat the flowering and fruit set. Blueberries have a tendency to over-produce, and the thinning will encourage larger and earlier berries of higher quality. The thinning process will set a compromise between total production and fruit size and quality. Earlier fruit will be set on smaller, slender lateral branches, so if earliness is important, a greater proportion of these branches will be left unpruned.

After three or four years, individual older, larger canes should be completely removed down to the ground to encourage development of new fruiting wood. This typically begins in year four or five and continues by removing one or two canes each year. The canes become increasingly branched over time, and the numerous small branches are less productive. As plants mature in years six through ten, relatively less pruning will be required. Pruning of more mature plantings will be directed at cane renewal and keeping the center of the plant open.

There are differences in vegetative growth, flowering, and fruiting cycles among varieties, and these differences may be exaggerated by temperature and cultural practices. These cycle patterns will also vary from area to area and year to year within a growing area. These suggestions are meant to serve as guidelines to southern highbush blueberry pruning management. Experience will be the best guide in a given area.

There are videotapes on blueberry pruning that may be ordered. Contact me for further information or for answers to specific questions.

Additional References:

Commercial Blueberry Production in Florida by Jeff Williamson and Paul Lyrene Publ. # SP179 available from IFAS Publications, University of Florida, P.O. Box 110011, Gainesville, FL 32611.1-800-226-1764.

Commercial Blueberry Culture, Circular 713, available from Publications Office University of Georgia, Athens, GA.

Highbush Blueberry Production, PNW 215, available from Publications Orders, Agricultural Communications Administrative Services A422, Oregon State Univ., Corvalis, OR 97331.1-503737-2513.

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