Stop Filling Tree Hollows and Painting Wounds – Here’s Why It Hurts Your Tree

Trees are living, breathing structures. Like us, they can get injured—and just like our bodies, they have natural ways of protecting and sealing wounds. But when well-meaning homeowners or contractors try to “fix” tree hollows or cuts with foam, concrete, tar, or caulk, they’re often doing more harm than good.

In this blog post, we’ll explain why these outdated practices can actually make things worse, and what you should do instead to keep your trees safe, healthy, and standing strong.

Why You Should Never Fill Tree Hollows

It may seem like a good idea to fill a hollow spot in a tree with foam, glue, concrete, metal, or caulk—especially if it looks unsightly or you’re trying to keep water or animals out. But these materials can create serious structural problems:

Trees move. When wind blows, a tree bends and flexes. Artificial fillers don’t move the same way. When a rigid material like concrete is inside the trunk, it creates a false fulcrum—a hard point that prevents natural flexing. That pressure can cause the trunk to snap during storms.
Filling hides the problem. Decay doesn’t stop just because you cover it. In fact, fillers trap moisture and allow fungus and rot to spread invisibly.
Trees grow over the filler. As the tree continues to grow, it builds around the object. This leads to internal stress and unpredictable failure.

If your tree has a large, rotting cavity, it’s likely a sign of internal structural decay. In most cases, the safest, most responsible option is to remove the tree and plant a healthy one.

Why Painting Tree Wounds is a Bad Practice

Painting over a fresh cut with tar or wound dressing might seem like good protection—but it actually interferes with how trees naturally defend themselves.

Trees seal wounds, they don’t heal like we do. Trees form natural barriers through a process called compartmentalization, which helps block decay. Applying tar or paint traps moisture and creates a breeding ground for pests and disease.
Paint can attract insects. Many wound coatings actually draw in bugs because of their smell or heat-retaining properties, making things worse.

The Exception: Oak Trees and Oak Wilt

There’s one situation where painting a pruning wound can be helpful: pruning oak trees during the growing season.

Oak wilt is a serious disease that can kill oaks quickly—and it spreads in two major ways:

Insects are drawn to fungal mats that form on dead or dying oaks. These mats give off a sweet scent designed to attract bugs of all kinds—not just beetles. Any insect feeding on sap can pick up the oak wilt spores.
If that same insect lands on a fresh cut on a healthy oak (which leaks sap), the spores are transferred, just like a bee spreading pollen.
Root grafts between oaks are another major pathway. Oaks of the same species that are close together often share root systems, allowing the disease to pass underground.

In these cases, painting a fresh cut on an oak immediately after pruning (only in spring or summer) is a lesser evil—not ideal, but sometimes necessary to prevent infection. Still, the best approach is to prune oaks during late fall or winter, when the disease risk is lowest.

Safety and Liability Risks

Using improper materials like concrete or foam to “repair” a tree doesn’t just put the tree at risk—it can put people and property in danger.

If a filled tree fails and causes damage or injury, the homeowner and the contractor who performed the work can both be held legally liable. This is a risk no one wants to take, especially when the work was avoidable and not based on modern arboriculture standards.

What You Should Do Instead

Consult a certified arborist. We have the training and tools to assess your tree’s true condition and recommend the best action.
Let the tree be a tree. Don’t cover or fill voids—let the tree compartmentalize naturally.
If it’s unsafe, remove and replant. A new tree planted in the right place with good structure will be safer and longer-lasting than one that’s already rotting inside.

Final Thoughts

Trees are strong, but they’re not made to hold concrete or caulk. Filling voids and painting wounds may seem helpful, but it can actually shorten your tree’s life and increase the chance of failure.

When in doubt, talk to an arborist who understands tree biology—not just appearances. Doing it the right way now can save you money, stress, and risk in the future.

Need a professional opinion?
If you’re unsure about your tree’s safety, or you’ve been told to “fill the hole with foam,” give us a call. We’ll help you make the best decision for your trees—and your property.