Landscaping to Protect Our Trees
by Betsy Washington
With all the talk about the need to protect our majestic trees and at the same time allow updating and adding on to our older Lake Barcroft homes, I thought it might be helpful to review a few strategies for optimizing the protection of existing trees and plantings when planning for an upcoming construction or landscaping project. This is particularly important in lakeside properties that fall under the auspices of the new Chesapeake Bay Resource Protection Act (RPA).
The health of our soil is one of the most important factors in protecting our trees and the health of our landscapes. Recent research shows that most tree roots are located in the top two feet of soil and extend out 2 or 3 times past the canopy or dripline of the tree. The most important roots are tiny threadlike roothairs that are primarily located in the top six inches of soil. These are the roots that absorb the oxygen, water, and nutrients required by living plants. Our soils truly contain a rich web of life beneath their surface. Soil fungi and bacteria form critical partnerships with the roots of virtually all of our forest trees and over 95% of land plants, effectively extending the surface area of a plant’s roots by 10 – 1000%, vastly increasing the amount of water and nutrients available to the plants. In addition these microorganisms play a crucial role in protecting our trees from disease and insect pests. Microorganisms cannot survive in soils that are disturbed or compacted by construction. In addition, oxygen is unable to diffuse down through compacted soils to reach the roots and the fine root hairs are unable to penetrate compacted soils and take up the life giving materials (water, oxygen, etc.) as before. The result is often disastrous for the health and even life of your trees.
Healthy soil structure is created over many decades by a combination of factors such as the freezing and thawing cycles and by thousands of microorganisms that break down organic matter such as fallen leaves and incorporate it into the soil. One single pass of heavy equipment such as a bobcat can completely destroy your soil’s structure. This is the reason it is so important to plan carefully before you begin construction or a new landscaping project so that you protect your soil and your priceless trees.
Here are a few rules to keep in mind when modifying your property:
First, inventory the existing soils and vegetation on your site and decide what should be preserved. It is better to save a group of trees and shrubs than to try and save isolated trees dotted here and there. Trees located immediately adjacent to areas slated for construction are least likely to survive, while healthy trees located farther away have the best chance of survival. Likewise, when planning new garden beds, try and group your plants together in planting beds
Second, plan to limit construction damage to as small an area as possible, perhaps near the road or driveway. Clearly fence off all areas that you want to protect, keeping in mind that most of the important roots of your trees are located in the top few inches of your soil and extend 2 or 3 times past the canopy of your trees. Insist that all construction vehicles, construction materials, and stored soil be kept away from tree roots and in designated construction areas.
Third, carefully remove and save your precious topsoil during construction. Many contractors remove topsoil from your site and sell it to others. The top few inches of soil are typically nutrient rich with organic material and microorganisms that optimizes plant growth. Topsoil takes decades to form naturally, while the heavy clay subsoil left from construction that is typically spread back over your garden, will not support healthy plant growth and is extremely difficult to improve even with lots of amendments!
Fourth, be sure to consider the impact of different types of materials and construction techniques when planning construction or landscaping. Stripping topsoil, re-grading soil surfaces, and excavating for footers and foundations for walls, patios, and walkways are all especially damaging to plant roots and soil microorganisms. If possible, consider using decking on wooden piers or flagstones or bricks set on a sand or stone dust base. This will have minimal impact on existing tree roots and will allow water and oxygen to permeate into the soil.
Fifth, never spread left over soil or ‘fill’ dirt over existing plantings; even as little as 2” of soil spread on top of existing plantings can smother delicate roots and ultimately kill your trees and plants.
Sixth, if you must have heavy equipment cross over plant roots, minimize damage by distributing the weight over as great an area as possible. Place large sheets of plywood over roots or spread a temporary 6” layer of coarse mulch over the ground. Wet soils are extremely susceptible to compaction and damage; avoid working when soils are wet or excessively dry.
Some of our most majestic native trees such as tulip poplars, white oaks, beeches and dogwoods are extremely susceptible to root disturbance from construction damage and soil compaction and should be protected from root disturbance. Likewise many of our most popular shrubs such as our native Mt. Laurel, rhododendrons, and azaleas have fibrous surface roots and require loose, airy soil and excellent drainage. They cannot tolerate compaction or having soil or fill spread over their roots. These plants make up a large percentage of the plant species that naturally cover our lakeside slopes, and play a critical role in preventing erosion. It is particularly important to take steps to protect them.