Trees in the Sustainable Landscape

By the National Gardening Association, www.garden.org

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Of all the plants that contribute to a sustainable landscape, trees are among the most important. Besides adding beauty to our landscapes, 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. It’s not hard to do; it simply requires following some good selection and planting practices and keeping up with a little routine maintenance as the tree becomes established.

Choose Carefully

Step one is to choose a kind of tree that is adapted to the conditions in your yard and whose mature size is appropriate to the space you have available. Your tree may be a spindly sapling at planting time, but it can live for decades, even centuries. Be sure you know what the height and spread of the tree you select will be at maturity. Consider a well-placed tree a gift not only to yourself but to the generations to come who will enjoy it spreading branches.

Likewise, be sure to select a tree that will thrive in the climate, soil, drainage, and light conditions on your property. Make sure your tree can take the winter cold or summer heat in your part of the country. A soil test can help you figure out the acidity or alkalinity of your soil, which will guide your tree selection. For smaller shrubs, flowers, and grasses, it may be practical to modify the soil to suit the plant, but not for a tree. As it grows, a tree’s root system will spread far beyond the planting hole. The tree needs to be adapted to whatever the existing native soil conditions are in order to thrive over the years. Share the information you gather about your landscape conditions with knowledgeable staff at a good garden center or nursery and they’ll be able to help you choose the kinds of trees that are most likely to flourish in your landscape.

Plant Properly

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. 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.

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.

With container-grown trees, slide the root ball out or cut away the container before setting the root ball in the planting hole. With a balled and burlapped root ball, once it is in the ground cut away the burlap from the top half and removes any nylon strings. If the root ball is enclosed in a wire basket as well, cut away the wire from the top half of the basket.

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 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.

Water Wisely

You’ll have some extra soil left over after the planting hole is filled. Use this to build a low dike of soil around the outer circumference of the root zone. This will help to contain the water you apply, allowing it to soak down into the root zone of the tree, rather than running off. Water well right after planting, putting on enough water to wet the entire depth of the root ball. Then let the top few inches of soil dry out again before watering deeply again. Don’t sprinkle the soil lightly and frequently; this will only wet the top couple of inches and won’t encourage the tree to develop a deep and healthy root system. It’s better to set the hose at a trickle and let it run for a while, or fill a five-gallon bucket with water and pour it on gradually, letting each addition of water soak in. Be sure to keep your new tree watered regularly during its entire first growing season.

Stake Successfully

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 rootball is crumbling, or they are planted on a slope or where they'll get whipped by the wind. 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.

Mulch Mindfully

Finally, spread mulch several 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 a couple of inches of bare soil between the trunk and the mulch.

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