How much crown to remove at one time: the figures extension services and forest services print
Maryland's Forest Service prints 25 percent of the foliage in a growing season. Kansas State prints 20 percent of the canopy in one pruning, Penn State calls a flat 25 percent too broad, and Missouri names 20 percent of the branches when planting a nursery tree.
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 lists the numbers the named sources print and what each one says the number applies to. It does not say how much to remove from any tree. The standards behind these figures are covered on the guide to pruning standards by country, which reports the ANSI A300, BS 3998 and AS 4373 scope without quoting their text.
The figures, source by source
| Source | Figure | What it applies to |
|---|---|---|
| Maryland Department of Natural Resources Forest Service, Tree Expert exam study guide on pruning | Not more than 25 percent | The foliage of a branch or limb cut back to a lateral, in a single growing season. The same guide says the Forest Service recommends not more than 25 percent of the foliage on a mature tree within a growing season. |
| Kansas State University and Kansas Forest Service, publication MF3551 (Tim McDonnell, March 2021) | No more than 20 percent | A properly thinned tree's canopy removed during any single pruning, typically for most species and circumstances. |
| Penn State Extension, Pruning Landscape Trees (updated November 5, 2021) | No flat figure | It says the total amount of live branches or foliage removed should be based on tree vigor, that up to 25 percent per year was the older guidance and is too broad for all trees, and that healthy young trees withstand wounding better than mature ones. |
| University of Missouri Extension, Pruning and Care of Shade Trees (G6866, reviewed March 2026) | 20 percent | Branches removed when planting a tree dug from a nursery. The sheet says removing more than about that share may reduce the tree's ability to produce food through photosynthesis and slow establishment. |
Other measures the same sources print
Several of the sources give a size ratio instead of a share of the crown. Maryland says the lateral a branch is cut back to should be at least one third the diameter of the removed portion so it can take over as the lead. The ISA Trees Are Good page on pruning describes reduction as cutting leaders and branch ends back to secondary branches large enough to take over the terminal role, at least one third the diameter of the cut stem, and says this keeps form and structure better than topping. The Arboricultural Association guide to tree pruning says BS 3998:2010 states that the diameter of the final cut should not exceed one third of that of the parent branch or stem, to keep the wound as small as possible.
What goes wrong when more is taken, in the sources' words
Missouri says excessive pruning may weaken a tree by removing too much leaf area and the food reserves stored in the stem, and that the apparent invigorating effect of pruning is only growth forced into fewer buds. Kansas State describes lion tailing as excessive thinning or removal of the interior branches, which leaves most of the foliage on the branch ends. It lists consequences: less photosynthesis, a surge of new growth that can raise water stress, possible sunscald on the tops of branches or the trunk, and branches with weight at the ends that fail in wind or ice. It says the practice is largely driven by public demand, and that homeowners tend to feel more material removed means more value. Maryland says lion's tailing reduces branch taper, sunburns bark and weakens branch structure, and that topping and lion's tailing are unacceptable practices.
How the sources define the work
Maryland's guide describes crown thinning as selective removal of live branches to reduce crown density, with an even spread of branches along each branch and through the crown, and with the percentage of foliage and the location and size of the parts removed to be specified.
Sources
- Maryland Department of Natural Resources Forest Service, Licensed Tree Expert Exam Study Guide: Tree Pruning (Chapter 4), read October 9, 2026
- Kansas State University and Kansas Forest Service, Lion Tailing and its Impact on Tree Health (MF3551), March 2021, read October 9, 2026
- Penn State Extension, Pruning Landscape Trees, updated November 5, 2021, read October 9, 2026
- University of Missouri Extension, Pruning and Care of Shade Trees (G6866), reviewed March 2026, read October 9, 2026
- International Society of Arboriculture, Trees Are Good, pruning your trees, read October 9, 2026
- Arboricultural Association, Guide to Tree Pruning, read October 9, 2026