No doubt as you drive or take a hike, you’ve noticed trees growing on slopes. Whether they’re volunteers or put there by landscape designers to hold the soil in place, certain factors must be considered to ensure they grow well in this unusual location.

Challenges for trees on slopes

Trees, or any plant for that matter, are destined to struggle when planted (by humans) on a slope. There are several reasons, but the most obvious is that the slope usually creates a microclimate where water fails to wet the rootzone evenly or completely. The key point in time for this phenomenon is when the rate of precipitation or irrigation exceeds the rate of infiltration. This occurs every time for moderate to severe slopes, and sometimes even for slight slopes depending on the soil type. In short, water tends to run off before it soaks in.

If trees on slopes are necessary, mulched and slightly terraced installations may serve the trees well.

PHOTO: JOHN C. FECH, UNL

Another challenge for trees on slopes is the competition with nearby vegetation—turf, forbs, groundcovers, etc. Plants with slightly shallower root systems than trees get first crack at rainfall; when trees are already stressed from being on the slope, competing with other plants is a double whammy. As well, since turf commonly needs more fertilizer and water than trees, these inputs are routinely applied in larger-than-needed quantities.

In addition to water application woes, it can be hard to fertilize without the product ending up somewhere other than the rootzone, at application and as the fertilizer granules or liquid move off target. Again, the steeper the slope, the more likely the product will be deposited on the downhill side of the applicator’s feet.

Treatment for pests such as emerald ash borer and foliar-feeding caterpillars is a bit dicey on a slope as well. It can be hard to treat for pests by soil drench, as when the slurry of insecticide formulation is mixed and poured on the root flare, it tends to follow the same fate as the fertilizer and irrigation applications. Pesticide spray applications often are difficult as well, as it can be hard to locate the spray rig on an even surface, and the down side of the tree can be hard to spray evenly.

Benefits of trees on slopes

In fact, there are fewer benefits to planting on a slope than challenges. Whenever “plant siting” is discussed, it’s wise to start with the concept of Right Plant, Right Place (RPRP), where it’s about putting well-suited plants in any landscape setting. In this odd location, there are two main benefits:

  1. Trees can hold the soil through their fibrous root system.
  2. Once, or if, they get established, it’s often not necessary to mow, which is always dubious on a slope.

I suppose establishment of trees on a slope could serve as a vista planting, sort of a backdrop plant, something to be viewed from a distance. Actually, this design technique is commonly used on golf courses, especially when – you guessed it – there’s a slope near a feature of high importance, such as a golf green or clubhouse.

Solutions for trees on slopes

Fortunately, there are some solutions for the quandary of trees on slopes in order to give them a fighting chance for success.

First, in order to overcome the likelihood that applied water will run down the slope and away from the roots, drip or micro spray irrigation systems can be utilized; ones that deliver water slowly and evenly instead of quickly and ununiformly. Either with a drip-by-drip emitter or via low-volume spray heads, this equipment applies water at a rate that is less than the rate of infiltration, thereby drastically reducing runoff.

If changing the irrigation equipment isn’t possible, a second option is to adjust the irrigation controller to operate with “delayed starts,” where water is delivered to the point just before it ceases to percolate into the soil, just before it begins to run off. The applied water should be allowed several hours to soak in, then another one initiated. Many delayed start cycles are necessary on severe slopes. Monitoring the equipment is necessary as the cycle timing is being established, as well as periodically during the growing season.

The amount of water and fertilizer needed to keep the turfgrass vigorous is likely to exceed the needs of the tree, often leading to shoot growth at the expense of root growth.

In order to slow runoff and increase the odds of successful tree establishment, it may be helpful to establish a low-input groundcover between the trees, preferably one that has similar fertilizer and watering needs as the tree. In almost all cases, in order to maintain high-input turf, water and fertilizer is applied at a rate greater than is desirable for woody plants. Care should be taken in terms of placement of the groundcover plants’ roots, such that they’re sufficiently far from the tree’s root mass that they don’t compete for water and nutrients.

Slippery Slopes

Slopes can be rough on trees; drip irrigation can help.

PHOTO: JOHN C. FECH, UNL

As a rule, arborists are quite used to applying mulch near the base of trees; this is yet one more incidence where mulching is a good idea. It’s best to think of mulch as a root treatment, not placing it on the trunk, but starting 5 to 6 inches from the root flare and extending it outward 3 to 4 feet. In some landscapes, it may be appropriate to terrace the slope in conjunction with mulching to create small planting areas that are resistant to runoff. Doing so will slow the rate at which water runs down the slope and help to keep lawn-mowing equipment away from the trunk, which is more likely to be a problem where footing is difficult to obtain. Ideally, this would be done at planting time, so that additional soil isn’t placed over existing tree roots, which would reduce oxygen penetration between soil particles.

When the need arises to fertilize trees, it may be necessary to weigh the advantage of fertilization through injection when needed. While it certainly isn’t desirable to deliberately wound a tree, once in a while, if really needed, it’s better than surface fertilization – especially if small holes are drilled and a decay-resistant tree is to be fertilized. Another approach would be to deliver nutrients through a water lance at low pressure to reduce runoff.

In some cases, tree roots on slopes fail to absorb adequate water to keep up with the evapotranspiration of the site.

Finally, it may be wise to consider alternatives to trees. If a “green mass” is called for, shrubs or other plant material may suit the purpose just as well. Again, with RPRP in mind, there are no good or bad plants, just well-sited plants.

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