The general advice used to be that trees needed help to heal their wounds after pruning or other injury. A wide variety of wound-sealant products were developed that contained an array of questionable ingredients, from petroleum-based and rubbery products to those containing asphalt.
There are even multi-purpose products claiming to be useable as tree wound dressing, including one used for sealing electrical boxes and fittings and sealing cracks in concrete and asphalt driveways.
It was also advertised for use on trucks and construction equipment: doesn’t sound much like something you’d want to put on living tissue! This isn’t surprising when you consider that it’s marketed as a rust and corrosion inhibitor.
(It becomes even clearer when you consider the safety warnings on some such products: may explode, causes eye and skin irritation, may cause cancer and genetic defects. That is not a product that anyone should be using for anything, never mind applying it to a living being.)
No, You Should Not Use Tree Pruning Sealant
Most Certified Arborists now agree that the use of tree wound dressing or pruning sealant is not advisable. Far from helping a tree to recover, these products actually make the tree more susceptible to rot and disease.
Not great for a product to do the opposite of why you bought it. This also applies to the new batch of “natural” products available which tend to include aloe vera gel, pectin, or collagen. There’s no evidence to suggest that any of these is beneficial to a tree.
To understand why, it’s important to understand exactly how a tree recovers from pruning or a wound.
Trees Don’t Heal Like People Do
If you have a wound, your body will rebuild living tissue to fill in the hole. This process takes time, and unless the wound is well-covered by a scab, it may need to be protected from dirt and bacteria.
However, when a tree is pruned or wounded, it does not regrow living tissue. Rather, it compartmentalizes the area.
What this means is that, through the formation of what’s called suberized and lignified wood, trees isolate the damage and work physically and chemically to prevent invasion by any pathogens.
Suberin and lignin are, as the International Oak Society describes, “two waterproof, phenolic polymers that are particularly important in woody plants.”
A callous then develops at the edges of the wound and spreads in toward the center. Although the wound may later be covered by subsequent growth, that isolated, calloused portion remains in place for the rest of the tree’s life, sealed off from the living wood.
An Exception: Oak Wilt Fungus
Trees have their own, natural forms of resistance against disease and attack by pests. One possible exception to this, and a case where arborists do still recommend wound sealant, is in the case of oak wilt fungus.
A lethal disease spread from tree to tree by Nitidulid beetles (also known as sap beetles), oak wilt fungus affects oak trees in approximately 24 US states.
The beetles are attracted to the smell of fresh sap, so cuts in trees act as beacons for the spore-laden insects.
In this instance alone do some experts suggest quickly sealing off tree wounds, but as the seal is only required for at most four days after a tree has been pruned or wounded, almost any old paint will do. (If you live in an area affected by oak wilt fungus, please do consult your local arborist before heading down to the hardware store to pick up a can of paint!)
Alternatively, some experts suggest a fungicidal treatment, but again, this is something to discuss with someone knowledgeable about the disease in your specific area.
What Tree Wound Dressing / Sealant Does
- Wound sealant or tree wound dressing traps in moisture. While some websites claim that this is a good thing, it isn’t! Trapped moisture leads to rot and decay.
- Some types of wound sealant can act as food for pathogens, be they insect, bacteria, or fungus.
- The application of a tree pruning sealant may prevent the tree from growing “wound wood” and negatively affect the process of compartmentalization.
- Tree sealant that forms a hard or even flexible barrier may eventually crack. If the tree has been unable to seal off the wounded tissue itself, these cracks may introduce the very pathogenic invasion that the sealant was used to prevent.
Trees Know What to Do
Trees have been “healing” themselves quite successfully for millions of years. Branches break and fall all the time; trees compartmentalize the wounds and form callouses over them.
The best way for humans to minimize the damage they do to trees and give their trees the best chance of an infection-free healing process is to respect correct pruning techniques.
How to Prune a Tree to Minimize the Risk of Disease
While tree pruning is a subject of its own, there are a few brief guidelines to follow.
1. Prune trees at the optimum time of year for the tree species, normally its dormant period. This will take into account its growth patterns, its susceptibility to pests and diseases, and the habits of those pathogens.
2. Practice proper disposal methods for any disease material, making sure that you completely remove branches and leaves from the area to prevent cross-infection or re-infection.
3. Sterilize pruning tools before and after use with a 70% isopropyl alcohol solution. This is something that most of us have to hand now!
4. Make sure that any cuts you make are smooth and clean. Use sharp tools suited to the task, and don’t leave large branch stubs. However, it is advisable to leave what’s called the branch collar: this is the slightly larger area where a branch joins to the tree’s trunk. Removal of this will leave the tree with a larger area to compartmentalize and may increase the risk of disease.
5. If your tree has a rough, jagged wound from a broken branch as the result of a storm or other event, either turn the wound into a neat, clean cut yourself, or (especially in the case of large breakages), enlist a qualified arborist to do it for you.
Ignore Aesthetics
Sometimes, in trying to please customers who feel that their pruned trees don’t look very nice, tree surgeons and arborists will make an exception to the no-sealant rule and say that it’s acceptable for aesthetic purposes. But the same problems still apply: it still isn’t good for the tree.
The ultimate aim is surely to protect the health of a tree rather than to ensure that it looks as we want it to all year round. As such, tree wound sealant applied solely for aesthetic purposes cannot be justified. Remember, the life of the tree is what matters, not its appearance!
Listen to the Real Tree Experts
People looking for more natural ways of sealing a tree’s wound, either because they want it to look nicer or they believe that a sealed cut is better for the tree, might stumble on several internet recipes for homemade tree pruning sealant.
The most prevalent one contains manure and diatomaceous earth and has absolutely no basis in fact whatsoever that I can find. Its original source is unknown and its ingredients questionable.
There are also lots of people giving advice on forums as to what they’ve used themselves, from beeswax to linseed oil. My advice would firstly be to listen to the experts who say that it’s best to let a tree deal with its wounds naturally.
Beyond that, if you do choose to use something, proceed with caution. Research natural ingredients if you’re going to try a recipe, and if you decide to buy something, always buy a product specifically designed for trees.
Apart from in some cases of oak wilt fungus where normal paint might be appropriate, don’t repurpose a product that wasn’t created for application to living tissue. Driveway sealer does not belong on a tree.
Leave Trees to Grow
Trees know how to look after themselves. If they hadn’t evolved ways to seal off and deal with damage, they wouldn’t have survived — and thrived — for millennia. The best we can do is to minimize our impact on trees and ensure they continue to live as free from our interference as possible.
If you have to prune, prune wisely. Just don’t think that the Band-Aid that works so well for you is of any use to a tree!
(My thanks to Dr. Linda Chalker-Scott for her work clarifying the modern horticultural position on tree wound sealants.)
Featured Image Photo by kylie1 at Pixabay
