Not all trees and shrubs are created equal when it comes to autumn colour
If April is the cruellest month, as T.S. Eliot wrote, then surely October is the most glorious, at least in nature. Beyond the harvests and holidays, many of us turn to a fascinating pastime known as “leaf-peeping” where we go for walks or drives and exult in the explosion of foliage colours, both in the wild and in public and private spaces. Who hasn’t been thrilled by the fall colours of a native red maple, for example, or the shimmering hues of an oak tree? We almost run out of names to describe those colours: scarlet, crimson, umber, carmine, gold, ochre, magenta, blaze orange … it’s like nature has one last hurray before going to sleep for the long winter months.
Why do trees change colour in the autumn? To understand this, we need to talk a little science, but don’t worry, there is no test at the end. All green plants contain chlorophyll in their leaves, which is used in the process of making food for the plant (photosynthesis). As the weather and the calendar moves into fall and daylength shortens, plants begin to shut down photosynthesis by slowing down the production of chlorophyll. This slowdown allows other compounds to show up as the green in the foliage fades.
There are three main compounds that contribute to fall colour: orange carotenoids; yellow flavanols, and the anthocyanins responsible for red and purple fall colour. Different tree species will have different amounts of each compound in their makeup, and hence the different shades in their fall colours.
Temperatures, drought, and other factors can play a part in when the colour change begins. If you see a maple starting to change colour in August or earlier, that tree is struggling, whether with disease, insect or weather damage, or growing conditions. Drought will also trigger an earlier change to foliage colour than what we might normally expect, and sometimes the hues won’t be as brilliant or last as long. If September is very warm, colours could be less showy as well.
This year, many of us have had a perfect summer and autumn for the colour display, with adequate moisture, cool nights and warm but not stifling days, and the rewards are before our eyes.
Trees don’t all start to change colour at the same time. The common sumac is one of the first to start, with a host of orange and red hues in its lacey foliage. The oaks tend to be late in the season, well into October before they begin. The various maples tend to be in the middle of the season, and their eye-catching colours are a feature in many leaf peeper photos at this time of year.
Speaking of maples, not all species are created equal. Two native species, Acer rubrum, the red maple, and Acer saccharum, our beloved sugar maple, are definite all-stars, but so are the Japanese maples (Acer palmatum) and a few other choice and easily found species. What are not stellar choices, however, are the Norway maples, which are non-native and invasive in nature, often crowding out and killing other, more desirable species. They are very inclined to tarspot, a disease that paints black blotches on the leaves, and their fall colour is usually a tepid yellow. Just to confuse the issue, there are cultivated varieties of Norway maples that are red or gold throughout the growing year, but again, they are not going to give the hues of the native maples and other species.
Did you know that there is a native conifer that changes its needle colour and then drops them? The common larch (Larix laracina), which also goes by hatmatack and tamarack, is what is called a deciduous conifer, dropping its needles every fall. Before they go, they turn a luminous shade of golden yellow, which shows off beautifully against a blue autumn sky. There is another genus of tree used in landscape trades that does this as well: the ancient dawn redwood, (Metasequoia glyptostroboides) is similar to larch in appearance, but its fall colour is pinkish-bronze and simply wonderful.
Great fall foliage colour by species
Not all trees and shrubs show off in autumn, but there are plenty that do. Here’s a baker’s dozen favourites, with both botanical and common names.
Acer rubrum, native red maple: these beauties are show-offs of the tree world, with pretty much all the autumn colours.
Acer saccharum, native sugar maple: if the colourful roadside trees in woodlands and along city streets aren’t our native red maples, chances are they are the equally spectacular sugar maples.
Amelanchier, serviceberry, chuckly-pear is a small tree or large shrub with brilliant red and orange fall shades.
Betula, the birches with their gorgeous gold or white bark and bright yellow foliage are beloved native trees.
Cotinus, smoke bush: An outstanding 3-season ornamental shrub, with fall colours similar to a maple.
Fagus grandiflora, American beech: golden fall colour, with foliage fading to a tan, and often remaining on (younger) trees well into winter.
Fothergilla, fothergilla is an ornamental shrub with fragrant flowers in spring and orange-red foliage in autumn.
Fraxinus americana, white ash is a striking tree which can show a variety of foliage colours in autumn, from purple and burgundy to luminous yellow.
Ginkgo biloba, ginkgo is an ancient and unique species, often used as a landscape or streetscape tree. Fall foliage is butter yellow.
Hydrangea quercifolia, oakleaf hydrangea, as the name suggests, has oak-shaped leaves that turn a handsome burgundy in fall.
Oxydendron, sourwood should be a more frequently used tree in landscapes, especially for its blazing fall colours in shades of red through gold.
Populus tremuloides, trembling aspen becomes an eye-catching golden yellow in autumn, and its trembling foliage looks to be shimmering in a breeze.
Quercus rubra, red oak turns shades of crimson in autumn before fading to bronze before leaf drop.
Rhus typhina, sumac is an early show-off, turning shades of red and orange beginning in mid-September.