| Key Takeaways |
|---|
| • Sealer does more harm: Pruning sealer traps moisture and blocks the tree's natural wound compartmentalization. • Oak and elm exceptions: Paint wounds on oaks and elms during risk seasons to deter disease-carrying beetles. • Prune cleanly instead: Prune in late winter, make clean cuts that keep the branch collar, and sterilize tools. |
For a long time, it was common practice to use a pruning sealer like tree tar when trimming trees, a seemingly common-sense step recommended by trained arborists and nursery workers alike. Today, homeowners are learning why painting on tree pruning sealer is a bad idea.
Research going back to the 1970s found the paint does the opposite of what the label promises: It traps moisture against the cut and blocks the tree’s own sealing process.
So let’s talk about why it’s now considered a gardening myth, when the rare exceptions apply, and how yesterday’s tree bandage became today’s bad practice. And if a tree job feels like more than a weekend project, call on LawnStarter’s tree care pros to handle the work.
What Is a Tree Pruning Sealer?
A tree pruning sealer is a waterproof product, applied by aerosol spray or brushed on, that covers the exposed cut on a limb after pruning. For years, arborists told homeowners that tree wounds had to be covered with a sealant.
A tree pruning sealant is a bandage of sorts meant to cover the wound. How does this “bandage” claim to help a tree heal? The packaging makes a few claims:
Aids in the healing of pruning cuts
Minimizes sap loss
Protects trees from decay, insects, and fungi
It was an easy DIY project, and the product was (and still is) in stock at garden and big box stores, so it became ingrained as a habit for homeowners.
Why Tree Sealant Isn’t Necessary
“Current best practices no longer recommend applying sealants [pruning spray or pruning paste] to tree wounds,” says Karl Flocke, former woodland ecologist with the Texas A&M Forest Service.
Today’s arborists are trained to consider pruning sealers bad practice because they make it harder for trees to recover after pruning.
While good in theory, scientific research conducted by Alex Shigo of the U.S. Forest Service in the late 1970s explained that pruning cuts should not be painted. Rather, the tree should be left to heal through its own devices after you prune it.
He emphasized the importance of following proper pruning practices. Let a fresh cut heal naturally instead of covering it with a wound sealer, he urged.
It’s important to understand that trees don’t really heal after they’ve been damaged. Instead, they isolate the damage through a process called compartmentalization.
Callus tissue develops at the edge of the wound and gradually grows in toward the center. When trees are pruned correctly, this new growth eventually covers the pruning wounds, protecting the tree.
These sealant products are typically petroleum-based, although some contain asphalt, and they trap moisture inside the wound, which may encourage more problems instead of preventing them.
Some products tout natural ingredients such as collagen, pectin, or aloe vera. Unfortunately, there isn’t any scientific proof that they benefit the tree, either.
The petroleum and asphalt-based wound dressings are known to:
Seal in moisture and decay
Sometimes serve as a food source for pathogens
Prevent wound wood from forming
Inhibit compartmentalization
Eventually crack, exposing the tree to pathogens
Read more: How to Treat Tree Wounds
Tree Sealant Impact on Human Health
Tree sealants are no fun for humans, either.
These products often contain a volatile combination of asphalt and solvent, and some are considered hazardous by OSHA.
According to the safety data sheet for one popular product, tree pruning sealers may:
Contain gas under pressure and may explode if heated
Cause serious eye irritation
Cause skin irritation
May cause genetic defects
May cause cancer
For this product, the label advises users to wear skin and eye protection and store it in a cool, well-ventilated place.
After learning those tidbits, it’s a bit easier to see why you should stop using pruning sealer on your trees.
Should You Seal Pruned Limbs at All?
The short answer is no. You should not use pruning paint to seal pruned limbs at all. They’re better left to undergo natural healing through their own plant defense mechanisms.
Plus, why would you put a product on your tree that’s also used to seal and waterproof rain gutters, roof flashings, wooden planters, and even asphalt driveways? If something claims to be effective for those outdoor uses, don’t put it on a living tree.
There are a few rare situations where a coating makes sense. Here’s a quick guide:
| Situation | Should You Seal? |
| Routine pruning cuts | No. Let the tree seal the wound naturally. |
| Storm or mechanical wounds on most trees | No. Trim the ragged edges properly and let the tree compartmentalize. |
| Any cut on oak during oak wilt season | Yes. Apply a thin coat of paint to deter sap-seeking beetles. |
| Any cut on elm during the growing season | Yes. Paint the wound to deter elm bark beetles that spread Dutch elm disease. |
What Can You Use Instead of Pruning Sealer?
For a normal cut, use nothing on the wound itself. Focus on prevention instead, and you can minimize problems that stem from pruning by following these tips:
Properly sterilize all pruning tools with a 70% isopropyl alcohol solution before and after using them. That applies regardless of the time of year you’re pruning or the type of tree you’re working on.
Control the spread of disease by properly disposing of all problematic limbs after pruning. Use only disease-free mulches or compost.
Make clean, smooth cuts, making sure not to leave stubs, but keep the branch collar intact.
Prune trees at the proper time, preferably in late winter while they’re still dormant and haven’t started forming leaves. This makes them less susceptible to infestations or infections.
See Related:
The Exceptions: Oak and Elm Trees
Oak trees in more than 20 states, from the Mid-Eastern states through Texas, can suffer from oak wilt. The damage done can be both devastating and irreversible.
The fungus that causes the disease spreads from tree to tree, carried by Nitidulid beetles attracted to the smell of the tree’s sap. The insects then move to another tree, laden with fungus spores, and spread the disease. Quickly covering pruned branches hides that fresh-sap smell before the beetles come running.
“Most forms of paint work equally well, and there is no need for specialized rubberized or petroleum sealants,” Flocke says.
He’s stationed in Austin, Texas, where oak wilt is such a concern that pruning oaks is discouraged from February through June. “The wound paint is only necessary for the first two to four days after the cut has been made,” Flocke says, “so there is no need to reapply paint or to go out and paint old wounds.”
Elm gets the same treatment for a different beetle. American, slippery, and rock elms are all highly susceptible to Dutch elm disease, and fresh wounds draw the bark beetles that carry it.
The Wisconsin DNR advises against pruning native elms from May through November for that reason. If a storm or a safety hazard forces your hand, paint the wound right away with latex paint or a non-toxic wound dressing.
How to Remove Pruning Sealer
You may be wondering whether using a pruning sealer has already caused irreparable damage to your trees, and whether you should remove it. This isn’t easy to answer, and surprisingly, there isn’t much discussion about it online.
I recommend if your trees’ pruning cuts had a tree sealer applied to them, you remove it if possible. Due to the chemical composition of the sealer, you can’t simply peel it off and throw it away, unfortunately.
Here is how to remove pruning sealer from your tree:
Make a new pruning cut
Remove the paint and create a new, open wound.
Pro Tip: A new pruning cut wouldn’t need to be made much below the old one. A quarter to a half of an inch is good if there’s enough wood to do that. Make sure it’s a clean, straight cut, and your tools are sterilized.
This new wound triggers the tree to begin compartmentalization, forming a new callus to protect the wound naturally.
Hire a Professional
Pruning trees can be a daunting task, especially if you were inspired to plant an orchard. Professional tree trimming typically runs $430 to $640 per tree, so hiring it out is a manageable way to get clean, safe cuts without buying gear or climbing a ladder. See the full tree trimming cost breakdown for details.
When in doubt, the best solution is to hire one of LawnStarter’s local tree care pros to handle the dirty work while you enjoy the shade.
FAQs
Prune healthy deciduous branches in late fall to winter, when the tree is dormant and leafless so its form is easy to read. Compartmentalization also happens fastest just before spring growth. Prune dead or dying branches any time of year.
Oaks are the main exception because of oak wilt. If a cut must be made during the growing season, paint each wound as soon as you make it. Always read the manufacturer’s label for instructions.
Water regularly to help the tree recover, aerate any compacted soil for air circulation, and add fertilizer to encourage new growth. Skip the wound sealer and let the cut close on its own.
See Related:
• How to Trim a Tree
• What Tools Do You Need to Trim Trees?
Main Image: Applying pruning sealer to a tree wound after cutting a branch. Photo Credit: encierro / Adobe Stock
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