Large trees and steep slopes can work together—but it takes thought.

By Annette Wilkus, FASLA

It’s interesting to watch the evolution of planting design alongside our profession’s affection for steep slopes. Using steep landforms allows us, as designers, to create dramatic rooms in small places. In any urban environment, the use of landform has become increasingly important to expanding our environment without increasing the square footage of its footprint. Slopes greater than 3:1 have become the sweetheart of the landscape architecture world—­and the steeper the better.

As space becomes smaller and landforms become steeper, clients are requesting larger plants that provide an instant landscape. The bigger the tree or shrub, the better. So we see a lot of enormous plants placed into sharp slopes as a standard practice.

In our practice, we’ve seen steep landforms become a challenge for contractors when installing large root balls, trying to establish the plants, and maintaining them during the warranty period. We’ve also seen misunderstandings among designers over the relationship between the steep slope and the root ball size associated with a large tree’s caliper. This challenge affects the success of the project and long-term maintenance.

The American Standard for Nursery Stock, ANSI Z60.1, is clear about the appropriate size of a root ball as it relates to the species and size of a tree. For example, the minimum root ball size for a multistem tree of 10 feet is 32 inches to 44 inches in diameter, depending on the habit of the species (Table 7 of ANSI Z60.1); the minimum root ball for a shade tree with a caliper of four inches is 42 inches in diameter and 25 inches deep (Tables 3, 4, and 5 of ANSI Z60.1).

Arborists recommend planting a tree with the root flare in its original growing relationship to the soil—this is of course with the understanding that the root flare is exposed in the nursery. If the tree doesn’t have its root flare exposed, the nursery must be instructed to expose the root flare prior to digging. Making sure that the root flare is exposed prior to digging ensures the root ball is dug at the appropriate depth and with an appropriate amount of root mass.

As a profession, we need to reevaluate our propensity for large plants on steep slopes. It is simply not sustainable.

We’ve seen several problems repeatedly where landform dominates the design and plantings are not sized properly for the slope. Most notable is the extent to which the landform is contorted to accept the root ball, both uphill and downhill. We consistently see grading plans with a smooth slope, and it’s clear the relationship between the size of the root ball and the grading plan is completely missed by the designer.

Should your client insist, even after you’ve explained the problems of installation, establishment, and maintenance, on planting large plants on steep slopes, you can still attempt to minimize the risks. But it requires different thinking. From our field experience, we’re seeing several developed details with a “terrace” in the landform to accept the root ball on the uphill side and mound around the root ball on the downhill side. Other options are using small, bare-root plants, bare rooting larger plants during planting, or using the Missouri Gravel Bed method, developed by Chris Starbuck at the University of Missouri, as a way to enhance the flexibility of the root mass.

The method we are most familiar with is terracing the slope to accept the root ball (see graphic above). The detail calls for a flat area created within the landform and the root ball placed on it. Once the root ball is placed, the grading on the uphill side is adjusted to avoid covering the root flare, and on the downhill side, mounding is performed to protect the face of the root ball. This method works for the initial installation, but we’re seeing short- and long-term consequences of this approach. There is difficulty in planting and establishment, and the need for maintenance techniques to serve the distorted grading over time.

First, the planting aspect. To simply provide the depth and width of the root ball, the contractor has to stabilize the surrounding soil while digging the hole—many times the contractor is in a hole three feet deep and four to five feet wide. We’ve seen geofibers blended into the soil and jute mesh placed after planting in efforts to stabilize the soil. Both of these methods accommodate planting, but over time they create their own maintenance requirements, which may include replacing jute mesh or chasing geofiber “dust bunnies” for several years after installation.

Landscape architects consistently overlook the difficulty of establishing plantings on steep slopes, where well-draining soils prevent plants on the top of the slope from getting enough water to establish themselves. We’ve had to manually operate a sophisticated irrigation system with a complex sequence of timing several times a day to ensure that the trees and plants at the top of the slope receive the requisite moisture and yet make sure that the plants at the toe of the slope aren’t overwatered. To accomplish this dual scenario, we’ve had to run the zone on the top of the slope for 10 minutes, turn it off for 10 minutes, and repeat several times a day during the acclimation period. Because the water is traveling downslope, we minimized any watering midslope and at the toe of the slope.

The third overlooked consequence of steep slopes is long-term maintenance. There is yearly cleanup. There is mulching. There is the special equipment the maintenance staff may need to reach all areas of the slope. These requirements all add to the cost of maintaining your design over time.

Using bare-root plants is not a new idea. The benefit of using bare-root plants over standard container or balled-and-burlapped plants is that when the trees arrive on site, the root system will be pliable and able to be adjusted to the designed slope. Many projects are enhanced by bare-root plants as long as the construction schedule accommodates smaller trees as well as early spring delivery and planting.

If those conditions are not a luxury, there are some exciting recent methods to accommodate large root balls and steep slopes. One of these methods is using an air spade to expose the roots of an existing tree either on site or near your site. The proximity of the tree’s final planted location is important for several reasons. One is the logistics of transporting it. Depending on the size of the tree, large construction equipment may be needed, and permitting may be necessary. Second is the time it takes to transport from excavation to installation. The bare roots will need to be kept moist and protected until they are in their new location, so traveling on a major highway may not be a practical option. One major benefit is that the root system will be able to be manipulated to respond to the steep slope while minimizing the need for contorted grading.

It is necessary to prepare early when bare rooting a plant by root pruning within the predetermined transplant root zone. Once the roots are exposed and excavated to a depth acceptable for lifting the tree, care must be taken while securing the larger roots with root ties to the trunk of the tree.

Another option for bare rooting a plant in anticipation of planting is the Missouri Gravel Bed method, which calls for bare rooting the tree and growing it in a matrix of pea gravel and sand while using drip irrigation. The benefit of this method is being able to plant trees throughout spring, summer, and fall—even in full leaf—with little shock, owing to the massive fine root systems developed (see “The Bare-Root Cause,” LAM, June 2011).

The practice of using bare-root plants is proving to accelerate acclimation, growth, and establishment. It argues toward an industry of bare-root stock as a norm. Landforms are important to design. They create space where none was before—providing multiple programming opportunities—and are made all the better, of course, with trees. The way to make it work is to realistically understand and embrace the relationship of the slope to the size of the root ball, and to think hard about establishment periods and all the maintenance that follows.

Annette Wilkus, FASLA, is the founding partner of SiteWorks LLC based in New York City.