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Monday, March 4, 2013

Spur or Cane Prune My Grapes?

Q. Every year my grapes on my Thompson seedless grape vine are small, about the size of a large pea. The taste is wonderful but I would like to get the berries larger. I understand that grape growers use a chemical to make them larger but I would rather not use chemicals if I don’t have to.

A. Getting better berry size on grapes means you have to reduce the number of total berries on the vine. The berries are small because there are too many berries for the size of the plant.

Grape bunches are triangular in shape.  To get larger berries in a bunch,
remove some of the berries from each bunch. This way each berry gets
more of everything from the vine and get larger. To remove berries, you
can pinch the bottom third off of the bunch when the berries are very small..
the size of a BB. This tends to make the bunch round when it is harvested
rather than triangular. Did you ever seed round bunches of grapes in the store?
This is why.
            The first step is to reduce the total number of grape clusters on the vine. Grape clusters are produced on last year’s growth. If you look at your vine, you will see that last year’s growth will be a lighter brown, or sometimes a reddish brown, compared to older growth.

            Once last year’s growth has been identified, then it must be cut back to reduce the total number of bunches the vine has to produce. If the grape is trellised, it is much easier to see where to cut. If the grape is left sprawling, then it is more difficult.

On a trellised vine, we normally want last year’s growth about 12 inches apart. If you haven’t finished your grape pruning, you still have time. You should be finished before you see new leaves emerge.

            There are two ways of pruning grapes; spur pruning and cane pruning. Spurs are created by cutting back last year’s growth so that only one or two buds remain. This means last year’s growth is reduced to an inch or less in length.

This is a grape spur. This spur had two buds. You can see the growth from
the buds. This will produce two or more bunches of grapes. If you are really
daring you can prune to leave only one bud and shorten the spur even more.
This type of pruning, spur pruning, is done to most grapes. However, if you spur prune a Thompson seedless grapevine, it is very possible you may lose most of your fruit. This is because the first ten buds or so on last year’s growth will not produce grape bunches. They are fruitless. In Thompson seedless, spur pruning cuts off buds that produce grape bunches.

Thompson seedless, along with Black Monukka, should be cane pruned, not spur pruned. Canes are just extra-long spurs. Where spurs are pruned so that only an inch or less of new growth remains (one to two buds), canes are pruned long enough so that you have ten to twelve buds remaining on this super long spur.

This “extra long spur” is no longer called a spur anymore. It is now called a “cane” due to its length. Once the grape plant begins to flower in the next couple of weeks, you must then reduce the total number of grape bunches. This is called “balancing the fruit load”. Balancing the fruit load really means reducing the total number of grape berries so that the leaves can produce enough sugar and energy to make the berries that remain, larger.

The number of berries are reduced primarily by two methods; reducing the total number of grape bunches and reducing the size of the bunches. When the berries are very small, the size of a BB, bunches are removed so that the remaining bunches are large and spaced about ten or 12 inches apart.

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