The time to prune your Grapevine trees depends on why you want to prune. Light pruning and removing dead wood can be done at any time. Otherwise, there are specific times of the year in which you want to prune your trees to accomplish certain things.
Pruning when the trees are dormant is the most common practice. This practice is usually done just after the coldest part of the winter. The results of pruning during dormancy produces new accelerated growth in the spring. This would be typical when you have a newer tree and want the tree to grow more efficiently or if you want to increase shade in your yard.
If you are experiencing early stages of overgrowth in your trees or you want to slow the growth of your trees, pruning should be done just after the seasonal growth is complete. The reason this slows the growth of the tree is that you reduce the total leaf surface which reduces the amount of nutrients sent to the roots. Another reason to prune just after the seasonal growth period is to remove defective limbs that can be noticed easier during this period and limbs that hang low due to the weight of the leaves.
Fungi spread their spores immensely in the fall and when you make cuts to your trees, they will heal a lot slower due to the spread of fungi than during this time. A good analogy to this effect is when you cut yourself and have an open wound. You have to keep the wound covered so that bacteria does not get in resulting in an infection. This is similar to cuts made on your trees so it’s better to make these cuts during times when fungi spores are low.