Trees Grow Best When Planted in Square Holes, but You’ve Known That for Awhile

An article published in The Guardian earlier this month has been making the garden news rounds, turning tree planting on its head with the notion that recent scientific research has finally, unequivocally, shown that it’s better to plant trees in square holes than round ones. At first I was like, “What the Sarah Jessica Parker is this?” but it turns out it’s true. It also turns out it’s been known for more than a century and a half.

“There is a difference of opinion as to whether the holes should be made square or round,” wrote horticulturist Robert Thompson for The Gardener’s Assistant, Practical and Scientific in 1859. “We much prefer the square form. In the first place, a larger hole can be sooner made; but there is a point of still greater importance. Although the ground may have been well dug, yet, when the (roots) come to the side of the hole, they have to penetrate a firmer medium than that of the more recently loosened soil the limits of the hole, whether this be round or square. In the former case, however, resistance is more direct.”

Let me break it down another way: roots are lazy and will always take the path of least resistance. They hate turning corners, and avoid it like the plague if they have a choice. In a square hole, roots have more space to strengthen and grow before they hit the obstruction of the firmer, un-dug soil, so they’re more able to psyche themselves up to the challenge of punching through. When the tree is planted in a circular hole, however, the roots hit the hard edge of the surrounding soil and, because they’re lazy, will follow the edge of the compact soil around and around, essentially staying in one little spot in the hole you dug instead of traveling the world in search of water, nutrients, mycorrhizae, and stuff like that. (It’s kind of funny that trees are a symbol of wisdom when they’re actually as dumb as well, a block of wood.)

Now you know to plant trees in square holes, but also make sure you dig it at least twice as wide as the root wad, so they have some space. And don’t plant the tree too deeply, or you can end up killing it; aim for keeping the soil just below the root collar. The Arbor Day Foundation has tips for planting container-grown trees, but you can get more bang for your buck with bare root trees; follow our tips for planting here.