Winter Pruning Workshops
Winter Pruning Workshops Focus on Tree
Preservation From Planting to Maturity
By Chad Giblin
Department of Forest Resources - University of Minnesota
Nearly six dozen arborists, landscape managers, parks workers, and maintenance staff joined University of Minnesota scientists and professional arborists to learn more about pruning trees in January, February, and March 2017. Workshops at both the Saint Paul and Duluth Campuses stressed the importance for timely and effective tree pruning beginning at planting and carrying forward into maturity.
Our Saint Paul workshops brought together staff from many different organizations. In late January, UMN Landcare and the Davey Tree Expert Company joined us for a full-day workshop to cover the basics of developmental and structural pruning during the first 10 to 15 years of life. In March, a second campus workshop provided training for parks and public works staff from the City of Eden Prairie, City of Minnetonka, City of Prior Lake, and Washington County Parks. These workshops provided a great opportunity for a diverse group of practitioners to team up and problem solve pruning challenges. Work focused on a variety species and many different sizes of young trees. Also included was an examination of different work positioning techniques to provide safe and efficient access to young tree canopies.
(Fig. 1 - Using simple rope access equipment and techniques gives arborists an ideal work position to make the best pruning cuts.)
Pruning young trees is really about proactive tree preservation. When the proper pruning dose is applied at the ideal time, care is directed at creating a mature form that is structurally sound and able to resist branch failures during wind, snow, and ice loading events. When trees are smaller than eight inches pruning should focus on the transition from frequent nursery pruning schedules to those that are much less frequent in the landscape. The challenge lies in developing a functional mature crown while maintaining an attractive young form. In many cases a compromise in aesthetics is required to establish a strong canopy.
Proper pruning dose is the volume of canopy that can safely be removed during a single pruning event. Dose is calculated using several factors including tree establishment status, growth rate, inherent resistance to decay, and the time between pruning cycles. A dose that is too low or too high can have equally negative effects on the developing crown in a young tree.
The first step in pruning young trees is to identify a strong central leader. Next, the height of the lowest permanent scaffold branch should be established; this may be variable depending on the species and location. Trees located in public boulevards may require higher permanent canopy than those located in parks or on private property. Branches below permanent canopy are considered temporary and will need to be pruned to a smaller lateral branch to reduce or suppress their growth and eventually thinned at the main stem. All temporary branches need to be managed to keep them less than one-half the size of the stem where they are attached until permanent removal. This will minimize the wound size when they are thinned at the main stem and promote quicker wound closure. The exact size of the pruning cut will vary depending on the tree species and its growth rate as well as resistance to decay.
(Fig. 2 - Young lindens often require several courses of reduction pruning cuts to slow the growth of large temporary branches and codominant leaders.)
(Fig. 3 - UMN Landcare, Davey, and UMN Forest Resources staff at the January workshop.)
On February 17th we returned to Duluth, MN for the Second Annual Northeast Pruning Workshop hosted at University of Minnesota - Duluth. UMD generously provided the Bagley Classroom and Nature Center for our home base while giving access to the entire campus for teaching subjects. Joining the teaching staff this year were Louise Levy of Levy Tree Care, Liam McClannahan of Branch and Bough Tree Service and Landscape Care, and Hannibal Hayes of the City of Minnetonka.
(Fig. 4 - Hannibal Hayes discusses developmental pruning on a young tamarack - conifers also benefit from these techniques!)
Returning to the UMD campus allowed us to really examine how trees have been responding to the pruning performed last winter and give instructors opportunity to discuss and demonstrate proper pruning dose. When approaching medium sized trees ranging from 8 to 12 inches in diameter, pruning cuts should be kept relatively small and in the outer periphery of the canopy. As trees approach maturity their growth rates level off and some species may respond poorly to large pruning cuts, especially when made on the main stem. Knowing tree species profiles and their ability to resist decay is critical in making these decisions. In many cases, if a large branch doesn’t need to be removed for clearance it can simply be reduced to direct resources to higher, more central portions of the canopy.
(Fig. 5 - Liam McClanahan and Hannibal Hayes demonstrate proper branch reduction in a pair of Autumn Blaze Freeman maples.)
(Fig. 6 - Hannibal Hayes and Louise Levy continue work on developing mature canopy in an Autumn Blaze Freeman maple.)
This year we had a great opportunity to discuss and demonstrate techniques for managing large and mature trees. This includes trees in the 20 inch and greater DBH range and those with permanent established canopy. Larger trees really need to be assessed for structural defects that create risk due to targets below. These defects may include decay, branch inclusions, and codominant leaders as well as branch unions that are actively failing. The goal is to prevent or mitigate the effects of failure. Much like the approach with medium-sized trees, pruning cuts should focus on the outer canopy to reduce the sail on weak or defective branches. This can reduce the lever action on poorly attached unions and prevent failure during loading events. Cabling can be used to reduce the load on these unions as well. Liam spent time with the participants discussing options for canopy preservation using both static and dynamic cabling techniques that complement the pruning techniques demonstrated throughout the day.
(Fig. 7 - Liam McClannahan discusses cabling options for mature trees.)
(Fig. 8 - Workshop participants had a chance to try out rope access techniques under the expert guidance of Louise Levy and Hannibal Hayes.)
I would like to thank the Minnesota Turf & Grounds Foundation and the Minnesota Society of Arboriculture for their financial and in-kind support of these workshops. Also many thanks to all of our attendees, it’s been great seeing so many new and returning faces at these workshops! Your attendance makes all of this possible.