Fall is prime time to plant trees in Minnesota

As temperatures drop and the days get shorter, Minnesotans think of putting the garden to bed for the season and perhaps putting their feet up for the winter. But there's one more task that homeowners can do now, with benefits that stretch far into the future.

Fall is the best time of the year to plant trees. The heat and dryness of summer are gone. And in fall, trees will focus on putting down roots rather than growing branches and leaves.

Evergreens should be planted very soon — they need to get their roots established before the ground freezes because they are sensitive to drying winter winds — but deciduous trees can be planted well into October.

Properly planted in a good location, trees may live for decades, so choose the tree and your site carefully. You don't want to plant a tree that could reach 60 feet in height under a power line. If you're a gardener, don't plant a walnut tree, which has chemicals in its roots that inhibit growth of other plants. Some trees grow best in a bit of shade, while others are better at coping with urban pollution or salt.

Forestry experts at the state Department of Natural Resources believe our changing climate will make it harder for trees like aspen, paper birch, tamarack and black spruce to thrive. Trees that should adapt well include lindens, black cherry, northern red oak, bur oak, sugar maple, red maple and eastern white pine.

Don't focus too much on buying a big tree. Smaller trees suffer less from being moved and may have faster early growth. The University of Minnesota Extension Service has a good guide to tree selection here: extension.umn.edu/tree-selection-and-care/recommended-trees-minnesota.

Once you have your tree, preparation is everything. Dig a hole that's a foot or two wider than the tree's container. Loosen the soil at the bottom of the hole. Now comes a critical step: figuring out what's going on with those roots before you plant.

Remove the tree from its pot, and gently scrape the surface soil away until you find the first pencil-thick lateral root coming off the trunk. Then take a sharp knife or pruning saw and "box" the roots — cutting the rounded sides of the root ball into a square. Cut a crisscross across the bottom to loosen the roots there.

It sounds brutal, but boxing will sever roots that want to continue circling as they did in the pot. Not doing this can lead to girdling roots, which eventually will strangle a tree.

With balled and burlapped trees, move the tree into the planting hole and cut off any twine that's wrapped around the trunk and top of the root ball. Cut away or push down as much of the burlap as you can without having the root ball crumble. Burlap that is sticking out of the soil will wick away water that the tree needs. If the tree is also is in a wire basket, it's a good idea to remove or push down wire that's near the top of the hole.

Tree trunks widen slightly at the bottom, and you should be able to see this "root flare" after you've planted. That first lateral root that you exposed should be at or near the surface of the soil. A tree that looks like a telephone pole after it's planted is a tree that likely won't live very long.

If a tree is grafted onto different rootstock — something that's sometimes done to create a hardier, tougher plant — there will be a prominent bump low on the trunk where the graft union was made. Unless you're told to bury that, always leave that knot above ground.

Gradually backfill the planting hole. Watering little by little as you fill the hole will help eliminate air pockets around a new tree. It's best to plant trees in the soil they will grow in, so amending the soil too much isn't a good idea. If you feel you must add something, mix a bit of compost into the backfill. But half or more of the soil that goes back around the tree should have been there before you dug the hole. If you change soil too much, you may create a basin where water doesn't drain normally, and root growth can be retarded.

Water and mulch about 3 inches deep around the base of the tree with wood chips, making sure to keep the mulch away from the trunk. Mulch that is too close to the trunk can create moisture problems and attract pests. Mulching helps the ground retain moisture and provides some insulation to the roots over that first winter.

New trees should be watered regularly right up until the ground freezes. The U of M recommends watering daily for a week or two after planting, and then watering every few days until the ground freezes. For a tree with a trunk that's an inch in diameter, use a gallon or two at each watering and about three gallons for a tree with a 2-inch trunk.

Young trees have tender bark that hungry rabbits and other rodents love to eat in winter, so protect your tree with a barrier of hardware cloth. You can also use tree wraps, but put those on only after it gets cold, and remove them first thing in the spring.

U of M Extension has a comprehensive guide to planting trees, with links that answer other tree questions: https://extension.umn.edu/how/planting-and-transplanting-trees-and-shrubs#container-grown-and-containerized-stock-1400311.

Mary Jane Smetanka is a Minneapolis freelance writer, a Tree Care Advisor and a Hennepin County Master Gardener.