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Odd tree growth: witches' brooms, burls and fused trunks

Dense tangles of shoots, swollen trunks and trees that have grown into each other draw questions on every job. This page sets out what the agencies and extension services say about each, with a page each for witches' brooms, burls, slime flux, fused trunks, fasciation, water sprouts, spiral grain and buttress roots.

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. The sources are agency, university and magazine pieces that quote agency scientists, and each cause named belongs to the source that names it.

Compared at a glance

GrowthWhat the sources saySource
Witches' broomA dense mass of shoots on a branch. Several causes are listed: fungi, phytoplasmas, mites, dwarf mistletoe, viruses, genetic mutation and bud-killing stress. Most do not appear to weaken the host, according to a Forest Service physiologist quoted by Northern Woodlands. See the page on witches' broom.Missouri Botanical Garden, Northern Woodlands, UC IPM, Oregon State
BurlA rounded woody swelling, usually on a trunk, with swirled grain. Maryland, New Hampshire and Northern Woodlands name bacteria, fungi, viruses or insects as possible triggers, and the exact cause is often unknown. See the page on burls.University of Maryland Extension, UNH Extension, Northern Woodlands
Slime flux (wetwood)Foul-smelling liquid or foam seeping from cracks in the trunk. Maryland, Utah State and South Dakota attribute it to bacteria fermenting in wounded wood, with gas pressure pushing the liquid out. See the page on slime flux and wetwood.University of Maryland Extension, Utah State University Extension, SDSU Extension
Fused trunks (inosculation)Two trees, or two parts of one tree, whose wood joins after bark rubs off at a contact point. South Dakota says it is common between trees of the same species and does not occur between unrelated species. New Hampshire says it has been seen between different species but less often. See the page on fused trunks and branches.UNH Extension, South Dakota Department of Agriculture and SDSU Extension, University of Minnesota Extension
FasciationA flattened, ribbon-like or fused-looking stem, sometimes with dense tufts of shoots. UC IPM says some cases are caused by a bacterium or viruses, some are noninfectious and possibly genetic, and the cause of many has not been identified. Washington State, Montana State and South Dakota State add genetic, hormonal and injury causes. UC IPM says it rarely threatens the survival of established woody plants and is mostly aesthetic. See the page on fasciation.UC IPM, Royal Horticultural Society, Washington State University, Montana State University, SDSU Extension
Epicormic shootsUpright sprouts from buds in the bark of a trunk or branch. The Forestry Commission note says they usually arise from suppressed buds. The sources name pruning, stress, thinning, light and injury. See the page on epicormic shoots.US Forest Service, UK tree note, Maryland, UNH, Illinois
Spiral grainWood fibres that wind round the trunk. The Wood Handbook and a 1956 Forest Products Laboratory report describe it as cross grain that affects strength and twisting in timber. A paper hosted by Forest Research says no proposed cause explained it adequately. See the page on spiral grain.US Forest Service, Forest Research, Alaska Science Forum
Buttress roots and flutesRidges where the trunk meets the large roots, and grooves down the trunk. The glossary says genetics may be responsible for fluting in some trees and environmental factors in others. A 2014 pull-test study on red maple reports a nearly direct linear relation between buttress roots cut and the force applied. See the page on buttress roots and flutes.Virginia Master Gardeners, tree journal, US Forest Service
Swelling at a mistletoe attachmentOregon State and UC IPM describe swellings of the branch where mistletoe attaches, and bole swelling and cankers with dwarf mistletoe.UC IPM, Oregon State

One growth, several causes

The sources agree on one point: a distorted shape does not name its cause. UC IPM says witches' brooms are not necessarily diagnostic of dwarf mistletoe because mites, viruses, fungi and genetic mutations also cause them. The Missouri Botanical Garden lists examples by tree: oak and hackberry with powdery mildew, incense cedar with rust, and rose with a virus.

Other forms

Galls, which have an insect, mite or bacterial cause, are on the tree galls page. Mistletoe itself, a plant, is on its own page. Cankers and decay in trunks are outside this page: see tree risk assessment and how a tree works.

Growth forms, one page each

Sources