Spiral grain (twisting wood fibres): what the sources say
Spiral grain is wood fibres that wind round the trunk instead of running straight up it. The US Forest Service Wood Handbook, a 1956 Forest Products Laboratory report, a paper hosted by Forest Research and a university newspaper column describe what it is, what it does to timber, and why no single cause is settled.
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
The US Forest Service Wood Handbook (Forest Products Laboratory report FPL-GTR-282, March 2021) says spiral grain is caused by winding or spiral growth of wood fibres round the bole instead of vertical growth, and calls it a form of cross grain. Its glossary says the spiral may run right-handed or left-handed round the trunk. A paper on Sitka spruce hosted by Forest Research (Northern Research Station, Roslin, with the University of York) defines the angle as the angle between the longitudinal wood elements and the axis of the stem, and says spirality is a common characteristic of almost all tree species, in both conifers and broad-leaved trees.
How it shows on a standing tree is described in a 1956 Forest Products Laboratory report by B.H. Paul on ponderosa pine. It says standing dead trees that have lost their bark often show checks in the wood running at an angle to the length of the tree, spiralling right in some trees and left in others, and that these checks show the direction of the grain on the surface of the trunk. The Alaska Science Forum column of August 25, 1986 (Geophysical Institute, University of Alaska) describes the same thing as spiral marks in old trees that crack open diagonally round the trunk.
What the sources say it does
| Source | What it says |
|---|---|
| Wood Handbook | In sawn products it often goes undetected by ordinary visual inspection, and any form of cross grain can have a deleterious effect on mechanical properties or machining characteristics. |
| Paul, 1956 | Grain direction in relation to the axis of a timber influences strength, warping during seasoning and twisting of posts and poles. Reversals of grain direction within a trunk indicate opposing forces that might influence the behaviour of wood, particularly the twisting of round products. |
| Forest Research paper | Spiral grain is strongly associated with twist in dried timber. The paper says it decreases the strength and stiffness of the wood and causes warp when timber is seasoned, and that for wood products large grain angles are a defect. |
Causes the sources name
The Forest Research paper says a large number of factors have been proposed as possible causes, and that none explained its occurrence adequately. It reports research on wind, temperature, soil nutrient status, rainfall, exposure and altitude, and quotes Harris (1989) that there is no support for the belief that regular patterns of spirality can be initiated by, or attributed primarily to, any environmental factor. It says despite intensive research the fundamental cause of grain angle changes in individual trees remains a mystery, and that, per Harris, grain angle is highly heritable although its expression may depend in part on the environment.
The same paper cites Kubler (1991) for two possible benefits to the tree: that spiral growth lets water from each root reach nearly every branch, and that spiral-grained stems and branches bend and twist more in strong wind. These are the paper's account of that work, not findings of the Wood Handbook or the Forest Products Laboratory.
The Alaska Science Forum column is a newspaper column, and its reader suggestions are labelled as speculation in the text. One reader proposes that westerly wind on a thicker south-side crown twists the tree over the years. A Geophysical Institute professor proposes the Coriolis effect of the earth's rotation, and the column says local effects such as topography play a much greater role than that very small effect. Both readers suggest checking trees in the southern hemisphere for the opposite twist. The column says the question had been asked in the same column over ten years before.
Where this ends
These sources are about wood quality, not about whether a standing tree is sound, which is a tree risk question: see tree risk assessment. How wood grows in layers is on the how a tree works page.
Other growth forms are on the odd tree growth page.
Sources
- US Forest Service Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (FPL-GTR-282), March 2021, read October 9, 2026
- US Forest Service Forest Products Laboratory, Changes in spiral grain direction in ponderosa pine (B.H. Paul, report FPL-2046), 1956, read October 9, 2026
- Forest Research, Modelling spiral angle in Picea sitchensis (Mavrou Ioanna, University of York with Northern Research Station), read October 9, 2026
- Geophysical Institute, University of Alaska Fairbanks, Alaska Science Forum: More on Why Tree Trunks Spiral (Larry Gedney), August 25, 1986, read October 9, 2026