My grape vine is about 8 years old. Every year the grapes fall before they are ripe, around late July. The plant looks very healthy and has a lot of foliage. It is growing horizontally on several cable lines against a concrete wall. I don’t water and fertilize the plant much except for half a bag or a bag of manure in mid- to late winter.
At first I thought the fruit wasn’t getting enough sun, so early on, I cut all the leaves around the grapes, but they all fell off the vine from mid-July onward.
I prune the grape vine very aggressively in fall, leaving only several year-old branches with several buds on them. The vine is very healthy and grows about 30 feet a season. Could the problem be due to too much foliage?
According to The Grape Grower by Lon Rombough (Chelsea Green, 2002), the premature dropping of fruit is called ‘shatter,’ and excessive shatter can be caused by nutrient deficiencies. It’s normal for a certain amount of unfertilized berries (i.e., grapes) to drop a week or so after the bloom stage, but dropping of fruit in July is a problem.
I wonder if you are giving them overly rich soil (by adding manure). If the manure is high in nitrogen, this encourages a lot of leafy growth, often at the expense of flowers and fruit. I grow grapes and have never fertilized them. The book mentioned above says not to apply fertilizer unless you know there is some kind of deficiency. Note in particular this quotation from the book:
“Excess nitrogen causes flower clusters to ‘shatter’ (flowers fall off), reducing fruit set. In fact, mature vines should not need any supplemental nitrogen when grown in a healthy soil with plenty of organic matter. A good general rule for fertilizing grapes is to use a mulch of well-rotted compost, which will supply small, but regular essential amounts of nutrients.”
Also, the usual time to prune grapes is in mid- to late winter–I do mine in February. Pruning in the fall may be contributing to the problem your vine is having with poor fruit set.