The lone voice of horticultural reason

“When planting a new tree, DO NOT AMEND THE SOIL you shovel back into the hole around the tree.”

Well gee, um, OK, I won’t, even though it goes against every shred of instinct I’ve developed over my years and years of gardening. But if you say so …

WHO says so? Why, just about every member of this year’s new crop of TV gardening experts, tired old garden writers and gullible radio gardening experts, that’s who. They have forwarded this myth along to the point it’s now considered the “in” fact to know about tree planting-except it’s wrong. Here’s the whole story:

Researchers at the University of Minnesota discovered that the best way to plant a tree was to plant it in as big a hole as possible. Instead of digging a hole twice the diameter of the pot or root ball, you should dig and rototill and break up the soil around the new tree as wide as possible. Otherwise, the roots of the tree start circling in the hole, where the soil is loose, instead of breaking through and heading out to establish a broad, long root structure.

The U also found that if the soil was amended in a too-small hole, this encouraged the tree’s roots to circle, and never leave the safe confines of amended soil. In these situations, the trees suffer and are prone to disease and drought problems because their roots never get very far from home.

They did not say to never amend the soil, and two months ago the top tree expert at the U’s School of Horticulture confirmed this to me in person.

If you dig a hole for a tree and the soil is junk, just crappy, inert, lifeless junk, the stuff you often find in a yard underneath the ribbon of black topsoil, it’s ridiculous to miss this opportunity to add some organic matter to the soil that you use to fill in the hole after planting. The tree needs organic microbes and fungi in the soil to survive. But don’t dig a small hole-dig as wide a hole as possible.

If you dig a hole for a tree-again, as wide as possible-and the soil is decent, loam soil, then you don’t need to amend it.

The last time I planted a tree, it had a root ball that was about twenty inches across and twenty-four inches tall. I dug a hole two feet deep and four feet across. Then I got the rototiller out and loosened the soil around that hole until the area I had busted up was a circle ten feet across. The soil wasn’t too bad, but it was pretty coarse and gravelly halfway down, so I amended the soil with compost and peat moss.

So what the U said was don’t amend the soil if you dig what was once considered the correct size hole-it’s going to encourage root girdling. Dig as wide a hole as possible, and if the soil is dead, amend it.

Your instincts were correct all along.

Don Engebretson
The Renegade Gardener