Planting Trees – Are Square Holes Better than Round Holes?

There have been several articles in the past few years that recommend planting trees in square holes rather than round holes. It is claimed that the square holes will result in more root growth into your native soil outside of the planting hole, which is good for proper tree development.

Does this work? Should we all start making square holes to plant trees?

The Problem: Girdling Roots

You have probably seen the problem either with purchased trees or perennials. The plant has been left too long in the pot and the roots have started circulating around inside the pot forming what is known as a root bound condition or girdling roots. Such root growth can result in the strangling of roots in future years and the death of the plant. This is a more serious problem with trees and shrubs since their roots remain woody and grow in thickness each year, but it also hampers the growth of perennial plants.

Provided that girdling roots are corrected at planting time, the new roots will grow out from the center of the tree, towards the edge of the planting hole. What happens then?

The theory goes something like this. Since the hole is round, just like a pot, and the soil outside of the hole has not been disturbed, the tree roots behave as if they are still in a pot. They start circulating around the hole, eventually creating a large mass of girdling roots.

The Solution: A Square Hole

When roots reach the edge of a square hole, they follow along the straight edge, eventually reaching a corner. Roots are not very good at turning a right angle, so they tend to grow out past the edge of the hole, into the native soil. The corners prevent the girdling root problem.

Flawed Logic With Square Holes

Is the logic of square holes sound?

Soil is Not Plastic

When roots reach the plastic pot they encounter a surface that is both smooth and hard. They can’t grow through it. They can only turn left, right ,up or down.

The edge of a round hole is not hard plastic. Even if the untouched soil is hard, it still has some porosity which allows roots to grow into it. Even if they can’t go straight, they will turn and look for a different opening in the native soil.

The Hole is Not Round

Humans have macro eyes – they only see the big picture and to us the hole looks round. But imagine being a root tip. You are very small and the edge of the hole looks like a giant flat wall. Because of your size, you can’t tell the difference between a  curved wall and a flat wall. This is very similar to being on a beach at the ocean. We see a horizontal horizon because we are too small to see the curvature of the earth.

To a root, the edge of the hole looks flat, whether it is round or square.

Roots Have a Hard Time Turning a Corner

One of the reasons square holes are claimed to work better is that roots have trouble turning a 90 degree corner. So when they reach the corner of the square it is easier for them to go straight through the wall, into native soil.

But …….. when roots grow out from the center in a round hole they hit the edge at less than 90 degrees. Turning in this smaller angle would be even harder for them than turning a 90 degree angle. If a 90 degree square hole makes roots grow straight, a round hole will be even more likely to make them grow straight. Which means round holes should work better than square ones.

The claimed logic is faulty.

Does Science Support a Square Hole?

We can debate the logic of using a square hole, but good science trumps logic every time. What does the science say?

There are numerous non-scientific articles online about the benefits of using square holes, but they all seem to be copied from one in The Guardian, a magazine that is notorious for promoting myths. This source did not include any references to scientific studies to support their claim, although it implies they exist.

I was only able to find one research study that looked at different shaped holes and it used oak (Quercus virginiana) seedlings. It tested square holes, round holes and star shaped holes. If the 90 degree corners in a square hole are effective, the smaller angles of a star-shaped hole should be even more effective.

They found that hole shape did not affect tree growth or the growth of roots into native soil.

They also tested round holes that were tapered towards the bottom and found they were slightly better than round holes without the taper. If you dig with a shovel, you naturally taper the hole a bit, which might be a good thing.

It is only one study, with one tree species, but it does not support the use of a square hole.

Suggestions for Planting Trees

I have developed a complete tree planting guide: Planting Trees the Right Way.

When digging the hole it is important to only dig it deep enough to accommodate the root ball. Don’t loosen the soil below the root ball.

It might help to roughen up the edge of the hole to make it easier for roots to grow out into the native soil.