Planting depth and root flare: what extension services and BS 8545 say

This page reports what the named codes, regulators and makers say. It is not training and does not teach the work. Each publication below is guidance from a university, a state forest service or a tree body, not a law. The standards for nursery stock and planting, with their publishers and who cites them, are on the planting and soil standards guide. The courses that teach tree work are on the certifications guide. Staking and watering are on a separate page and mulch on the mulch page.

The root flare in the sources' words

ISA's Trees Are Good planting page defines the trunk flare as the place where the trunk expands at the base of the tree. Kansas State University (MF3313, July 2016) calls it the area where the roots and trunk merge, slightly thicker on most trees. University of Illinois Extension (Tree and Shrub Planting Guide, updated September 2020) says the flare is the part of the trunk that starts to taper out as trunk tissue turns into root tissue.

Illinois warns that the depth of the original container or root ball is misleading, because, it says, the flare on most nursery stock commonly sits below the top of the pot or root ball. For a flare that is not easy to see it says to dig into the container or root ball to find the first root or roots coming out of the trunk, which on a young tree with little taper are the best indicator. It says the most common mistake is a hole that is too deep, and that most tree roots lie in the top 12 to 18 inches of soil. ISA gives the top 12 inches (30 cm).

Five publications and one city specification side by side

Source	Flare and hole	Circling roots and wrappings
ISA, Trees Are Good, Planting a Tree	Flare: Trunk flare partly visible after planting; excess soil removed first if the flare is not visible. Hole: 2 to 3 times wider than the root ball, only as deep as the root ball.	Wrapping removed from root ball and trunk; wire basket removed or one or two rings cut off; circling roots straightened, cut or removed
Kansas State University, MF3313, July 2016	Flare: Container tree: slightly above or even with the ground; if the hole is too deep, the tree comes out and soil goes in. Hole: Shallow and saucer-shaped, 2 to 3 times the root ball diameter, no deeper than the root flare.	Balled and burlapped: top one-third to one-half of burlap and wire basket removed. Container: circling roots pruned, teased apart or the outer edge shaved, because left alone they can restrict root growth and girdle the stem
Colorado State Forest Service, How to Properly Plant a Tree, Part 2	Flare: 1 to 2 inches above ground level, because disturbed soil settles; the hole depth is the root ball height minus 1 to 2 inches. Hole: 2 to 3 times the width of the root ball.	The sheet says a circling root is cut before the point where it turns, and that slicing through a root after it has started to circle was shown by research to be ineffective. It describes shaving about 1 inch off the outside of the root ball
Auburn University and Alabama Extension, FOR-2192, January 2026	Flare: Slightly above the surrounding grade. Hole: Not deeper than the root ball or root system; 2 to 3 times the root ball diameter, soil in the pit loosened.	Shave or cut 1 or 2 inches from the outer root ball and tease out circling roots; all ropes, wires and synthetic burlap on balled and burlapped trees removed
University of Illinois Extension, updated September 2020	Flare: Even with the soil surface. Hole: Wider rather than deeper, edges of the hole roughened.	Not covered
City of Toronto, TS 5.30 Construction Specification for Planting, September 2021 (a city contract specification, not a general guide)	Flare: Trees planted at a level that places the trunk flare above finished grade, as the contract drawings specify. Nursery stock is rejected if the flare is not exposed above the root ball on arrival, and the specification puts the root collar within the upper 50 mm of the soil. Hole: Depth and width as the contract documents specify.	Root system free of stem girdling roots over the root collar and kinked roots. For balled and burlapped stock, the top half of the jute wrapping and wire basket is cut away and burlap is not pulled from under the root ball. Backfill goes in 150 mm lifts.

What goes back in the hole

Kansas State says not to amend the backfill with organic matter, because that may change how water moves into and out of the root ball and does not help establishment. The Colorado State Forest Service sheet gives a rule of thumb of no more than 5 percent organic matter in the backfill, and says a rich compost can make roots reluctant to grow into the native soil. Kansas State says not to bury the root flare when backfilling. The Colorado State Forest Service sheet says no soil is put on top of the root ball, and says that undoes the work of finding the flare.

Safety lines inside the sources

Kansas State and ISA both tell planters to call 811 before digging; ISA gives 811 as the national call-before-you-dig number in the United States, and Kansas State says to call at least two full working days ahead. The call before you dig guide covers the locate services. The Auburn sheet recommends protective gloves and eye protection when cutting tools are used on a root ball.

United Kingdom

BSI lists BS 8545:2014, Trees: from nursery to independence in the landscape, as current and under review, published February 28, 2014. BSI says it gives recommendations for transplanting young trees with a view to the tree reaching independence in the landscape, and covers planning, design, production, planting and management of young trees. It applies to trees with a distinct crown prepared in the nursery, not to whips, transplants or seedlings. BSI's catalogue page does not state a planting depth, so the depth rule in that standard is not reported here. The page is the BSI Knowledge entry for BS 8545.