Of course, prevention of wounds is the best practice in the first place. There are many ways that homeowners can prevent wounding and decay of urban trees. First, trees should be planted in proper locations, with adequate space relative to other trees or obstacles. For example, trees should be located sufficiently far enough from buildings, sidewalks, driveways, and power lines. Furthermore, ensuring that a tree is planted properly and in a high-quality, well-drained soil will go a long way to minimize wound healing problems. If the site has poor soil, that soil should be amended with composted mulch to improve conditions for the root system and the tree overall. Mulching trees also discourages activity involving lawnmowers and weed wackers, which are common agents of damage at the base of the trunk, since mulched areas are not meant to grow grass.
Pruning and Care of Tree Wounds
Maintaining a healthy looking, yet safe tree is an important plant health care challenge for homeowners. Although trees are strong and durable, they are susceptible to physical injury such as that caused by ice and windstorms, as well as lawnmower and other mechanical wounds. When trees are wounded, the newly uncovered tissue is exposed to canker pathogens and decay microorganisms that infect the tree. The latter microorganisms digest tissues that are responsible for nutrient and water translocation (phloem and xylem) or structural support (heartwood), resulting in unhealthy, unsightly, or unsafe trees.
Despite these limitations, at times homeowners need to prune (and thus wound) trees to maintain aesthetic characteristics, remove diseased or dead limbs, or improve structural stability. Proper approaches to pruning and wound treatment can go a long way to minimize the issues mentioned above. These approaches are based on an understanding of tree biology and the wound healing process.
Callus Formation
Trees attempt to close wounds naturally by forming callus tissue (Fig. 1).
Callus formation rates vary for different tree species and are affected by environmental conditions such as nutrient and water availability. Some trees may never completely close their wounds, depending on these and other factors, like wound size. However, numerous studies demonstrate that favoring callus formation can significantly reduce infection and colonization by decay organisms and other pathogens.
There are several ways that callus formation rates can be enhanced, or at least not inhibited. First, it is essential to avoid limiting oxygen availability to the wounded tissues. Oxygen is necessary for the proper healing process to take place. It thus follows that painting a wound with any kind of material that interferes or impedes oxygen access will delay or even prevent wound closure by callus formation. Thus wound painting is no longer recommended, with one exception: in areas where oak wilt disease occurs, wound paints may be useful in preventing insect spread of the oak wilt fungal pathogen.
Second, trees that are planted in well-drained, quality soils, with good texture, structure, and containing adequate levels of organic matter, grow in a physiologically more balanced fashion that favors the healing process. Thus, when planting trees, homeowners should be aware of soil effects on tree growth and take measures to improve soil quality, for example, by applying adequate amounts of good composted mulch.
In rare cases of extreme soil nutrient deficiency, homeowners may wish to apply mineral fertilizers for rapid, reliable growth responses, including callus growth. However, before fertilization is even applied, care should be taken to ensure that proper application rates are used so that issues associated with over-fertilization, such as burning of roots, are avoided. Furthermore, if fertilizers are over utilized, tree growth rates may be excessively rapid, which may lead to lowered structural stability as well as increased susceptibility to pathogens and pests.
Why Pruning?
Trees are pruned for a variety of reasons, for example, to develop proper canopy shape or remove potentially hazardous limbs. Although the optimal time for pruning varies by region and tree species, the best time to prune many trees is when rates of growth, and therefore callus formation, are the greatest. In Ohio, this means that pruning should take place in the spring to early summer (March–June), although hazardous tree limbs should be removed regardless of the time of year to minimize the risks of injury and damage. There are some caveats to these basic recommendations depending upon the type of tree. Examples include pruning for optimal fruit production and problems for some tree species with any pruning in winter months due to increased chance of winter freeze injury. If significant pruning is necessary, homeowners should check with their local professional garden center, Extension service, or certified arborist for advice.
Main branches emerge from the trunk at various angles. The strongest branches are those that are placed at near 90-degree angles with the main trunk (Fig. 2).
Oftentimes branches form at very acute angles that are nearly parallel to the trunk. Such branches are structurally weak and may need to be removed. Crossed, diseased, dead, or broken limbs should also be removed in the context of a proper tree care program.
If a tree has obvious severe decay, the local certified arborist should be called to assess the situation and determine if the tree needs to be removed altogether.
Should I use a wound paint on pruning cuts?
Should I use a wound paint on pruning cuts? The short answer is ‘No’! 15 years ago the answer may have been yes but there has been much research in this period of time which has definitively concluded that it is not only unnecessary but may prevent the tree carrying out its normal protective processes. In fact, this research started 50 years ago and avoiding the use of wound sealants was first recommended as far back as the 1970s.
Pruning sealants include Mastic gum, and petroleum and asphalt/bitumen products. They can seal in disease, decay, insects, microbes and moisture. Bitumen sealants can heat up the area to which it is applied, damaging the plant tissue beneath or crack open under heat and stress allowing pathogens and moisture to enter. Sealants also interfere with the trees’ compartmentalisation process which allows wound wood to form.
Tree wounds do not heal so do not need a healant. They isolate the damage and grow calluses over wounds, beginning at the outside and growing inward until the wound is covered. This is as true of fruit trees as it is of forest trees. So allow your trees to use their own natural defence mechanisms to protect themselves.
- Our role is to:
use sharp, sterilised tools and clean again before moving on to another tree or clean after pruning a diseased limb - make good clean cuts at the right time of year
- remove stubs and cut as close to the collar as possible while leaving it intact. The collar is the swelling on the the lateral where it joins the main branch or trunk (see photo)
- remove all prunings from beneath the tree (in order to remove any diseased wood that may reinfect the tree)
- use disease-free mulches beneath the tree. Some tree loppers who prune mainly Council trees for safety purposes supply disease-free wood chip.
Many sealants are carcinogens so a bonus is that not using them also protects your health.
Written by Robin Gale-Baker
Pruning Paints Debunked
When my turn comes up to blog for the Garden Professor site I like to reflect on the horticulture in my own gardens and orchard. Right now I am focused on pruning my old apple and stone fruit orchard. It has suffered bear attacks, drought, and mismanagement before we arrived in 2018. The previous owners were very aware of the need to treat pruning cuts large and small. The remnants of tree wound dressings are found all through our orchard and range from white latex paint to silicone caulk. Unfortunately there has never been good research evidence to support pruning paint use. Despite the lack of any published evidence, for their usefulness, pruning paints are still available in garden centers and there are no end of do it yourself preparations that gardeners continue to use on pruning wounds.
So why paint the cuts on your fruit trees after pruning? One idea is to keep the surface protected from infection by pathogens. Plant pathogenic fungi and bacteria can cause disease that may lead to blight, cankers, or wood decay.
Wounds are often implicated in pathogenesis or disease development. Many horticulturists believed that wound dressings provide a barrier to entry of pathogens and insects. Fruit trees are easily decayed by a number of fungi which cause white and brown rots in their wood. Wood decay organisms enter through wounds created when branches break from excessive fruit loads or when pruning wounds expose heartwood or significant amounts of sapwood. So painting cuts became a very common practice advocated by gardening columns and various books over the last century.
Over one hundred years ago Howe (1915) recognized that pruning paints did not help wounds to close, in fact, they retarded the development of callus wood especially in peaches. Howe called into question the necessity of using wound dressings at all. Still the use of wound dressings has prevailed to this day.
Shigo and Shortle (1981) showed that wound dressings do not prevent decay nor do they promote wound closure. If the poor pruning practices that harm trees are abandoned, then wound dressings are unnecessary (never mind that they don’t work). Shigo often maintained that tree genetics determine the extent of decay forming in a given species. His work conclusively showed that flush cuts would lead to more decay than cuts that were made outside the branch collar or bark ridge.
Expanding foam? As far as I know there is no research on expanding foam but lots of anecdotes and observations of how it is often used to fill tree cavities. Filling cavities with cement to prevent or limit decay is a practice that subsided some decades ago and is generally not recommended as part of modern arboricultural practice. By the time decay has caused a cavity it is usually well entrenched in the wood of a tree and is not controlled by filling in the void. The best way to limit decay in trees is to prune them frequently so cuts are never large and the tree (fruit or shade) develops a strong structure that is unlikely to fail.
Chalker-Scott, L., and A.J. Downer 2018. Garden Myth Busting for Extension Educators: Reviewing the Literature on Landscape Tree. Journal of the NACCA 11:(2) https://www.nacaa.com/journal/index.php?jid=885
Howe, G.H. 1915. Effect of various dressings on pruning wounds of fruit trees. New York Agricultural Experiment Station, Geneva, N.Y. Bulletin No 396.
Shigo, A.L and W. C Shortle. 1983. Wound dressings: Results of studies over 13 ykears. J. or Arboriculture 9(10): 317-329.
Shigo, A.L. 1984. Tree Decay and Pruning. Arboricultural J. 8:1-12.
