Even after a tree is selected and installed based on the site conditions of sun, shade, soil drainage, proximity to other trees and shrubs, nutrient availability, desired size, slope, surroundings, adjacent activity and more, it can fail to thrive.
Sometimes that’s because the tree wasn’t chosen well and sometimes it’s because it wasn’t planted well. But even more critical to the tree’s success is where it was planted. A tree’s proper location usually will determine whether it becomes an asset or detriment to the landscape.
Tried and true: RPRP
The gold standard of “Right Plant, Right Place” still reigns supreme.
Most arborists would say failure to mind this guiding principle begins with a lack of consideration for the tree’s size. Many reasons exist for this, including lack of foresight and denial that a certain species will actually get that big or wide. Many property owners are too lazy or self-centered to consider the future of their land and the potential ramifications associated with it.
The next consideration is the square footage of available rooting space required for adequate growth, nutrient absorption and structural and support. Although it’s a gross generalization, the average shade tree requires about 1,000 square feet of unimpeded surface – either covered by turf, mulch or groundcovers – in which to grow. In many locations, this much space isn’t provided.
The desired shape of tree – vase, cylindrical, rounded – also can play a role in RPRP. Some sites allow these forms to grow and develop well, and others don’t. The ones that require the largest available space are often limited on site.
The availability of sunlight is a key consideration when choosing tree species, as many require full sun exposure, while others are tolerant of, or actually prefer, shade. Species such as serviceberry, pagoda dogwood and redbud are often poorly sited as a result of this factor.
Tough sites and why
Unfortunately, many sites are compromised in one way or another. These are the most common tough sites.
Slopes: Gentle slopes are desirable; a 2 to 3 percent drop-off facilitates water movement away from buildings, yet generally allows for water movement downward through the water profile. When the degree of slope is 5 to 10 percent or greater, problems commonly arise in tree performance and landscape maintenance. At least four undesirable outcomes are associated with a severe slope:
- Decreased infiltration rate: On slopes, natural rainfall and irrigation water isn’t absorbed as quickly as at the top or bottom. Trees growing on the face of the slope often suffer from inadequate moisture in the root zone.
- Difficulty in mowing: When turfgrass is grown on the slope, an increased chance exists for the mower to slip and slide, potentially striking the tree trunk or lower branches, causing damage.
- Difficulty in application of fertilizers and pesticides: After landscape maintenance products are applied on slopes, there’s a tendency for them to move downward, especially when granular products are used and/or moderate to severe rainfall occurs after application.
- Difficulty in mulch retention: Over time, mulch pieces tend to drift downward or sideways on slopes, moving away from installed landscape plants.
Hell strips: The thin, narrow and sometimes oddly shaped portions of the landscape often provide an inhospitable location for trees to grow well. The lack of available rooting and absorptive space is the main limiting influence. In northern climes, hell strips (also called tree lawns or devil strips, depending on your region) are often the space between the sidewalk and a street; in these situations, applied salt and sand for winter traction control causes damage and adds to the lack of rooting. In general, the siting of woody plants in these locations should be carefully considered.
Parking lots: Parking lots offer many of the same negative influences as hell strips. Salt, sand, radiating heat – along with the added occasional misfortune of cars, juvenile delinquents and shopping carts running into the trunk – are the major ones. The main differences between the two are that parking lots generally offer a bit more rooting space and a whole lot more interaction with pedestrians.
Middle of turf: Generally, trees and turf don’t mix. In most scenarios, turf requires more water and fertilizer than trees; in a mixed planting, if moderate amounts are applied to keep the turf thriving, excessive amounts of both are received by the trees. As well, in the midst of an island of turf, tree trunks are prone to mower blight, especially by youngsters and turf maintenance professionals who are in a hurry. The key message to deliver to them is to stop the movement of the mower before it reaches the tree, not after.
Next to concrete and rock: These materials have their place in the landscape, but it’s hard to overlook their negative impact on trees. They have a warming effect on the soil, don’t facilitate horizontal root growth as well as organic materials, provide no soil replenishment and are just so-so on moisture retention and weed suppression as compared with organic mulches and materials.
Compacted sites, high traffic spots: Settings where the soil particles are routinely compressed are tough locations for trees. Common locations for compaction of the soil are those that receive high traffic such as parks, campus grounds and shopping malls.
Adjacent to tough sites: Locations that are adjacent to the tough sites of compaction, hell strips, parking lots and other concrete surfaces may appear to be in good shape, or at least have the potential to produce healthy shoots and roots, however they are still adjoining and share a root zone. At best, sites adjacent to poor locations are half and half – half compromised and, hopefully, half conducive in terms of healthy soil, adequate space and overall growing conditions.
Close spacing: Often a scenario where the original property owner didn’t take size into account and planted way too many trees in way too small of a space. Close spacing is really an issue of trees competing for sun, nutrients and water.
Extremes of sunlight reflection: Commonly noticed when one side of a tree – the side that faces an office building – becomes blighted by excessive sunlight that can cause desiccation of the bark, stems and leaves.
Extreme shading: Opposite of sunlight reflection, absence of sunlight can cause etiolation, or a stretching for adequate light to support sturdy shoot growth. Growth that occurs in a heavily shaded location is usually thinner and weaker than when grown under ideal conditions.
So, what to do?
Identification and understanding is the first step in dealing with difficult sites; four other actions are next as making a difference in a customer’s landscape:
First, it’s important to evaluate the status of the tree in question. Inspect for tree hazards and document defects that will influence future actions.
Second, decide whether to keep the tree in the landscape or remove it. Consider the number of issues that weaken its structural integrity and limit the aesthetic value to the customer. If the tree doesn’t contribute toward the goals for the property, perhaps it shouldn’t remain on site.
Third, if removal is chosen, possible replacement choices for each site can be contemplated. This is especially true for specimens that are not performing well due to an incompatibility with the size, sun, shade, soil and slope specifics of the site. For example, if a large tree is growing where a small one is called for, the potential replacements should be chosen from that group of options. In terms of possible selections for new specimens, local botanic gardens and arboretums are good places to gather information on suitability.
Fourth, avoid planting in tough locations in the future. Advise clients of the microclimates where trees simply aren’t a good choice. In many scenarios, tall shrubs, groundcovers, perennials, native grasses and other landscape ornamentals are better options.
PHOTO: JOHN C. FECH, UNL