This is a good question that everyone should ask yourself towards the end of the growing season. This also varies depending on when you get your first frost of the fall. Here is what I like to warn people of…
When deciding when to do the last pruning of the season, you need to pay attention to when you usually get your first frost. Why I say this, is because when pruning a bush during the growing season, you are encouraging new growth. If you prune within a week or two of getting a good hard frost, you can severely damage the new growth and either make the bush look ugly, or kill off certain branches completely. Obviously, this also depends on what type of bush you are pruning. You should know by now which of your bushes grow the fastest and which are very slow at new growth. Knowing that is crucial to knowing when to do the last pruning of the year.
Here is my solution to this problem. If you need to prune at the end of the season, do so at least a month before the first hard frost. This will allow the new growth to come out and harden up before they get damaged with the frost. Or, you can wait till after that first frost and prune them while they are going dormant. After a good frost, every plant starts going into dormancy for the winter. They release most of their sap out of their branches and into the ground so they can rest and focus on root development. If they are a deciduous plant, this is the time that they drop their leaves, because there is no sap to hold them on.
So by pruning after they are dormant, the plant does not know that it got pruned and will not put off new growth till the spring time and will come back very naturally. Just pay attention to what you are doing, if you give the plant a bald spot, it will be bald until spring! This is also a good time when they are dormant to do any of those ‘hard’ prunings you have been wanting to do. If a plant is way over grown and you want to take it back down to normal size, do it in the winter. The plant in the winter is not depending on photosynthesis to grow strong and will recover nicely in the spring with the new growth.
If you have any questions on specific pruning issues, don’t hesitate to ask me.