August 10, 2007 Articles No Comments

By Mike Allen

Written for the Lee Valley Tools newsletter August, 2007

Gardeners often face tough decisions regarding the sizes of their mature trees, especially if the trees are crowding each other and casting heavy shade over shrubs, perennials and grass. Once crowding sets in, the health of those plants will be questionable at best. As a certified arborist and urban forester for over 36 years, I find this problem is one of the most common faced by homeowners who have planted trees in small yards. Colorado blue spruce saplings may appear dwarfed by a 40′ x 30′ yard, but in 10 years they will have grown approximately 15′, in 20 years they will have grown over 30′, and in 30 years they can reach nearly 50′ tall. And once that spruce reaches maturity, its canopy will be approximately 20′ – 25′ in diameter. So, why do we let it happen?

The More, the Merrier?

The most common reason we let overcrowding happen is that we purchase most trees and shrubs when they’re relatively small and our new yard has a lot of space available. Why be content to plant just a few, when there is space for a dozen or more? It’s especially tempting to fill garden space with small, woody plants—particularly in areas around foundation walls and along perimeter beds adjacent to fences—since masses of woody plants are useful as privacy screens. For many gardeners, the goal is to foster growth in order to fill the yard and create a special, private space. And for many, the sooner this goal is achieved, the better.

However, many gardeners are surprised at how fast the trees and shrubs they plant can grow. Many people know that if they plant an ash sapling or a Colorado blue spruce, these have the potential to grow into tall trees with either a broad crown, as is the case with the ash, or a broad mass of boughs, as with the spruce. Placing the ash beside the smaller spruce may seem like a good idea at the time. You may even have an unspoken intention to transplant one of the trees to another location before it gets too large. More often than not, though, that intention is simply forgotten or you find better things to do with your time.

Survival of the Fittest

The problem with planting trees close together is that their roots compete for scarce nutrients and water and this may result in one growing faster and larger than the adjacent one. The competition may be so fierce, the weaker plant can’t cope and becomes unable to take up sufficient nutrient energy to sustain its life processes. This plant will be the first to die. So, in a sense, the overcrowding problem in this instance is solved. As a general rule, deciduous shade trees will usually outcompete coniferous evergreen trees of comparable size and age.

To remedy this, it may be a good idea to hire a certified arborist who can examine all of your woody plants and provide an opinion about the removal of the weakest and least healthy. Physically removing a dying tree is often a better solution than trying to coax it into marginal health. Sometimes pruning certain branches of a larger tree can resolve localized shading issues for nearby smaller trees, shrubs and flowering perennials. However, pests and diseases may be present on the plants you want to save and treatments must be undertaken. Consult bookstores, libraries, gardening magazine editors, horticultural departments at colleges and universities, or talk to your certified arborist. (As an aside, always check that your arborist has general business liability insurance.)

Buyers Beware

When starting a new garden or refurbishing an old one, people often consult the staff at the garden center where they purchase their plants. Some of these centers even provide simple garden plans or offer custom landscaping for a fee. But gardeners can get overzealous and buy more plants than can properly fit into their landscape plan. I find that it’s better to plant five to eight woody plants in an average urban back yard, rather than 15 to 20.

Another problem is that you may not know how to maintain the cultivated varieties of trees and shrubs that have been selected on your behalf. I have assessed the woody plants in thousands of gardens and I’ve noticed that almost half the plant species fail to thrive in the first five years after establishment. An individual plant’s growth and pest / disease control needs are very specialized, and often homeowners have not been appropriately informed on how to adequately deal with problems that may arise. Out of desperation, they may resort to spraying insecticides or fungicides and hope the problems go away. After these methods fail, they may hack away at branches, ignoring proper pruning and wound sealing techniques.

Planning Makes Perfect

Here is a tip I recommend to homeowners who want to establish a garden with an assortment of trees and shrubs. First, determine the height of the plant at maturity. Its root system, on average, will extend half of its height in all directions unless it is physically blocked from doing so. Secondly, tall-growing coniferous evergreens such as spruce, pine and fir, along with tall shade trees such as ash, poplar, maple, oak and linden, should be planted near the center of the yard, not within 15′ of a fence or any buildings. (Do not subject your neighbors to branches overhanging their property from a tree located on your property!)

There is a good argument to have an assortment of young woody plants, trees and shrubs, in a new garden. As woody plants grow older and larger, the compassionate gardener will select and remove some of the struggling plants early in their competitive life to make room for the fitter and more desirable ones. Keeping many struggling plants in competition with each other year after year is unhealthy and obviously detracts from the potential beauty of the plant arrangements in the garden.

Michael Allen
Urban Forester, Certified Arborist
Viburnum Tree Experts

Written by treeexperts-mb