Fused trunks and branches (inosculation): what extension services say
Inosculation is two trees, or two parts of one tree, growing into each other so their wood joins. A University of New Hampshire forester, a South Dakota forest health update and a Minnesota oak wilt folder describe how it starts, which trees do it, and what the joins carry.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say. It does not diagnose a tree, say whether any tree or growth is safe or unsafe, or tell anyone to cut, keep or treat one. Each cause and claim belongs to the source that gives it.
What it is and how it starts
University of New Hampshire Extension (Cheshire County Extension Forester Matt Kelly, February 2, 2022) defines inosculation as two individual trees growing in close proximity becoming morphologically joined. It stresses that this is a naturally occurring event, unlike grafting, which is a horticultural technique used to grow fruit and ornamental trees.
The same article gives the steps. A wound exposes the cambium, xylem and phloem when bark is removed, for example when a branch breaks in wind, snow or ice, or when two trunks rub against each other as they sway. The tree answers with callus, a scar tissue that later becomes woundwood. When the friction scrapes the bark off both trees at the point of contact, the callus from each pushes outward, which raises the pressure between them. The article says that pressure, together with sticky sap or pitch from the wounds, reduces movement at the contact. The cambium layers then touch and the vascular tissues connect, which it says allows an exchange of nutrients and water, though it says it is unclear whether the two trees exchange genetic material.
Which trees the sources name
| Question | What the sources say |
|---|---|
| Same species or different? | New Hampshire says it usually happens between trees of the same species, but has been seen between different species, which is less common and sometimes produces "false grafts" with no union of the conducting tissues. South Dakota's forest health update of February 5, 2020 says it is a common occurrence between two trees of the same species and does not occur between unrelated species such as honeylocust and cottonwood, where it describes one outgrowing and strangling the other. |
| Which species are more prone? | New Hampshire names eastern hemlock, white pine, maples, birches, ashes and American beech. |
| Trunks, branches or roots? | New Hampshire says the focus of its article is on branches and trunks, and that natural grafting between roots is common. |
| What counts as a "true graft"? | New Hampshire quotes a 1938 paper by Henry Baldwin, which calls a true graft one with complete union of the conducting tissues, so that the start of the connection is obscured. |
What a root graft can carry
The University of Minnesota Agricultural Extension Service's oak wilt folder (Extension Folder 310, revised 1980, D.W. French and Ward Stienstra) shows what a join can pass along. It says the roots of oaks form natural grafts, so once the oak wilt fungus enters a tree it can spread to nearby trees through them, and that the fungus then moves outward from the first tree and involves more and more trees. It calls the resulting group of dead and dying trees an infection center, and describes a graft as one that lets trees interchange water, and with it the spores of the fungus. Oak wilt is covered on its own page.
Whether the sources call it harmful
None of the three sources calls the fusion of two stems a defect in itself. New Hampshire describes it as something to marvel at and notes that naturally joined trees have been called "marriage" trees in many cultures. South Dakota describes a case of fusion between unlike species as a struggle in which one tree outgrows the other. The Minnesota folder treats the root graft as a path for a disease, not as a growth problem. For defects in trunks and unions, see tree risk assessment.
Other growth forms are on the odd tree growth page.
Sources
- University of New Hampshire Extension, Inosculation: Making Connections in the Woods (Matt Kelly), February 2, 2022, read October 9, 2026
- South Dakota Department of Agriculture and SDSU Extension, Pest Update February 5, 2020, read October 9, 2026
- University of Minnesota Agricultural Extension Service, Oak Wilt (Extension Folder 310, D.W. French and Ward C. Stienstra), revised 1980, read October 9, 2026