Gabbi Edwards
Urban Forestry Specialist
Iowa State University Extension and Outreach
With the unseasonably warm weather we have been experiencing this winter, you might be dreaming of spring tasks to get a head start on: what vegetables should you plant in your garden, what about that fire pit you have been meaning to build, maybe you should re-stain the deck? Along with the excitement of an upcoming spring, another topic that should be on your mind is pruning your trees and shrubs. Pruning trees and shrubs is important for safety and overall appearance and health.
Pruning Trees
Late winter is the optimal time to prune trees and shrubs since the plant is dormant and there is less chance for disease and pest damage. Late winter is December-mid March; it is not quite too late as long as pruning is completed as soon as possible. Pruning in March, especially with such warm weather, will produce heavy sap flow in maples, walnuts, and birches. This heavy sap flow will not damage the tree but it will be unsightly and may produce some mold on the sap stream; thankfully it is not warm enough to have active insects which could be a potential disease vector attracted to the heavy sap flow. It is best to prune maples, walnuts, and birches in late August-early October to avoid excess sap flow.
Another species that requires special consideration is oaks. Do not prune oaks during active picnic beetle months. Picnic beetles (nitidulid family) spread oak wilt which is a devastating disease resulting in death within 1-7 years. An easy way to verify if picnic beetles are active is to pour a little bit of beer into a bowl and leave the bowl outside for 15-20 minutes. If there are small beetles in the bowl after 15-20 minutes avoid pruning oaks. As we move further into March, be extremely cautious about pruning oaks; it is better to wait a year and prune in the late winter than to risk oak wilt infection.
Which limbs to prune?
Selecting which branches to prune can be confusing. Below is a prioritized list of pruning to help assist in deciding which branches to prune.
Multiple leaders: It is important to overall canopy structure to have only one main vertical stem (central leader) on every tree, with the exception of some shrubs with multiple stems. Retain the strongest, straightest, most central stem and remove any other competing branches.- Dead or dying limbs: Dead or dying limbs act as pathways for decay, disease, and pests to affect the main stem of the tree. These branches also pose a potential safety risk.
- Rubbing or crossing limbs: Branches that are rubbing or crossing will create weak points in the canopy over time. Branches that rub will grow together forming a weak union that could fail and fall at any time.
- Obtaining proper branch height: Due to the activity in yards and the overall structure of hardwood trees, lower branches are removed over time. Envision the tree several years down the road; will a branch that is three feet off the ground be desirable in fifteen years? Lower branches need to remain for the first 3-5 years after planting, as these branches are crucial for stem development and growth. Wait to remove these lower branches until you can see a flare at the bottom of the stem near the soil. Remove lower branches when they are between 1” and 2” in caliper.
How much pruning should be completed in one year?
Remove no more than 25-30% of the total canopy in one year. Take a step back, view the canopy as a whole, and then visualize 25-30%. Removing more than 30% of the total canopy of a tree in one year can greatly stress the tree and affect nutrient storage and production. The only exceptions to this rule are when there is a significant amount of dead or dying limbs that need to be removed for safety concerns, or when pruning fruit trees. Fruit trees used for production purposes can be pruned more heavily to promote fewer but larger fruit.
What is the proper way to prune a tree?
The first and most important part of pruning a branch is to identify the branch collar. This is the small swollen joining tissue between the branch and the main stem. The branch collar contains the callus tissue that will seal the wound and prevent disease and decay. The branch collar usually ceases at a 30-60° angle; it is critical not to cut this tissue. For aesthetic purposes, most people want to prune a branch off parallel to the main stem but this straight, flat cut will remove most or all of the callus tissue and the wound will not heal properly. Use a clean, sharp pruning hand saw to make all of the cuts. When pruning a branch from a tree, use the three-cut method.
Make a small cut, several inches up from the branch collar on the underside of the branch. This first cut should be approximately ¼ of the way through the branch. This initial cut will prevent any bark from tearing off along the underside of the branch during removal of the heavier outer portion.- Complete a removal cut outside of the first cut. From the top of the branch down, cut all of the way through the branch, removing the majority of the branch.
- Cut the remaining branch stub away from the main stem, just outside of the branch collar. Be extremely careful not to cut into the branch collar tissue. It is sometimes easier to use a permanent marker to draw out where the final cut should be creating a visual line for the cut.
Should wound dressings be used after pruning?
The use of wound dressings was quite common in the past, however wound dressings are no longer recommended. The chemicals found in wound dressing treatments cause harm to the tree. If the pruning cut was correct and the callus tissue remains intact the wound should heal well on its own. The only time wound dressing is recommended is if oaks must be pruned in the summer. Since of the high risk of oak wilt transmission during the summer, it is better to artificially seal the wound. Apply a coat of latex-based interior paint to the wound; this paint will cover the wound so that beetles cannot transmit oak wilt, but will wear off by winter and allow the tree to start the natural healing process.
Pruning Shrubs
Pruning shrubs requires different techniques and timing compared to pruning trees. Keep these differences in mind when pruning shrubs. The timing of shrub pruning is dependent on the type of shrub, bloom time, and overall condition.
Spring-flowering shrubs: prune healthy lilacs and other spring-flowering shrubs immediately after flowering. Pruning after blooming allows for enjoyment of the flowers but also stops seed production, which retains energy for new growth the following season. Unmaintained or unhealthy spring-flowering shrubs require extensive pruning, often foregoing flowering for 2-3 years, in late winter to early spring (Feb-mid April).
Summer-flowering shrubs: Prune in late winter to early spring (Feb-mid April). Since these shrubs bloom from current year growth, they will still bloom that summer.
Deciduous shrubs without desired flowers & evergreen shrubs: Prune late winter to early spring (Feb-mid April).
What is the proper way to prune a shrub?
There are two different shrub-pruning techniques depending on the condition of the shrub. Renewal cuts are best for standard trimming and pruning of healthy shrubs to promote vigorous growth and good quality shape and flowers. Utilize rejuvenation pruning on unhealthy, unmaintained or overgrown shrubs.
Renewal pruning: renewal pruning occurs over three years, cutting away one-third of plant material each year. In the first year cut the oldest one-third of stems of back to ground level in March to early April. The following year cut half of the remaining old wood (1/3 of beginning shrub mass) back to ground level. The third and final year, remove all remaining old wood back to ground level. Throughout the three years, thin any new shoots to the preferred size and shape. At the end of three years, the shrub consists of all new or young wood, renewed for future growth.
Rejuvenation pruning: rejuvenation pruning is a severe form of pruning used to revitalize unhealthy shrubs. During March to early April, trim all stems back to approximately four inches above the ground. This will result in vigorous sprouting that growing season. In the following late winter, retain several strong shoots based on preferred size and shape and remove all other shoots to ground level. This is a great method to turn an overgrown shrub into a trainable young shrub and does not require removal and replacement.
Pruning trees and shrubs improves appearance and health while also lowering hazard potential. Get out and prune your trees and shrubs today or make plans to remove branches next year. If you have any questions or concerns, please contact Gabbi Edwards at (515)-294-1153 or gedwards@iastate.edu.