Q: In last week’s article, you commented that one of the factors of plant deaths over the winter is the timing of fall pruning. I thought we were supposed to prune our trees and shrubs in the fall: Can you go into more detail on when we are to prune trees and shrubs?
A: Since we are still in spring, let’s start there. We prefer to prune spring-blooming trees and shrubs in the first few weeks to two months after they bloom. We can prune them while they are blooming, and in some cases, a few branches can be pruned early to allow the flowers to open up indoors in a vase.
The reason we prune at this time is that these plants develop their flower buds in the summer and fall. Any pruning in the fall or winter will cut off next spring’s flower buds. Plants, such as lilacs, that have flower buds on the ends of their branches can lose all their flower buds if pruned in the fall.
The exception to this spring pruning is fruit trees. We want to encourage growth on some branches and not others to maximize fruit production, so most fruit tree pruning is done in late winter, just before spring growth.
Some summer-blooming plants bloom on branches produced the previous year and are best pruned after blooming. Some hydrangeas and clematis fit in this category. Other summer bloomers produce flowers on new branches that grew during the spring. They can be pruned right after blooming or during the winter and even early spring, since all of the new growths will produce flowers.
Fall-blooming trees and shrubs produce flowers on new growths and can also be pruned in the winter and early spring, as all of the new growths will produce flowers.
During the spring and summer, plants produce many chemicals, including sugars and carbohydrates, that are consumed during the winter and used to start next spring’s growth. Fall pruning that removes larger branches takes away these stores of plant food.
During the fall, plants stop producing chemicals and start storing them, about six to eight weeks before the average killing frost. The plant is going into what appears to be a rest period, but it is still actively moving chemicals from the leaves into branches and roots. The woody tissue in the branches must mature and harden before a frost arrives.
Pruning during this time can cause the plant to begin growing again to replace the missing branches. The new growth will be too tender to survive freezing weather. Cold weather will prevent the plant from healing the wounds.
Even if the pruned plant doesn’t send out new growths, it may not properly move from the rest stage into a dormant state. If it isn’t dormant when the coldest temperatures arrive, the plant may lose branches or the entire top may die.
Once plants are dormant, they cannot begin growing again until specific environmental conditions are met. For some plants, a specific length of dryness or darkness must be met. For the plants we are talking about, specific amounts of cold are required. This is called a plant’s chilling requirement. It is actually a bit complicated, but one method of measuring a chilling requirement is simply to total the number of hours below 45 degrees Fahrenheit.
Once a plant has met the chilling requirement to break dormancy, it can begin growing in the spring. Warm winter days above 60 F are great for humans and show a promise of spring, but a plant that has met its chilling requirement may begin to grow a little on that day. If the next night or following days are very cold, the plant will not go back into dormancy and it can have freeze damage.