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We just got into a new home and I wanted people to notice my flowers. They have, but besides compliments I've gotten some warnings. Is it true that I'm hurting my trees by planting flowers around the trunks? - L.M. -
If it's a young tree and you're planting right in or close around its root ball, that can't help and can hurt the tree. Its roots must grow new tips so the tree can establish itself, but competition from fast growing annuals right in that same space or grass at the outer edge of the root ball can slow or even prevent that. (A study by a tree planting specialist we greatly admire, Gary Watson of Morton Arboretum, measured the effect at a one-third loss or greater over 5 years.) This applies to all tree species, even the "tough" ones.
Clear and mulch around a new tree so it's at the center of a circle that's two feet in diameter for every one inch of trunk diameter. Keep that area plant-free.
For a tree planting step-by-step see Oaks got flare.
Increase the size of the circle as the tree grows, to eliminate plants above the new roots at the edge of the root zone. Loosen the soil in that new outer ring.
Expect the roots to extend themselves about twice as far as the branches if all grows well. (The tree pictured at the top of this page is sad for having its roots confined to the surface, but it's normal for the breadth of the root zone.)
If it's a species with sensitive roots (fleshy, easily broken roots that don't regenerate rapidly) such as magnolia, birch or dogwood, the stress of root disturbance is also a very big set-back. Planting annual flowers may seem to be very minor digging yet still can cause significant set back to the sensitive-root tree, even after it is well established.
So don't plant into a new tree's root ball or into the space just outside the ball. The best plantspeople urge us to eliminate the lawn in that circle. Give the tree at least a year to solo in its space.
The bigger the new tree is to begin with, the longer its solo time must be. If its trunk is up to one inch in diameter, one year may be enough time to establish, which means to grow enough roots to support its top. A tree with a two-inch trunk needs at least two years, a three inch tree three years, etc.
A tree with a trunk one inch in diameter at six inches above the ground is called a "one inch caliper tree."
After a tree's established but while you're still cheering it on, use minimal disturbance plants there. Choose groundcovers or long-lived perennials that don't have to be replanted every year and don't need frequent dividing.
While we're on the topic, if you're shopping for a new tree, buy small. It's the best return for your money and nets the lowest care tree.