Halifax Tree Project

 Nuisance Shade

Kate Thompson, IDPhD Program, Dalhousie University

2021-05-06

I adore our shady backyard. In spring, I wait for the canopy of basswood, maple, and ash trees to reveal itself. I enjoy seeing the dappled sunlight filtering through the red oak and red maple in my neighbour’s yard. There is not a better place to relax and cool down on a hot, sunny afternoon. 

I’m in good company. Shade is generally perceived as one of the most important values provided by trees in the city, along with values such as aesthetics and air quality (Ordonez et al., 2016). Shade provides essential benefits – primarily cooling, reducing exposure to harmful UV radiation, and reducing energy costs (Quinton, 2020). 

I will also admit to sometimes being conflicted about the amount of shade in my yard, and I can understand why people might view shade as a nuisance. I can’t grow sun-loving flowering plants and shrubs, and I have no chance of cultivating anything resembling a vegetable. Other urban residents share my dislike of shade in some locations, for similar reasons and others. A city brings together a diversity of people, so there is a diversity of views about shade from trees. Some of these may be related to cultural differences about the role of shade trees in private landscapes. For example, in a survey of shade tree preference, people with a British cultural background strongly favoured the aesthetics of shade trees, while those with an Italian background preferred fruit trees for the food they provided (Fraser & Kenney, 2000). 

Nuisance shade is just that – mainly an annoyance: it’s not a hazard, as are falling branches, and doesn’t affect health, as pollen or VOC production do. Fortunately, most of the perceived negative aspects of shade can be managed or prevented, and others we can learn to live with. 

What problems does shade create? A dense tree canopy can create areas of darkness or low illumination in parks, which may be perceived as unpleasant or ugly (Von Dohren & Haase, 2015). Management of the trees by pruning can mitigate this problem. Other shade issues are most apparent where people live, and relate to aesthetics, impacts on what plants can be cultivated, or the need for maintenance. Where shade makes a home’s interior too dark, removing lower branches can often help. In future, issues can be avoided by a careful choice of planting location for street trees, and selection of a species that is less likely to produce nuisance shade, in consultation with homeowners.                

Where a mature tree casts shade on a grassy lawn, many a Halifax homeowner has begun a fruitless war to kill the moss that takes its place, in the mistaken belief that the moss is outcompeting and killing the grass. Some people take the more extreme action of removing trees. There are better alternatives. A judicious pruning of the lower branches of a problematic tree will allow more skylight to illuminate the area beneath the tree. Usually, this amount of light is sufficient to cultivate shade-tolerant grasses or shade-loving ground covers. 

As the grasses planted in lawns are generally sun-loving varieties, the mosses that grow in shade are simply better suited to the growing conditions than grass. Why not give up the battle and enjoy a low cost, low maintenance, verdant carpet of mosses, which is both pleasing to the eye and to walk on – and much better for the environment? I have embraced another solution – planting and enjoying some of the shade-tolerant or shade-loving, native Nova Scotia shrubs and plants or their cultivars, such as rhododendrons, viburnums, or ferns. 

Trees that shade the sides and roofs of homes provide cooling benefits, reduce exposure to UV light, and may prolong the life of the roof and reduce the need to re-paint siding. When a tree’s branches are very close to a roof, the shade can encourage the growth of moss and lichens. Some people find this a charming addition to the roof; others dislike the unkempt appearance. The manufacturers of roof coverings such as asphalt shingles warn that this growth leads to increased weathering and deterioration of the surface (IKO Industries Ltd., 2021). This may be true, but I have been unable to find support for the claim in the research literature on building materials. If it is viewed as a problem, the growth can be removed from the roof and further prevented by trimming the nearest overhanging tree branches.

In Halifax, some nuisance shade is the unwanted gift of one tree species planted extensively as a street tree and whose seedlings have populated many private yards. We met the Norway maple in a previous article in the urban forest series (Duinker, 2020). Norway maples leaf out earlier than most other tree species, grow rapidly, and form a dense canopy. The heavy shade they cast inhibits vigorous growth of other young tree species and vegetation. Compare the dense canopy of the Norway maple in the left of the first photo to the high, open canopy of the large red oak on the right. The red oak allows some sunlight to pass through, permitting plants at lower heights to thrive. Managing growth by frequent trimming will help to reduce the shade cast by the opportunistic Norway maples. Fortunately, Norway maples are declining in number in Halifax, as they are vulnerable to strong winds and often die young. In the other photo, a Halifax homeowner is planning to remove the large Norway maple to improve growing conditions – including sunlight – for the sugar maple planted beneath.

It’s impossible to discuss shade as a nuisance without also acknowledging what we gain from it. We can apply good foresight, knowledge, and management to deal with what is largely an annoyance. I’m willing to accept the minor downsides of shade from trees in our urban forest, if it’s the price I have to pay for its enormous benefits.