In the Garden:
New England
March, 2012
Regional Report

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Get your tree off to a good start by planting it at the correct depth.

Plant for Success -- Tree Planting Guidelines

Of all the plants that contribute to a sustainable landscape, trees are among the most important. Besides adding beauty, trees help the environment by taking up carbon dioxide, the greenhouse gas that is such a big driver in climate change. They provide shade that helps to reduce the energy costs of cooling our homes in summer, act as windbreaks to help cut winter fuel bills, and provide food and habitat for creatures big and small that share our landscapes.

All of the benefits a tree provides increase as the size and maturity of the tree increases. A healthy tree is a long-term investment. You'll reap the biggest dividends if you take the time to get your young tree off to the best start by planting and caring for it correctly. Consider a well-grown tree a gift not only to yourself but to the generations to come who will enjoy it spreading branches.

Plant at the Proper Depth
Tree roots need water, but they also need oxygen. The root system of a tree that is planted too deeply will slowly suffocate. So it's very important to plant your new tree at the correct depth.

Find the Root Collar
The best way to ensure this is to make sure you identify the root collar on your tree before you put it in the ground. What do I mean by root collar? Start by looking at a tree that has grown naturally in the landscape. You'll see that the base of the trunk gradually widens or flares out as it enters the ground. If you pull back the soil at the base, you'll see the tops of the main order roots spreading out. You want to plant your own tree so that this junction of trunk flare and main order roots, known as the root collar, is right at the surface of the soil (or even slightly above in heavy soil). The flare of the trunk is not as noticeable on a sapling as it is on a mature tree, but if you look closely, you'll see it -- that is, if it's not buried.

And there's the problem. With both trees grown in containers and those sold with balled and burlapped (B & B) root balls, the root collar frequently gets buried in the course of digging or repotting. When the root balls of B & B trees are dug up, quite a bit of soil often gets thrown up around the base of the trunk before the ball gets wrapped in burlap. An arborist told me of having encountered some B & B trees with root collars buried as deep as 10 inches! And when container-grown trees are repotted, they may end up with their root collars below soil level.

So forget the old advice to simply measure the height of the root ball to figure out how deep to dig the planting hole. Find the root collar by carefully scraping away the soil around the base of the trunk until you find the trunk flare and the tops of the main order roots. Then dig the planting hole only as deep as the distance from the root collar to the bottom of the root ball.

Why only this deep? You want to set the root ball on undisturbed soil so that it won't settle after it's in the ground and end up down too deep.

Make a Broad Planting Hole
As for the width of the hole, make it broad, at least two to three times the width of the root ball; as much as five time the width in compacted soil. Slope the sides of the planting hole outwards and rough them up with the edge of your spade. This is especially important in clay soils where digging can leave the sides of the hole slicked over and hard for plant roots to penetrate.

Set the Tree in the Hole
With container-grown trees, slide the root ball out or cut away the container and spread the roots out before setting the root ball in the planting hole. With a B & B root ball trim away as much burlap as you can and remove any twine; if the ball is in a wire basket, cut and fold back the wire from the top half of the ball. Sometimes this is easier to do once the ball is in the hole. Basically, you're trying to remove as much non-plant material as possible without causing the root ball to fall apart. If you notice any roots that are kinked or encircling the root ball, trim these away.

Be sure not to hold the tree by its trunk as you move it into the planting hole. And be careful not to drop the ball into the hole, as this can break off roots within the ball. If the root ball is heavy (and soil does weigh a lot!) try using a tarp under the ball to drag the ball carefully into the hole, using a plank of wood as a slide if needed.

Backfill with Native Soil
Now it's time to backfill the planting hole. Your first impulse may be to add lots of fertilizer and organic matter to the soil you put back in the hole. But stop! Your tree will do best if you use only the native soil to refill the hole. Why not improve this backfill soil? Because doing so will encourage roots to stay within the pocket of hospitable improved soil rather than crossing the boundary into the native soil to become established, which will ultimately interfere with the long-term health of the tree. And if you are planting in heavy soil and you fill the planting hole with lighter, more porous amended soil, the roots can drown as the hole fills up with water during rainy weather, just like a bathtub, with the water held in by the heavier soil outside the hole. So stick with the policy of what comes out of the planting hole goes back in.

Start by filling the hole about halfway with native soil, breaking up any large clumps with your shovel while taking care not to damage any tree roots. Then add 5-10 gallons of water to the hole and let it drain through, settling the soil. Add the remaining backfill, using the leftover soil to create a low berm around the edge of the planting hole to contain water. Then add another 5-10 gallons of water. Firm the soil in the hole with your hands, not your feet or you can compact the soil and restrict the growth of the roots.

Stake Only if Necessary
Staking is something else that most gardeners think is a must. But in many cases this is a task you can skip. Only stake your newly planted tree if necessary. Most trees with trunks smaller than two inches in diameter don't need staking unless their root ball is crumbling, they are planted on a slope, are sited in a very windy location, or have a badly bowed trunk. The natural movement of an unstaked tree helps it to develop a sturdier trunk and a more robust root system. If you do stake your tree, be sure to remove the stakes once the tree is established, usually by the second season in the ground. Also remove any tags or trunk guards on the top of the tree at planting time.

Mulch Mindfully
Finally, spread mulch 2-3 inches deep over the root zone of the newly planted tree to help conserve soil moisture and keep weeds down. But don't make a mulch "volcano." Piling mulch up against the trunk can lead to rot and disease. Instead leave 4-6 inches of bare soil between the trunk and the mulch.

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