When I was a younger landscaper working in our London garden centre, I planted a lot of trees. If the root ball was 36 inches wide, I probably chose a planting width of about 40 inches.
That’s the way most landscapers do it because it just saves so much time. The thing is, you’re really preparing an entire area for your shrubs and trees. If you just go for a planting width of 40 inches with straight sides for a 36-inch root ball, the roots aren’t going to be very excited about leaving that hole.
In compacted soil, they may just turn around when they hit the edge of your hole and grow in a circle within the soil, resulting in an unstable, unhealthy, unsafe tree.
If you’re trying to do the best job possible when planting trees and shrubs, go wider with your planting width and make sides that have a more gradual slope. I now dig my holes at least 1.5 times wider than the root ball.
I suggest going 2-3 times wider in compacted soil, although if I’m digging with just a shovel in compacted soil, for a big root ball, I may not want to spend that many hours digging one hole, so I can’t really tell you to. But I will always go at least 1.5 times wider.
Prepare a tree ring around the tree as wide as you’re willing to go. Improve the soil with compost and other amendments, either double digging or rototilling.
For depth, you need to look for the trunk flare at the base of the tree. It may not have a discernible flare, in which case you need to find the top most major root and make sure it’s within 2 inches of the top of the root ball.
If it’s not, remove some of the top of the root ball. You may kill some surface roots, but it is more crucial for the trunk flare to be exposed. It can’t be buried or the plant may die a slow death that can take many years. The same goes for the graft – make sure it’s above ground.
The depth of the hole should be slightly less than the distance from that top root to the base of the root ball. That would mean about 5% of the root ball is above ground. If your soil is going to be very wet or compacted, you might plant so it’s 10% or even as high as 20% is above ground. Planting too high is much better than planting too low. I don’t want to plant even an inch too low.
Planting high ensures the roots will get enough air and lessens the chance of too much water ending up in the planting hole. It also makes it almost impossible to develop roots that circle and strangle the trunk, which happens when planting trees too deep.
If you’re planting in a loose, sandier soil, you may only need 1 or 2 inches above ground for your trees. In a compacted soil with a big root ball, you may have 6 inches above the ground.
Don’t loosen up the soil underneath the root ball. You want it solid so the plant doesn’t sink. If it’s very compacted or rocky, you may drill some holes down in there to encourage some roots to go deep to anchor the tree.
If you’re on a slope, the top root should be even with the ground at the top end of the ball. The bottom end will be high above the soil and will need a berm under it.
If you have lots of roots circling on the outside of the root ball, the traditional thing to do is to slice them vertically with a spade or saw, as much as 8 times around the ball. The idea is that it encourages the roots to move out into the soil instead of continuing to circle.