Urban Trees

Honey Locust

Honey locust (Gleditsia tricanthos) is an arborescent legume, like the acacias of the tropics. It is however, a hardy temperate trees of eastern North America, that extends in its natural range up to southern Ontario. The tree grows fairly slowly into a 30 metre tall giant with spreading irregular crown, like the savannah trees of Africa. Originally the honey locust tended to live in fairly well drained areas, with full exposure to sunlight. It often grows on regenerating flood plains, and in fallow fields. It was not originally an extremely common species. Although of wide natural range, it did not form very many pure stands. The disturbed habitats created by human activity, pasture clearance and road construction, have been a bonus for the species.

Honey locusts have very hard wood, and durable branches. Dead branches remain intact for long periods of time. These trees have delicate doubly compound leaves. These leaves turn a vivid yellow before falling in the autumn. In the spring, early summer, it blooms in beautiful white pea-like flowers. Honey locusts can have large pods up to 45 cm long with large beans about a centimetre across. The seeds are generally too large for native animals to swallow, although many animals eat the pulp between the seeds. And most wild specimens are very thorny. The thorns which sprout on the branches and trunk often form dense clusters of spears several decimetres in length. The thorns are modified twigs, and sometimes they sprout foliage. The monster beans and thorns, have suggested to some botanists that honey locust was once spread by American elephants, in particular by the mastodon. For the seeds and thorns of this legume are much like the acacias spread by elephants in Africa. These African 'acacias' also tend to have thorns to dissuade elephants from stripping the bark.

Honey locust is very similar to the more southern Texas locust (G. texana). This southern version has smaller beans. In southern swamps grows the waterlocust (G. aquatica), which has rather small few-seeded pods. Apparently, the intermediate Texas locust originated as a natural hybrid of the waterlocust and honey locust.

Many varieties of honey locust have been created by artificial selection and hybridisation. Cultivars have been propagated by grafting. These varieties are often exclusively male or they are sterile, thus they do not produce the giant pods which could litter lawns. Furthermore, thorn-less varieties have long ago been developed. Some cultivars are of southern extraction, and are suited to warmer climates, others of northern origin are more winter hardy. These varieties and cultivars are now widely available at nurseries, and are commonly planted as urban trees. Cultivars include: the weeping Bujotii, the compact Elegantissima, the red leaved Rubylace, the broad Shademaster, the pyramidal Skyline, the yellow leaved Sunburst, and many others.

Japanese Maple

Japanese maple (Acer palmatum) is a maple native to temperate eastern Asia. Like all maples they have two seeded winged keys as fruit. (Like most maples one to three seeded keys can occur also.) The leaves are palmate, like many maples. On the Japanese maple these leaves have deeply cleft pointed lobes, like those of the west coast vine maple (A. circinatum). The leaves have a vague resemblance to marijuana leaves. Generally these small trees or shrubs do not grow more than 4 or 5 metres tall. The tree prefers moist rich forest soils.

Asia and North America share many genera of trees, maple (Acer spp.) being one of them. Although, at the species level all of the native trees are different. Thus it is that North American travellers to Japan and China often comment that the trees seem 'strangely familiar'.

Since the leaves of Japanese maple are so fine and delicate, the maple has a great appeal as an ornamental. There are many cultivars and varieties of the maple. 'Atropurpureum' has red leaves, 'Senkaki' has yellow-pink leaves, and the 'Dissectum' variety has very fine leaf-lobes with deeply cleft leaf serrations. The 'Weeping' Japanese maple has decumbent branches. The Fullmoon maple is actually a different species (Acer japonicum), although it is often called 'Japanese maple'. Fullmoon maple leaves are broad with deeply cleft sub-lobes on each lobe.

Japanese maples are sub-canopy trees in the wild. In cultivation they should likewise be given plenty of shade. As rule-of-thumb the more showy the variety, the more sensitive they are to extreme weather conditions. The dry summers of the last few decades (1990s +) have stressed the maples, in the Toronto area. This has caused many people to over-water them, in an attempt to compensate for the noted wilting. Although they thrive in rich soils, it is a mistake to over-water this species. These maples are also fairly prone to the Phytophthora root fungi. This disease is exacerbated by moist soil. Conversely, they are prone to Verticillium fungi, when soils are dry. What ever the actual cause, I have noticed that Japanese maples often go into sudden decline in the Toronto area. One is best to be cautious in planting these trees north of their recommended Hardiness Zone (5-8). Planting these maples in Toronto, or further north, is not recommended. (Toronto is in Hardiness Zone 4.)

Magnolia

Magnolia trees are broad and squat with magnificent spring flowers and handsome summer foliage. Only a few species reach 20 metres in height. Most are better classed as shrubs. They are typically deciduous with large waxy leaves. The buds are large thick and downy. The six petal flowers, which bloom before leaf-buds expand, are quite large and showy. Floral colours range from white to greenish, to yellow, and to pink, with all manner of hues between. These flowers attract the earliest insect pollinators of the springtime. The large rounded orange-red seeds are arranged in cones, each seed in a purse-like pod. They are often bottomland trees, but some grow on forested mountains.

The flowers have all of the full traits typical of angiosperms, with few of the specialisations and modifications exhibited by other angiosperm families. The petals and sepals tend to be very leaf-like. The stamens of some magnolia are even leaf-like in form. It has long been held that most flower parts are derived from leaves. Since magnolia flowers are seemingly primitive, it has been assumed that magnolias are primitive as a whole. This old idea is only partly justified. Genetic studies indicate that the magnolia family (Magnoliaceae) is very diverse. They are not particularly primitive, taken as a whole. However, the flowers can be considered primitive in structure.

There are many species of magnolia. Magnolia, and its close relatives, have a broad bio-geographic range, from Austral-Asia to the Americas. Species of note include:

Magnolia acuminata grows in the northern Carolinian forest, even in southern Canada. This cucumber-tree has rather yellowish-green flowers. It is seldom planted as an ornamental, but enjoys some popularity as an arboretum curio.

M. grandiflora is the popular southern magnolia. It has large thick leaves and vivid white-yellow flowers. The large thick leaves have reddish hairy undersides. It fairs poorly north of Illinois.

M. fraseri is the mountain magnolia of the Appalachians. Its pale yellow flowers have long thin petals.

M. virginiana is the sweet bay of the eastern Carolinian forest. Its leaves have silvery undersides. It grows fairly well in Canada's far south.

There are many other magnolia species, some of Asiatic origin. Many of the species have been crossed to create splendid ornamental varieties.

Magnolias do not suit every environs. Most Canadian cities have winters far too cold for magnolia. The prairies are too dry, unless the trees are frequently watered. In addition, the large flower petals litter lawns by springtime's end. These petals are slow to rot away. If one cares about the lawn under magnolias, the petals should be raked up. Magnolia have rather brittle wood, they can be damaged very easily. On the plus side, the fungal encrusted bark of old magnolia trees are seldom symptomatic of any serious problem. (Fungi and magnolia seek peaceful co-existence.) In short, magnolias do not suit everyone's taste, nor are they suitable for all climates.

Rhododendron / Azalea

Rhododendron is a genus of shrubs and small trees in the heath family (Ericaceae). The fork-stemmed shrub is often nearly as broad as it is tall. Typically they are evergreen with thick waxy leaves that are retained for a few years. 'Rhodos' have large buds and stout hairy twigs. The showy flowers have five petals, and often white to pink. Most flower in the late spring. The fruit is usually a five chambered capsule.

Over 1000 rhododendron species grow all over Asian and North America. In the warmer temperate regions of Asia they are most common. Some species of note include:

Rhododendron lapponicum grows above the tree-line. It can even be found on the south shores of Hudson Bay. Unfortunately, this sub-arctic rhododendron does not make a suitable ornamental shrub.

R. maximum is the rose bay of the Appalachians. The flowers are light pink. There are cultivar variations flower colour. It forms an under-storey on the mountain slopes of the eastern mountains, and becomes ever more common in the southern Appalachians.

R. catawbiense is a higher mountain somewhat smaller version of the rose bay. It makes a better ornamental in southern Canada because it is more winter hardy.

Rhododendron X spp.. the azaleas are usually hybrids of various Asiatic species. These cultivars seldom reach more than half a metre in height. They can have red to purple flowers.

Ornamental rhododendrons are almost entirely derived from under-storey warm temperate species. Almost none of them can be considered winter hardy. In Toronto smaller rhododendrons and azaleas should be covered in leaf mulch over the winter. If they are large they should be wrapped in burlap to protect them from 'winter burn'. (This defeats the aesthetic benefit of their being evergreen.) A few of the hardier rhodos can be over-wintered without special cloches. Such rhodos must be planted in shaded locals well protected from the wind. (Rhodos are also potential carriers of the sudden oak death phytophthora.) By all reasonable estimates Toronto is too wintery for most rhododendron cultivars. If one has the time for extra work, or the money to pay professional gardeners, rhododendrons are the plant of choice. Nevertheless, rhododendrons are beautiful and their evergreen foliage gives some early colour to springtime. This is especially true if you don't wrap them up!

Chinkapins & Chestnuts

Elms & the White Elm

Elms (Ulmus spp.) are members of the Ulmaceae family. They are related to the hackberries (Celtis spp.), planertrees (Planera spp.) and tremas (Trema spp.). The leaves are simple, serrate and acuminate, with strong venation. The flowers are 'perfect', with both male and female parts. These flowers unfold in the early spring, sometimes before the leaves unfurl. The fruits are flattened 'samaras' - or winged seeds. The paper-thin samara are easily dispersed by the wind. Many species have corky bark, which is often manifest as 'winged twigs'. Elms have an alternate two-ranked (distichous) leaf arrangement. The alternating shoots that develop as a consequence causes the trunk to fork repeatedly. This architecture gives the elms a spreading broom-like crown. A grove of elms can create a wonderful vaulted enclosure - like a green cathedral!

There are over 150 elm species worldwide. Some of the species are rather hard to tell apart at first glance. Most occur in the northern temperate regions. They are quite tolerant of drought, and many species have made good urban trees in the prairies. They have even been successfully planted quite far west of their native range, in the Great Basin and beyond.

There are many North American, European and Asian elm species which are suitable for the urban milieu. The American white elm (U. americana) has large smooth leaves with a marked asymmetry. The wych elm (U. glabra) from Europe has leaves which often have forward pointing serrations - like 'claws'. The ubiquitous Siberian elm (U. pumila) has leaves only a few centimetres long. There are a bewildering number of elms from China, such as U. szechuanica and U. davidiana. These Asian elms are potentially important as they are an untapped source of disease resistant urban trees.

Elms are fairly tolerant of air pollution and water stress. But they do have some important drawbacks as urban choices. Elms often bleed sap through insect borer holes, they suffer from 'gummosis'. Furthermore they are subject to two serious diseases: the Dutch elm disease fungus and the elm yellows phytoplasma. Unfortunately, the well beloved American white elm is especially susceptible to these two diseases.

References

Miller, Fredric and Ware, George H. 2001. Host suitability of Asiatic elm species and hybrids for larvae and adults of the elm leaf beetle (Coleoptera: Chrysomelidae). Journal of Arboriculture. 27(3): 118-125.

Ware, George H. 1995. Little-known elms from China: landscape tree possibilities. Journal of Arboriculture. 21(6): 284-288.

London Plane & American 'Sycamores'

Planetrees, buttonwoods or what Americans call 'sycamores' are members of the single genus (Platanus spp.) that comprises the Platanaceae family. Planetrees occur in Eurasia and North America. There are only about six species of planetrees in the world today, with diversity greatest in the Americas.

The bark on younger stems peels in plated scales, forming a mottled calico of pastel shades. This bark looks a lot like that of the Eucalyptus trees. Sometimes the older bark has cubic scales like 'alligator hide'. The leaves are arranged alternately, and are palmate and very 'maple-like'. Planetrees are deciduous, the autumn leaves turn brown before falling. The leaves and twigs are covered in a minute fuzzy hairs. These hairs can cause allergic reactions in some people. Staminate and pistillate flowers occur in separate globular balls on the same tree. From the pistillate flowers develops the 'buttonhead', a ball of elongated seeds arrayed radially. Most species have several such buttonheads strung out in racemes, the American sycamore has but one seed ball per petiole. The seeds are small and generally require muddy earth to germinate. Most species prefer bottomlands, but many can grow in seasonally arid stream beds. The Arizona sycamore (P. wrightii) grows in streambeds in the deserts of Chihuahua Mexico and Arizona. The American sycamore (Platanus occidentalis) grows in the Carolinian forests of North America. It is one of the quintessential trees of the Mississippi floodplains. It is also common in the river valleys of Ontario's deep south. The America sycamore can grow into a broad spreading tree of up to 40 metres height. It prefers open areas with plenty of sunlight.

Sometime in the 1600s, in western Europe, plantings of the exotic Asian P. orientalis and American P. occidentalis formed hybrids accidentally. The result of this fortuitous hybridisation was the 'London plane' (Platanus x acerifolia). The popularity of this hybrid spread along with cuttings and seedlings to all over Europe. Eventually it became one of the favoured trees of European gardens. When the hybrid was reintroduced to the Americas, the hybridisation process continued. Today most London planes in the Americas are descend from backcrosses. They are somewhat more native than 'pure' London planes.

London plane proved to be more urban hardy than either of its parent species. London plane is also fairly anthracnose resistant and it can tolerate drought quite well. This has made London plane one of the more favoured urban trees. The tree is somewhat prone to canker, but otherwise is rather tough and tolerant. The eucalyptus-like bark of young planes is interesting. And the tree has an overall beautiful form.

Kentucky Coffeetree

If you think the locust tree looks tropical and exotic, then you should see the Kentucky coffeetree! Up close the Gymnocladus diocus is a rather strange looking plant. It has unusual leaves, monster bean-pods and oddly thick twigs. Although, from a distance it looks like a normal shade tree. It is a legume with only one close relative, the G. chinensis of China. Originally the Kentucky coffeetree had a bizarre spotty natural range. It occurred in isolated patches in the Mississippi floodplains, in other river valleys, and in spots west just into the prairie margin.

Kentucky coffeetree has enormous bipinnate compound leaves. Each leaflet is only a few centimetres long, but the whole leaf can be almost a metre in length! The leaves flush late in the spring. In the autumn the leaves turn a beautiful clear yellow. There tend to be very few leaves per twig. The buds are large, the twigs stumpy and without thorns. It has small racemose clusters of pale yellow flowers. Usually male and female flowers are on separate trees, but not always. The bean-pods are about a decimetre in length. The individual beans are two or more centimetres in width. (While not uncommon for a tropical legume, such large beans are unusual in temperate North America.) The overall tree form is like that of the locust tree. However, the bark is less rough, and the crown can be broader than a locust tree's. The tree can reach 20 metres in height.

The beans of the female Kentucky coffeetree were used by rural Americans as a kind of coffee substitute. If roasted the ground beans have a very coffee like flavour, sans caffeine. Roasting is necessary if the beans are to be eaten. The raw beans are very high in toxins such as saponins and alkaloids. This discourages animals from eating the seeds. Even the pulp of the pods is toxic enough that very few animals will eat it a second time.

The shell of each bean is so thick that people should crack it, if they want planted seeds to germinate. In nature the beans do germinate with difficulty. However, since even squirrels do not cache them, the seeds tend not to spread far from their mother tree. It is a wonder that such large seeds occur in temperate North America. It is possible that Kentucky coffeetree was once spread by now extinct megafauna. The beans are so thick shelled that they could easily pass through the gut of a large herbivore. (Most herbivores can tolerate some plant toxins. Many plants are toxic to a degree.) Possibly mastodon or ground sloth once munched on the pods and passed the beans in their dung. The extinction of these megafauna could explain the relative rarity of the wild Kentucky coffeetree nowadays.

In Toronto there are a few 60 plus year old Kentucky coffeetrees in excellent condition. They are magnificent trees. They do produce root suckers. In the autumn the leaflets fall first, then the rachises fall afterwards. The beans of the female are enormous, and worse than the honey locust's to rake up. Fall clean-up can be a hassle. However, one can buy a male coffeetree from the nursery, and avoid some of the problem. Once established the Kentucky coffeetree is very hardy.

Cypresses

Cypresses are the handsome pointed conifers so typical of the Mediterranean region. The Italian cypress, cipresso or cyprès (Cupressus sempervirens) being the quintessential example. Cypresses have distinct rounded cones with close-fitting hexagonal scales. Leaves are tiny and arranged in close 'sprays' which are tighter than a juniper's. The crowns are often tightly pointed, columnar and quite dense. These conifers come in two closely related genera in the Cupressaceae family. True cypresses (Cupressus spp,) have rounded leaf-sprays and large cones, they are native to Europe, North Africa and western North America. False cypresses (Chamaecyparis spp.) have flat leaf-sprays and small cones, they are native to the Americas and to eastern Asia.

Hinoki cypress (Chamaecyparis obtusa) is perhaps the most suitable cypress for the Toronto area. This Japanese native has a rather rounded crown with a height of up to 18 metres. It does quite well in cities, it is popular for ornamental gardens. It is a wonderful tree with its sea-fan like sprays and luminous green hue. The Japanese Sawara cypress (Chamaecyparis pisifera) is similar, but it is taller and less commonly planted.

Leyland cypress (X Cupressocyparis leylandi) is a hybrid between the Alaska cedar (Chamaecyparis nootkatensis) and the Monterey cypress (Cupressus macrocarpa) of California. Evidently, both 'false' and 'true' cypresses are similar enough to hybridise. The Leyland cypress is hardy and has a beautiful form. However, its low-winter hardiness makes it unsuitable for Ontario.

Lawson cypress (Chamaecyparis lawsoniana) is native to Oregon. The cypress can grow to 60 metres tall, in a few centuries. It just barely tolerates southern Ontario's winters. It comes in a bewildering number of ornamental varieties. Cultivars such as the narrow Columnaris, the golden leaved Stewartii, the drooping Intertexta, and the fast growing Triumph of Boskoop have been marketed. Again, hardiness tests suggest that these varieties are not quite suitable for Toronto's winters.

Cypresses planted in Ontario are often burlap-wrapped to protect them from 'winter-burn'. Perhaps is simply better to admit that Ontario is not the Mediterranean. One should plant only the more winter hardy Japanese cypress species in this snowy clime.

Birch Trees

Birch trees (Betula spp.) are members of the Betulaceae family. They are the common ‘paper bark’ trees of the Canadian north, with their smooth white bark. The older layers of bark peel and roll-up, like paper. Only very old trunks loose this papery texture. (The Algonquin people fashioned canoes out of birch bark.) Birches have simple serrated leaves, catkin flowers and small nut-like fruits. Male and female flowers are separate, but occur usually on the same tree. The tiny ‘winged’ nuts are arranged in cone-like clusters. The seeds tend to fall in the winter. The wood is fairly soft, and only a few species are valuable for lumber. Birches thrive in disturbed bottomlands, they are pioneer species which grow before other trees shade them out. These trees do not live very long, within sixty years, or less, they become rotten, and start to fall apart. Birch trees seldom grow more than eleven metres tall.

There are about fifty birch species in the world. In Ontario the paper birch (Betula papyrifera) is very popular, because of the shining white bark. It grows in both the boreal and montane forests. It is very similar, in appearance, to the European birch (B. pendula). The European birch is more pendulant, and comes in many more cultivars than the native birch. The yellow birch (B. alleghaniensis) has duller bark, but more durable wood. It grows in the Laurentian and Carolinian forest zones. Yellow birch bark looses its papery texture earlier than does paper birch.

Birch may look like an attractive ornamental tree when it is young. However, as it ages it becomes more of a high maintenance tree. An old birch drops branches frequently, and heartwood rot becomes increasingly likely. Birch almost always becomes a hazard when it nears the end of its lifespan. A dwarf cultivar of European birch may be a preferred choice, if future hassles are to be minimised.

Fir Trees

Fir trees (Abies spp.) are members of the pine family. They have short needles, about two centimetres long, which are succulent, waxy and glossy. Like spruce the needles are arranged spirally, but like a yew they are twisted so that they appear to be in flat splays. Fir leaves are much more plump and juicy than a spruce’s. The female cones have smooth scales, and project very vertically. The spire of the come often remains after the scales have fallen. The bark on young firs is smooth, and has resinous ‘blister’ bumps on it. The crown form is spruce-like, but more perfect. Firs make excellent Christmas trees. In Ontario firs can attain up to twenty metres height. The alpine fir (Abies lasiocarpa), of the Rockies, can reach over thirty metres.

Firs are mostly either bottomland trees, or alpine trees. In either habitat they tend to require fairly ample soil moisture. Balsam fir (Abies balsamea) is a common species in the Canadian boreal forest. White fir (A. concolor) is a more commonly planted fir. White fir hails from west of the prairies, and tolerates more arid habitats than most other fir species. Nordmann fir (A. nordmanniana), a native of the Caucus region, is another widely planted exotic.

In my opinion, fir trees are not suitable as urban lawn ornamentals. Some species are better ornamentals than others, but overall they tend to become ratty in yards. Fir trees also have relatively shallow roots, and they can blow over, if planted out in the open. If you insist upon a fir, choose a white fir or some other city hardy species.