Site icon Deep Green Permaculture

The Truth About Pruning Sealers: Do They Help or Harm Trees?

Angelo (admin)
pruned cherry tree trunk

Pruning is a common practice that’s used to maintain the health, shape, and productivity of trees. However, it creates wounds that can expose trees to infections and pests. To protect trees, it was once standard practice to paint fresh pruning cuts and bark damage on trees with a bitumen-based pruning sealer. This was done to prevent bacteria and fungi from entering the wound and causing decay, and to also create a barrier against infestations of pest insects such as borers.

The practice has largely fallen out of favour in the arboriculture world after certain research in the 1980s concluded that pruning sealants were not particularly helpful. The study “Wound Dressings: Results of Studies Over 13 Years” was published by scientists Alex L. Shigo and Walter C. Shortle from the USDA Forest Service in the Journal of Arboriculture in December 1983.

Their conclusion was as follows:

“Many materials were used in and on experimentally inflicted wounds in many studies over a 13-year period. No material prevented decay. The individual tree had a greater effect on the wound than the treatments. Some individual trees of a species closed and compartmentalized wounds rapidly and effectively, regardless of treatment, while other trees did not close and compartmentalize treated or control wounds.”

The original study can be downloaded here – “Wound Dressings: Results of Studies Over 13 Years” by Alex L. Shigo and Walter C. Shortle, USDA Forest Service.

Prior research, as early as the 1930s, had also found that tree wound dressings did not prevent decay, and were of limited benefit for wound closure, so the matter had been raised half a century before these results were published.

Regardless, pruning sealants still are in use today, and there are many more modern formulations available with promises of improved performance.

In this article, we will look at how trees naturally repair damage they sustain, what effects pruning sealers have on tree healing processes, whether any pruning sealants are worth using, and what preventative measures we can use when pruning to help trees heal faster and more effectively.

Animal Healing vs Tree Sealing

One of the reasons gardeners think it’s necessary to cover tree wounds created by pruning cuts is because that’s what we naturally do for ourselves when we cut or scrape our skin. Covering our own wounds reduces bleeding, keeps wounds clean while keeping harmful pathogens out, and generally helps promote healing.

The only problem with this reasoning is that trees are not animals like us, they are plants, and they therefore function and repair themselves very differently from the way that we do.

In biology, humans and trees belong to different kingdoms and therefore occupy different branches of the taxonomic classification system.

The Five Kingdom Classification proposed by Robert Whittaker is used to classify and organize living organisms into five major groups based on their characteristics. (Source: Dept of Biology, Lewis and Clark College)

In biology, when organisms belong to the same taxonomical categories, they are more closely related, and therefore share more similarities. When they’re from completely different taxonomic kingdoms, they naturally tend to differ vastly in appearance and function.

The way we humans repair bodily damage is that we heal our wounds, while trees seal their wounds. Our skin is capable of repairing itself, and after wounds have healed, the skin usually regenerates, restoring the wound site back to the way it was originally, with damage no longer visible.

Trees in their natural environment are vulnerable to injury through physical damage from fire, frost, winds, and falling trees. They may also be attacked by animals, insects, fungi, and bacteria. In areas of human habitation, they can be accidentally damaged by lawn equipment such as brush cutters, line trimmers and mowers, as well as by vehicles. Also, when trees are pruned to establish and maintain their form and shape, injury is also incurred as the pruning cuts create wound sites.

When trees are damaged, the wounded tissue releases certain phytochemicals (plant chemicals) such as terpenes, phenolics, aldehydes and phytohormones, many of which play a protective role, but their presence may also attract the attention of pests and diseases that are drawn to the exposed, unprotected tissue beneath the bark.

Many pests and diseases intentionally seek out trees in distress, as the wounded and newly exposed plant tissue offers a convenient entry point for them. Unhealthy trees also have weakened natural defenses and are less capable of protecting themselves, making them even more attractive targets.

Since trees have existed on the planet for around 370 million years, they have evolved natural defense mechanisms to manage such problems in order to ensure their continued survival. However, some people feel the need to “manage” the natural processes that plants have developed over eons. In the next section, we will explore the intricate processes that trees have evolved to repair their injuries.

How Trees Compartmentalize Damage

Tree pruning cut showing wound closure over time, with tissue known as wound wood growing up and around the edges of the wound

Trees do not heal injuries in the same way animals do. Instead, they rely on a defensive process called compartmentalization, in which damaged or diseased tissues are sealed off from healthy wood. This involves the formation of suberized, lignified tissues that physically and chemically deter pathogens. Around the edge of the wound, callus (or “woundwood”) develops and gradually grows inward. Because bark does not regenerate the same way skin does in animals, any resulting scars remain within the tree for its lifetime.

This strategy prevents decay or pathogenic organisms—such as bacteria, fungi, and other microbes—from spreading unchecked throughout the trunk or branches. By building a chemical and structural barrier around the injury, compartmentalization allows the tree to continue growing around or even over the damaged area. In practice, this is sometimes referred to as “wound closure,” but it is distinct from true healing because the compromised wood remains contained rather than restored to its original condition.

When a tree is wounded—by pruning cuts, fire damage, storm breakage, or pest/disease invasion—it cannot replace the exact lost tissue. Instead, it surrounds that damaged section with new, healthy wood. The end result is a lifetime sequestration (compartmentalization) of any decay or pathogens within the injured zone, rather than their elimination. Over time, the injured tissue becomes an isolated compartment, visible later on if the trunk or branch is cut open.

How this process unfolds can be understood through four key steps (as described in the CODIT model). In essence, the tree forms chemical and physical barriers both within existing wood and at the perimeter of the wound, creating a stable compartment that slows or prevents the spread of further damage.

1. Initial Detection and Response

  1. Wound Signal and Cell Activation
    • When a part of the tree is physically damaged or infected, nearby living cells—particularly parenchyma cells in the xylem (wood), phloem (inner bark), and rays—detect the injury.
    • These cells become more metabolically active, initiating biochemical defenses to reduce the spread of infection or decay.
    • Parenchyma cells are found in the rays (horizontal structures in wood) and around vessels. They store nutrients and play a key role in producing antimicrobial compounds and forming tyloses or gum deposits to block the spread of pathogens.
  2. Chemical Changes at the Site
    • The tree often increases the production of antimicrobial compounds (e.g., phenols, terpenes) in the area surrounding the wound.
    • These compounds help slow or prevent the growth of bacteria, fungi, and other pathogens within the injured tissue.

2. Formation of Reaction Zones (CODIT Walls 1–3)

Trees set up walls to seal of or compartmentalize damaged or infected tissue. Source Tayte (2014). Tree Life Functions

In Dr. Alex Shigo’s CODIT model, the tree sets up “walls” internally to reduce the movement of pathogens along the sap stream:

  1. Wall 1 – Plugging Vessels
    • To limit the movement of pathogens up and down the trunk, the tree forms physical blockages in its vertical water-conducting cells known as the the xylem.
    • These blockages typically arise when tyloses (sac-like outgrowths of parenchyma cells) or gums develop inside vessels or tracheids (specialized cells within the xylem), effectively plugging these conduits. While xylem cells normally transport water and minerals, this compartmentalization response uses tyloses and chemical deposits to confine decay organisms to the damaged area, preventing them from spreading further through the trunk.
  2. Wall 2 – Blocking Inward/Outward Movement
    • This involves the horizontal arrangement of cells, especially the ray parenchyma. The tree fortifies these cells with chemicals or physically altered cell structures that help block or slow the side-to-side spread of decay.
  3. Wall 3 – Limiting Lateral Spread
    • Similar to Wall 2 but typically in a different orientation within annual growth rings. The tree can deposit barriers (e.g., lignin, suberin) to stiffen cell walls and limit the radial expansion of pathogens.

These “reaction zones” are not perfect barriers. Their role is to “compartmentalize” rather than eliminate the decay, keeping it confined as much as possible within one section of the wood.

3. Formation of the Barrier Zone (CODIT Wall 4)

After the tree has set up the internal reaction zones, it begins producing new tissues on the outside of the wounded area:

  1. New Wood and Bark Production
    • The vascular cambium—the thin layer of meristematic (actively dividing) cells that produces new xylem (wood) inward and new phloem (inner bark) outward each year—produces new xylem and bark cells around the wound.
    • As the cambium divides, it creates a strong barrier zone that physically separates the injured or infected tissue from the new, healthy wood being laid down.
    • Phloem (inner bark) cells transport sugars and other nutrients. They also help detect wounds and can mobilize resources for woundwood formation.
  2. Woundwood (Callus) Formation
    • At the edges of a wound, the tree produces what is commonly called “callus” or “woundwood.”
    • This specialized tissue grows over the wound’s perimeter, gradually reducing the exposed area over time.
    • Chemically, woundwood is often rich in protective compounds, giving it added strength and resistance to decay.
  3. Long-Term Sealing
    • As the tree adds annual growth rings, the barrier zone becomes even more robust.
    • Over several growing seasons, a well-compartmentalized wound will be sealed off internally and may be entirely closed externally by overlapping woundwood.
    • Bark is the outermost protective tissue that can thicken around a wound. While bark itself can be damaged, new bark formation around the wound margin (edge) helps protect the healing zone from further infections or environmental stress.

4. Lifelong Sequestration (Compartmentalization) of Damaged Tissue

Because the old, injured wood cannot “heal” in the same way an animal might heal a wound, the damaged tissue remains inside the tree forever, locked behind these compartmentalizing walls. If the trunk or branch is cut open years later, one might see:

Key Points

In summary, compartmentalization is how trees “manage” injuries. Instead of healing them in the animal sense, trees create internal and external barriers to contain decay or infection, then form new layers of wood and bark around the wound. This process allows the tree to safely coexist with an internal column of damaged tissue, often for its entire lifespan. As long as the walls remain intact and the tree’s health is generally good, the overall structure and longevity of the tree remain largely unaffected by the old wound.

Stopping Bleeding Sap from Tree Pruning Cuts

Another reason gardeners think pruning cuts need to be painted with pruning sealer is to help prevent the tree from losing too much sap, and literally ‘bleeding to death’. This is an understandable concern, but trees don’t actually ‘bleed’ as they don’t have blood, they have sap, which performs a very different function.

The blood in animals is critical to their survival because it carries oxygen to all the cells within the body to keep them alive. By contrast, plant sap carries water, minerals and sugars, which serve as food, or an energy source, but doesn’t carry oxygen. Therefore, trees can lose a fair bit of sap, and still survive without major injury.

Sap loss can weaken a tree because it’s losing stored energy, but the dripping sap does run back into the soil where the tree’s roots are, so it might be able to reclaim some of the water and nutrients from the ground.

Preventing Bleeding Sap When Pruning

To prevent excessive sap loss, avoid pruning deciduous trees and vines too late in the winter dormant season, when the weather warms and the sap begins to rise. Grape vines and mulberry trees in particular can bleed copious amounts of sap if pruned at the time when buds are beginning to swell.

Regardless, pruning sealers aren’t effective at all in reducing the dripping of sap from a pruning cut, as the pressure of the sap flow just forces its way through the layer of sealer before it hardens or sets. I’ve tried it, and can confirm it doesn’t work!

The sap flow will just stop naturally of its own accord when the tree compartmentalises the wound, and as part of that process, seals off the vascular tissue (tubes in its tissues that carry the liquid) around the wound site.

Effect of Pruning Sealers on Tree Healing Processes

Trees do not heal wounds in the same way animals do. Instead, they compartmentalize damage—forming barriers within the wood and producing callus (woundwood) around pruning cuts to contain decay and pathogens. When you apply pruning sealer to a fresh cut, the artificial coating can:

Reasons to Avoid Pruning Sealers

Research and practical experience have led many arborists and horticulturists to conclude that pruning sealers generally do more harm than good. Key reasons include:

  1. They Are Unnecessary: Trees have evolved natural defenses—suberized, lignified tissue and callus formation—to protect against pathogens and decay. Pruning sealers do not enhance these defenses and can disrupt them.
  2. They Can Be Harmful: Petroleum- or asphalt-based sealers can trap moisture, foster decay, and release potentially toxic substances into the wounded tissue.
  3. They Are Often Ineffective: Sealers do not reliably prevent disease or insect infestations. Pruning sealers do not deter beetles that spread vascular wilts, as these insects can still detect the scent of the wounded tree and bore through the sealer. They do not repel borers, which simply find other entry points on the tree or create new ones by chewing through the layer of sealer. Finally, pruning sealers do not promote healing, as they do not stimulate the growth of new tissue or improve the tree’s resistance to stress.

Exceptions: When Pruning Sealers May Be Warranted

Although rarely recommended, there are scenarios where a pruning sealer (often a latex-based product) may be used:

While research supporting this advice is questionable at best, it may be justifiable to use a fungicide or insecticide during spring or summer pruning. Ideally, avoid pruning susceptible trees during active disease periods (usually spring/summer) whenever possible. Dormant-season pruning (late fall or winter) reduces both insect activity and disease pressure, lessening any need for sealers.

Alternatives to Using Pruning Sealers

Instead of relying on pruning sealers, proper tree care and maintenance are generally more effective for promoting healthy wound closure:

  1. Prune Correctly and at the Right Time
    • Timing: Prune during the dormant season, when the tree is less vulnerable to infections and pests.
    • Technique: For the best results, prune with sharp, clean tools that make smooth, precise cuts, allowing the tree to seal over the cuts more effectively. Follow the tree’s natural shape and structure, avoiding topping (indiscriminately cutting the main trunk or large branches to stubs, usually for height reduction), stubbing (leaving an overly short branch remnant), or flush cuts (cutting too close to the trunk or parent branch and removing the branch collar). Pruning should be done sparingly and selectively—removing only dead, diseased, damaged, or crossing branches—while keeping the main trunk and branch collar intact.
  2. Provide Optimal Moisture and Mulch
    • Watering: Keep soil evenly moist (not waterlogged), especially during dry periods, to help the tree maintain vigor, which aids natural wound closure.
    • Mulching: Apply a layer of organic mulch, such as wood chips or compost, around the root zone (but not against the trunk) to retain moisture, moderate soil temperature, suppress weeds, and improve soil structure.
  3. Use Appropriate Fertilization and Protection
    • Fertilizing and protecting are optional but helpful measures to enhance the growth and resilience of the tree.
    • Fertilizer: Fertilizing should be done in spring or autumn with a balanced and slow-release fertilizer, preferably organic, to provide the tree with the necessary nutrients for its development and defense.
    • Physical Protection: Trees can be protected with physical barriers, such as tree guards or wraps, to shield the tree from mechanical injuries, sunscald, frost cracks, or animal damage. Protecting should be put in place only when needed and removed when not.

By following these steps—pruning properly, choosing the right timing, and supporting overall tree health—you can help your trees recover naturally without the drawbacks associated with most commercial pruning sealers.

How to Help Tree Pruning Cuts Heal Faster

There are various ways we minimise pruning damage to trees, allowing them to heal faster or more easily, as detailed below:

Retain the Branch Collar: Flush cuts that are level with the surface of the trunk leave a larger wound opening, and damage trunk tissue, leaving the tree vulnerable and susceptible to pests, diseases, and decay. A proper pruning cut is a clean cut made at a 45-degree angle, outside of the branch collar.

Prune Earlier Rather Than Later: When we need to reshape a tree, we should make those formative pruning cuts earlier in the tree’s life. If we cut off an unwanted side branch on a young tree when the branch is thin, it will leave a very small pruning wound that’s only as wide as the branch itself. That pruning wound may only be 1cm (3/8″) for example, which will heal fairly fast and seal over quite easily. Pruning wounds don’t grow in size as the tree grows, they stay the same size.

On the other hand, if we wait many years, and cut off the same branch when it’s 10cm thick, this will leave a 10cm wound, which is much larger, causes more tissue damage to the tree, and is harder for the tree to seal off.

With some simple mathematics, the area of a circle can be calculated using the formula A = π x r2, where π=22/7 or 3.14 and r is the radius, or half the diameter of the circle.

Shape trees by training (pruning them to the desired form) them when they are young. Cutting small branches when the tree is young avoids the need to cut larger branches later. The large wounds produced by removing large branches create greater injuries to the tree that are more difficult to repair.

How Long Does Tree Wound Recovery Take?

The time it takes for a tree to close a wound can vary dramatically among species and depends on environmental conditions, genetic capacity, and the overall health and vigor of the tree. In some cases, a tree may never fully seal a wound due to genetic limitations or lack of resources. Nonetheless, research consistently shows that quicker wound closure reduces the likelihood of long-term health problems, making rapid healing beneficial.

Healthy, vigorously growing trees—especially younger ones—typically seal wounds more quickly. Good soil conditions (including adequate drainage, favorable soil texture and structure, and sufficient nutrients) also help support faster wound closure. Therefore, when planting new trees, it’s important to consider site selection and soil quality, as these factors can greatly influence a tree’s growth and its ability to recover from wounds.

Conclusion

Decades of arboricultural research—most notably by Dr. Alex Shigo—demonstrate that pruning sealers are generally unnecessary and can even harm a tree’s natural healing processes. While marketed as a way to prevent sap loss, disease, and insect infestations, sealers often trap moisture and decay organisms, interfere with callus formation, and fail to keep pests out. Trees rely on compartmentalization to isolate damaged tissue and grow new wood around the wound; sealers do little to improve or speed up that process.

The sole exception is when pruning highly susceptible species (like oaks prone to oak wilt) during active disease-transmission periods. In such cases, a thin coat of latex-based paint—or a fungicide or insecticide—may offer partial protection. Even then, dormant-season pruning is strongly recommended to minimize disease and insect activity from the outset.

Ultimately, the best defense against infections and pests lies in proper pruning practices:

In short, let trees do the work they’ve evolved to do. Healthy trees, pruned correctly, can generally compartmentalize wounds effectively—without the added complications of synthetic sealants.

References

Exit mobile version