Fall is nearly here. That makes it a good time to plant a tree.
“The most important job for a newly planted tree is growing roots,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “If you plant it in early autumn, the soil will still be warm enough for root growth for at least a couple of months.”
Fall planting agrees with most kinds of trees, such as buckeye, catalpa, crabapple, hackberry, hawthorn, honey locust, elm, Kentucky coffee tree, linden, maple, oak, sycamore, pine and spruce.
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Some tree species should be planted in spring because they need a whole growing season to get well established. They include bald cypress, American hornbeam, ginkgo, larch, magnolia, hemlock, sweetgum, tulip tree and willow. Broad-leaved evergreens, such as rhododendrons, are best left for spring too.
Here are some tips from the Plant Clinic for fall tree planting. Learn more about tree planting at mortonarb.org/planting-trees.
Plant early in the fall: Get the tree in the ground while there’s still plenty of time for those roots to grow. “August to September is best,” Yiesla said. If the autumn is warm, you may be able to push the deadline to mid-October.
Don’t plant if the soil is too wet: Fall rains are usually a blessing to trees, but if there has been too much rainfall over the summer, the soil may be too wet to dig.
Buy a tree in a pot: A container-grown tree will have all its roots. One that has its ball of roots wrapped in burlap was grown in a field and dug up, losing much of its root system in the process. “Wait until spring to plant a balled-and-burlapped tree,” she said.
Check that it’s healthy: Choose a tree that is fresh stock, or that has been watered and well cared for throughout the summer. A good garden center or nursery that guarantees its plants should be able to give you that assurance.
Select the right site: Be sure the place where you plan to plant will have space for the tree after it has grown to its full, mature height and width (check the label for that information). Look around for power lines and make sure it won’t interfere with them when it grows big.
Don’t plant too deep: This is the single most important thing about planting a tree. To figure out the right depth, locate the root flare —the place where the trunk flares out into the big anchoring roots, the way your neck flares out into your shoulder. That flare should sit right at the level of the surrounding soil and no deeper. Be aware that the root flare may be buried in several inches of soil in the pot when you buy the plant. Dig down to find the first fat roots growing out sideways; that’s the root flare. Remove all soil above that point.
Take two measurements: Measure the width of the root ball (on a potted tree, this will be the width of the pot). You’ll want to dig a hole at least two to three times as wide as that measurement. Then measure from the root flare to the bottom of the root ball; your hole must be no deeper than that measurement.
Dig a wide, shallow hole: It should be shaped like a low bowl, with sloped sides, so the roots have room to grow out sideways. Spreading a tarp to dump the soil will make it easier to refill the hole later.
Place the tree in the hole: Slip the pot off and place the tree in the hole. Double-check that the root flare is at or just an inch or so above the level of the surrounding soil. If it’s lower, remove the tree and put some soil back in the hole so it can sit at the right height.
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Make it straight: Have someone steady the tree while you walk all around to check that it’s upright. “Tree trunks are rarely perfectly straight, but the overall direction should be vertical,” Yiesla said.
Refill the hole: Fill in soil around the tree in two or three layers, gently tamping it down between layers.
Water the tree: Pour at least 10 to 15 gallons of water gently onto the root ball. You will need to water the young tree often, whenever the soil is dry, so check the top inch or two of the soil for moisture every few days until the ground freezes.
Spread mulch: “It should be an even layer, not a pile,” Yiesla said. Rake it out to make a wide circle about 3 to 4 inches deep and at least 3 feet wide. The mulch will insulate the tree’s roots from fall hot spells and also keep the soil warmer after the air turns cold, so the roots can keep growing. “The more roots the tree can grow in fall, the better its chances to thrive next spring and for years to come,” she said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.