Why You Shouldn't Put Screws or Nails Into a Healthy Mature Tree

Putting a nail, screw, hook, lag bolt, or other piece of hardware into a healthy mature tree is easy to dismiss as a minor injury. Trees are large, wood is strong, and a small fastener may seem trivial. The problem is that a tree is not a post. It is living tissue that must respond to every wound while continuing to grow around whatever has been inserted into it.

A small hole is still a wound

The biologically active tissues of a tree are concentrated near the outside of the stem. Driving hardware through the bark penetrates those tissues and creates an injury the tree must compartmentalize. Healthy trees can often limit the spread of dysfunction around small wounds, but compartmentalization is not the same thing as healing the way animal tissue heals. The injured wood remains part of the tree.

The risk depends on the size and type of fastener, where it is installed, how long it remains, the tree species, tree vigor, and whether repeated attachments are made in the same area. One small fastener is unlikely to kill a vigorous mature tree. That is not a good reason to make unnecessary wounds.

The tree keeps growing around the hardware

A mature tree adds new wood around its circumference. Hardware that is left in place can become partially or completely engulfed. Years later, a person pruning, removing, milling, or processing that tree may have no idea that steel is buried inside the trunk. Embedded metal can damage chainsaws, chippers, sawmill equipment, and other cutting tools and can create a serious safety hazard for the people operating them.

Cables, signs, lights, birdhouses, hammocks, and landscape features also create another problem when their attachments are forgotten. Wire, rope, chain, or rigid hardware can eventually constrict expanding tissues or become engulfed by the tree.

Professional support systems are different

There are legitimate situations in arboriculture where hardware is intentionally installed in a tree. Structural support systems, including certain cabling and bracing systems, are designed for a specific mechanical purpose and should be selected, positioned, installed, documented, and periodically inspected by qualified professionals. That is very different from attaching decorations or structures to a trunk because the tree happens to be convenient.

Use the tree's surroundings instead

Whenever practical, signs, lights, cameras, fencing, birdhouses, and similar objects should be supported independently on posts or structures outside the trunk. Noninvasive temporary attachment systems may be appropriate in limited circumstances, but they still need to accommodate growth and be inspected and removed before they constrict the tree.

A mature tree can represent decades or centuries of accumulated growth. Avoiding an unnecessary hole is a small decision that respects that investment and reduces problems for whoever eventually has to care for the tree.

Research context

Tree wound biology is better described as compartmentalization than healing. After injury, trees form anatomical and chemical boundaries that can limit the spread of dysfunction and decay, but the wounded wood is not restored to its pre-injury state. The effectiveness of compartmentalization varies with species, genetics, wound characteristics, tree condition, and subsequent exposure to microorganisms.

Sources and further reading

Shigo, A.L. (1984). Compartmentalization: a conceptual framework for understanding how trees grow and defend themselves. Annual Review of Phytopathology 22:189–214. doi:10.1146/annurev.py.22.090184.001201.

Shigo, A.L. & Marx, H.G. (1977). Compartmentalization of Decay in Trees. USDA Forest Service, Agriculture Information Bulletin 405.