Plant but don’t prune trees and shrubs in the fall

Fall is a good time to plant trees and shrubs but not, contrary to popular belief, a good time to prune them, a University of Missouri horticulturist said.

“Most people believe spring is the best time to plant a tree, and while spring is in fact a good time to plant, there are a number of benefits to fall planting as well,” said MU woody ornamentals specialist Chris Starbuck.

“The key point to remember when planting a tree or shrub is that for the new plant to survive and grow, it must grow roots out into the ‘backfill’ soil surrounding its original root ball. The faster and more extensively roots grow into the backfill, the faster the plant will become established.” Unlike trunks and stems, roots never go completely dormant but grow whenever conditions are favorable, Starbuck said. “Assuming the tree or shrub is planted into a non-compacted, well-drained, moist soil, autumn soil is warm enough for roots of most woody plants to grow well into the winter. This rooting head start can be a real advantage in years like 2000, when we have a hot dry spring.”

Trees and shrubs generally alternate top growth and root growth, he said.

“When the foliage matures, it manufactures carbohydrates through photosynthesis, some of which are mobilized to the roots. Unless there are other limiting factors, these carbohydrates usually promote a ‘flush’ of root growth in late summer or fall.”

Many homeowners mistakenly believe autumn is also an optimal time to prune trees and shrubs.

In the fall, Starbuck said, “one thing we may see is that some of the plants grew a lot over the summer and have gotten somewhat out of bounds. There is often a strong temptation to get out the pruning saw and start fighting back the jungle. While a fall pruning frenzy may make things look a bit more tidy, you may be killing your trees and shrubs with kindness.”

For several reasons, Starbuck said, “September is the absolute worst time of year to do major pruning of woody plants.” Pruning invigorates woody plants and could stimulate late growth that is susceptible to winter injury. “Even if new shoots do not develop in response to pruning cuts, cambial growth may be prolonged, increasing the likelihood of stem freezing injury.”

In the fall, woody plants begin moving minerals and carbohydrates from the foliage into the stem and roots for storage. “If a significant part of the foliage of a tree or shrub is removed, these nutrients are discarded before they can be stored away for the next season,” he said.

“A third reason to resist the temptation to prune in fall is that pruning wounds will take much longer to callus over than in spring. Pruning wounds are an open invitation to potential insect and disease pests.” A little fall pruning doesn’t hurt, Starbuck said. “You may want to cut back some of the long canes on roses to make them less susceptible to wind damage over the winter. A fall touch-up of the privet or yew hedge will probably not lead to significant winter injury.” Gardeners who yearn to lop off large branches, however, are advised to “avoid the urge,” he said. “Go back in the house and write ‘pruning’ on your gardening calendar for March of 2001.”