Grape Problems: Flowering and Production

Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County

Client's Request. I live in mid-County. I have a controller that waters regularly, but not a lot (once a week deep watering). The grapes don't get as much sun asthey could. South is partly shaded. The soil is amended clay, typical of mid-County. But the grapes are in a coarse gravel amendment as they are adjacent to a massive concrete retaining wall and the backfill gravel. While the grapes have produced moderately in past years, the last several years they have not flowered or produced grapes. What is wrong and what can I do about it?

MGCC's Help Desk Response:  Thank you for contacting the UC Master Gardener Program Help Desk with your questions about grape culture.

Based upon the information you have provided, here are several reasons your grapevines may not flower and produce grapes:

• Grapevines need full sun to activate the flower blossoms. Without sufficient sunlight, the flower buds won't develop properly.
• Pruning the vines incorrectly may affect the blossoms. Most wine grapes are pruned severely in the winter, leaving only short spurs on the woody trunk and main branches. However, some grape varieties, such as "Concord," "Crimson Seedless" and "Thompson Seedless" are cane pruned. These varieties require longer branches, or canes, because the lowest buds may not produce fruitful vines. The non-productive vines should be trimmed back in spring and early summer to prevent them from shading the developing flower buds and blocking the sunlight, thus reducing the number of flowers produced that season.
• Grapevines thrive in well-drained, rich soil. However, if the soil contains too much nitrogen, or if you've over-fertilized, the grapevine may put all of its energy into foliage instead of flowers and fruit.
• Grapevines harbor a variety of pests that can affect the development of flowers and fruit.
• Grape blossoms are mostly wind pollinated, however, varieties that require both a male and female plant require cross pollination to produce fruit. This may be accomplished by wind or insects

Following are several links from UC leading to further useful information about grape culture and pests. These links also cover and expand on the reasons for lack of flowering and production listed above:
http://ipm.ucanr.edu/PMG/GARDEN/FRUIT/grapes.html
http://cagardenweb.ucanr.edu/Growing_Grapes_in_the_California_Garden/

Hope that information helps. Please do not hesitate to contact the Help Desk again if you have further questions.

Help Desk of the UC Master Gardener Program of Contra Costa County (SLH)

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Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer our gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523, although we will be moving this spring. We will notify you if/when that occurs. We can also be reached via telephone:  (925)646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/  MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (//ucanr.edu/blogs/CCMGBlog/)