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.
I get that you’re trying to make the point that soil profile is the same whether you cut it into a square or circle and the root tip will find its way through it. But the soil compaction is not the same and the parent soil is not the same. So the analogy is it soil is not plastic doesn’t really work because when there is a texture interface the route will struggle and change direction. Apical root tissue will change direction when faced with a texture barrier(common with tree planting). The point you are making is that soil is soil and roots will find their way through it which is true, But you are glossing over the fact that a square hole does in fact make for a better root system. No, soil is not plastic. But for the sake of science what happens when you grow the same tree in a round pot versus a square pot? The square pot doesn’t suffer from girdling.
So I would call this a 1/2 truth or a more than plausible myth.
Why is the soil compaction any different in a square hole or a round hole? The compaction and soil texture will be the same in both. The point I am making is that the root can’t tell the difference between the two holes.
If a square hole makes a better root system, as you claim – where is the scientific evidence?
Conjecture.
Fantastic article: Formative, informative and utterly entertaining.
Square hole? Where do you find this stuff, lol.
With four hundred plus trees to plant in a months time thank you for putting my mind at ease.
Here in Central Florida with sandy soil there is no such thing as transplanting with a root ball. When I move bushes or tree saplings,I dig around the specimen about 2 feet out from the trunk in order to cut any lateral roots. Then I remove enough soil in a circular trench to loosen any tap roots. Finally I am able to lift the bush out with totally
bare roots! If it won’t be replanted right away, such as, when it will be relocated to another property, I wrap the roots loosely with wet newspaper or an old towel. Seldom does a bare root transplant fail.
Of course, they need to be covered from the hot sun until they start
to grow new leaves. Roots have no problem spreading out in sandy soil.
In fairness the Guardian article is more nuanced than the clickbait headline – anyone who’s observed horizontal root growth in thin topsoil on top of hard clay should get the point.
I love science and a skeptical mind
Interesting article, but I would have thought a bit of initial nutrient in the dug hole would help the plant establish. It doesn’t last long; soon leached out. Then later add nutrients to the perimeter of the hole to encourage the roots to grow outwards.
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