This is the time for pruning trees to prepare for new growth

By Ryan Pankau University of Illinois Extension Services

The callus tissue that has formed on this 2-year-old pruning cut, creates a symmetrical-looking “donut”, indicating that the cut was properly aligned and wound closure is progressing. (Photo by Ryan Pankau/University of Illinois Extension)

If you are just dying to get into the garden these days, there is one essential practice that is best done during winter dormancy. Pruning is perhaps the most important landscape maintenance task performed in any space that hosts woody plants. There is a large list of benefits to plant health, which is the most important reason to prune, but it’s often essential to prune woody plants so they maintain the form and structure we expect.

Dormancy is the best time for pruning because it has the least impact on health, which maximizes the benefits of the practice.  There are certainly other times of the year when it’s acceptable to prune plants, but winter is ideal and becomes especially important when plant health is a primary concern.

Making a proper pruning cut is central to the effectiveness of this practice.  Just as pruning during dormancy preserves health, a properly applied pruning cut can save valuable plant resources that are expended on wound closure.

Where to cut

The location of each pruning cut needs to be planned with respect to tree biology and not necessarily the shape of the plant.  Too often I see past pruning cuts that were simply made at a convenient location on the limb.

Such as ground clearance pruning on shade trees where every cut is right above head height.  That makes sense when trying to meet the objective of more height clearance without drastically affecting visual appeal with a big change, but significant tree resources will be unnecessarily tied up in the wound closure response.

All pruning cuts should be located at a branch union or, on smaller limbs, right at a bud.  The reason for this placement is that cells exist at both locations that are ready to initiate growth and close over the wound.

Arborists commonly say, “Trees don’t heal, they seal” to remind folks that trees cannot regenerate tissue like other organisms and their response to wounds, such as pruning cuts, is to grow over the wound and encase the damage in new growth.

Every branch union on a tree has tissue referred to as the branch collar.  On some species, this tissue is easily identified, but the growth habit of some can make it less apparent.  It also tends to be harder to identify on smaller limbs as opposed to large, mature branches.

A properly applied pruning should be located right outside the branch collar, leaving all the collar tissue intact.  This cut alignment can sometimes feel unnatural because the collar is often slightly raised and not necessarily flush with the adjoining limb or trunk.

If left fully intact, the collar tissue reacts to initiate new growth at the cut surface, called callus tissue.  I often refer to callus tissue as the “donut” that forms after a pruning cut.  On a healthy plant, with a proper cut, the donut grows together over time to “seal” over the wound.  This sealing process is important because it blocks out potential pathogens, forming a protective barrier of new wood.  The faster the donut can close, the less chance that harmful, rot-causing pathogens can enter the plant.

However, if any of the collar tissue is removed with the cut, wound closure is slowed.  If too much tissue is left beyond the collar, additional tree resources are needed for additional growth outward to encase any non-branch-collar tissue.

The observation of donuts from past pruning cuts can certainly tell a tale.  If a nicely symmetrical donut forms, it’s a good bet that all the collar tissue was left.  If an uneven donut forms, the area with less new growth indicates that some of the branch collar was inadvertently cut off with the pruned limb.

Ryan Pankau is a horticulture educator serving Champaign, Ford, Iroquois and Vermilion counties