Issue: July 23, 2005
Pruning backyard grapes in summerQuestion:
I recently moved into a house in Albuquerque and have a number of grape vines that are taking over half of my backyard. The previous owner said that they produce a lot of grapes. I wonder how much I can or should trim the grapes back and when I should trim them. I would like to get as many grapes as possible but limit the size of the plants as much as possible. I also would like to know if the grapes should be thinned in the beginning of the summer as you would a peach or apple tree.
Answer: Grapes should be pruned in the late winter, about a month or less before the last frost. This is because grapes, like roses, can be damaged by a late freeze after new growth has begun. There will be some delay after pruning before the new growth begins, so waiting until just before the last frost increases the chances that the new growth will not be present if there is a late freeze. Some people prune in the autumn, but we may have a prolonged warm period during the autumn that may stimulate growth, resulting in freeze damage. The mid-winter warm-up we often experience may stimulate growth that will be damaged by our inevitable return to cold weather. If the vines are unpruned, the new growth will be stimulated at the ends of what will be cut later so no damage will occur. If they have been pruned, the buds most likely to grow are those you do not want to freeze. Leaving the vines unpruned through the winter protects the plants through the natural process of apical dominance ö the buds at the ends (apex) of the vines have priority in growth and will inhibit growth of buds further down the vine. If there is a freeze, the apical buds can be sacrificed because they will be removed later when the vines are pruned. This protects the lower buds you will want to grow after pruning.
Many local growers do some summer pruning by cutting back the new growth once they have exceeded 2 - 3 feet in length. This directs the carbohydrates (sugars) produced by photosynthesis into fruit production rather than excess growth. Overgrown, unpruned grape vines often produce much less fruit than those that are properly pruned.
If you choose to remove the ends of the current seasons growth (they have probably already exceeded the 2 - 3 foot length), you can induce root formation in the cutting you removed (if the plants are not currently under plant variety patent protection).
I have a seedling Concord grape that I cut back a few weeks ago. I cut a 4 - 12 inch section from the end of several new growths, dipped the basal ends of the cuttings into rooting hormone powder (available in nurseries), then put these cuttings into pots of moist potting soil and covered them with plastic bags in a bright location (not in direct sunlight). After 3 weeks, they had produced roots. You may wish to try this propagation technique with your vines. Do not trim your vines all the way back to the fruit clusters; allow several leaves to remain between the cut and the fruit.
Grapes are an excellent source of late summer fruit. The vines are excellent producers of shade to enjoy if you create a shade trellis rather than a vineyard type trellis. Each spring or late winter you must trim the vines back to the basic plant structure, and each summer the vines will regrow to create an abundance of shade and fruit. Information on proper pruning techniques is available at your local NMSU Cooperative Extension Service office. Ask for publication H-303: "Pruning Grapes to the Four-Arm Kniffin System." You can modify this form of pruning to create a shade trellis if you desire.
With regard to thinning, many grape varieties will not need to be thinned but some may benefit. Thompson seedless grapes tend to remain very small unless some clusters are removed and the berries thinned in the remaining clusters. To determine (or at least estimate) which varieties you have or which you may wish to purchase, you can get publication H-309: "Grape Cultivars for North-Central New Mexico" from your local NMSU Extension office. Both publications are available on the NMSU College of Agricultural, Consumer and Environmental Sciences website as well at http://cahe.nmsu.edu/pubs/_h.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!