Structural pruning of young trees: what extension and research sources say

This page reports what the named codes, regulators and makers say. It is not training and does not teach the work. The three sources are an Iowa State University Extension page, a Purdue University Extension bulletin and a technical report from a commercial tree-care research laboratory. None is a law or a standard. The pruning standards that name the cuts are on the pruning standards guide, and the courses that teach the work are on the certifications guide and the chainsaw tickets guide.

What each source says, side by side

Point	What the sources say
When it starts	Iowa State: at planting, limited to removing weak, dead, diseased, rubbing or injured branches, with training pruning taking over about one year after planting. Purdue: at planting and through the first years of establishment, longer if needed. Bartlett report: once the tree has established and resumes normal growth, which it says is generally two years after planting.
How long it runs	Iowa State: each spring for the first five years. Bartlett report: yearly inspection for the first ten years after establishment, up to 25 years on large-maturing species, with cycles of 2 to 4 years from year 11 to year 25. Purdue says the goals may be reached in two growing seasons or take several years.
The leader	Iowa State: one central dominant leader unless the tree is naturally multi-stemmed, with the weaker of two leaders usually removed. Purdue: a vertical stem near the center of the crown free of wounds, with the competing stem removed, and the leader twice the diameter of its attached branches. Bartlett report: a single stem for half the mature height, so at least 35 feet on a species that reaches 70 feet and 10 feet on dogwood.
Branch size	Bartlett report: no branch larger than 50 percent of the stem diameter at its attachment, with larger ones reduced to slow them, which it calls subordination. Purdue: leader twice the diameter of attached branches.
Branch spacing	Iowa State: 24 to 36 inches apart on alternating sides of the stem. Bartlett report: 12 to 18 inches apart at maturity on large species and 6 to 8 inches on small trees.
Lower branches	Iowa State: left as long as possible, and if not permanent, removed before they exceed one inch in diameter. Bartlett report: foliage kept on the distal two-thirds of the stem and of each permanent branch.
Season	Iowa State counts the training years by spring. The Bartlett report says winter, when leaves do not hide the branches. Purdue gives no season.

Why the sources give for starting early

Iowa State says the time and expense of training a young tree are always much less than corrective pruning of a neglected mature tree. Purdue says the same about cost, and adds that a strong central leader is one of the most important factors in preventing structural failure in storms and wind. The Purdue bulletin defines codominant stems as two or more main stems of the same or nearly the same diameter from one point on the trunk, and says included bark in the union marks a more advanced stage that is more at risk of failure.

Which species the sources single out

Purdue says trees with a decurrent, rounded crown such as most maple and elm tend to have weaker clustered branching than trees with an upright, excurrent form such as sweet gum, pin oak and most conifers. The Bartlett report says the same of decurrent species including elm, honeylocust and redbud, and adds that opposite-branched species including maple, ash and dogwood more often develop codominant stems when young. Its table of species that need frequent structural attention lists maple, birch, katsuratree, redbud, yellowwood, dogwood, ash, honeylocust, crapemyrtle, crabapple, pear, willow, pagoda tree, linden, elm and zelkova, each with a short note on its habit. For pear it says multiple leaders on nursery trees are difficult to correct and that pruning for structure must begin in the nursery.

Older trees

Purdue also covers medium-aged trees, which it defines as 15 to 50 years old, and says most healthy trees of that age can still seal wounds and redirect growth to a new leader. It says that where a branch removed is less than half the diameter of the trunk, wound closure is expected, and that wounds over 4 inches can be difficult to close. For a large stem it describes reducing in increments over several sessions rather than removing it at once.

Where the sources differ

The dates for the first cut differ: Iowa State and Purdue begin at planting, while the Bartlett report waits until the tree has established. The sources measure spacing differently too, since Iowa State gives a fixed gap of 24 to 36 inches while the Bartlett report ties the gap to tree size. Planting itself is covered on the planting depth guide, and what extension services say about topping and wounds is on the topping guide and the wound dressings guide.