Urban Trees

Broadleaf Trees

ailanthus / tree-of-heaven - apple / crab-apple- ash trees - azalea - European beech - birches - box-elder - buckeyes - catalpa - cherries - sweet chestnut - chinkapins - horse-chestnut - Kentucky coffeetree - cottonwood - American elm - elms - figs - shagbark / hickories - katsura - linden - honey locust - magnolias - Japanese maple - Manitoba maple - Norway maple - silver maple - sugar maple - English oak - red oak - white oaks - London plane - American sycamores - plums - prickly pear cactus - rhododendrons - serviceberry / shadbush - sweetgum - tulip-tree - walnuts - weeping willow.


white cedar / arbor-vitae cedar - cypresses - firs - Douglas fir - larches - jack pine - red pine - Scots pine - white / Weymouth pine - black spruce - blue spruce - Norway spruce - white spruce - tamarack. -

D. Andrew White, 01, 01, 2015
Cottonwoods in Nevada
Old cottonwood trees in Nevada, near Reno.

Northern Red Oak

pin oak

Northern red oak (Quercus rubra) is a member of the Fagaceae or 'beech' family. It is a New World oak of the 'red oak' subfamily (Erythrobalanus). There are many red oak species, some differing only slightly from northern red oak. Red oaks differ subtly from the oaks of the 'white oak' subfamily (Lepidobalanus). Most of the red oaks have pointed leaf lobes. White oaks tend to have rounded lobes, as do most European oaks. Red oak wood is darker and less durable than is white oak wood.

Northern red oak has male and female flowers on the same tree, but the flowers are not combined. Female flowers are small and solitary, and the male flowers are arranged in dangling catkin-racemes. Flowering follows close behind leaf flush in the spring. Unlike white oak, red oak seeds (acorns) are bitter tasting and require two years on the tree to mature.

oak & pine opens
Oak opens or oak-pine savannah at High Park in Toronto.

Mature northern red oak can tower 30 metres tall, and live for over a century. While old oaks are often hollowed-out by fungal rot, the sapwood is very durable, and old trees are not easily felled by windstorms. It can grow in fairly dry soil. In some sand-dune areas along the Great Lakes it is a dominant along with pine. It can even be the dominant species on rocky ridges. Northern red oak ranges from the Great Lake-St. Lawrence forest region south to the deciduous forests of Georgia U.S.A..

Northern red oak has many close taxonomic relatives. Black oak (Q. velutina) is a similar oak with a range that overlaps with red oak's in the south. Black oak was once common in the "oak openings" of southern Ontario. Pin oak (Q. palustris), while similar in range, prefers swampy habitats. Northern pin oak (Q. ellipsoidalis) grows to the north-west, its range overlaps also with red oak. Scarlet oak (Q. coccinea) is more common in the Great Smoky Mountains. Shumard oak (Q. shumardi) is most common in the Mississippi bottomlands. Maple leaf oak (Q. acerifolia), which has an almost maple-like leaf, it grows wild in the Ouachita Mountains of Arkansas. In the Dixie states southern red oak (Q. falcata) is the most common form. Some west coast 'live oaks', such as Q. agrifolia, are red oaks. The various red oak species often hybridise where they overlap. Probably they all had a common ancestor only a few tens of millennia ago.

Although not as esteemed in the lumber trade as white oak, northern red oak is more valued in urban forestry. Many people say that it does not have the rugged Romantic beauty of white oak, yet it has other features that make up for what it lacks in appearance. It is hardier than most white oaks, and does not require as much water. It is very stress tolerant, and is far more likely to survive in a cityscape than are most other oaks. Red oak was one of the first New World species to be widely planted in European cities. In Europe this pointy leafed oak is an exotic. It is now a common urban tree in Paris and London.

The ISA Species Rating: 81%.


Crab-apples after ice-storm, Dec. 2013

Apple (Malus pumila) is the most commercially important species in the rose family (Rosaceae). The apple is of Eurasian origin. It is suspected that the apple most likely developed by artificial selection from Malus sieversii a crab-apple native to Kazakhstan. But M. sieversii might be merely a subspecies of M. pumila. Several od the Asiatic ‘species’sargentii, orientalis, prunifolia and niedzwetzkyana are probably really subspecies of M. pumila. However, the apple has many American and Eurasian relatives that are distinct species. These trees are collectively referred to as 'crab-apples' (Pyrus spp. & Malus spp.). Apples and pears are very similar. In fact, apart from the true pear (Pyrus communis) and the apple, it is difficult to decide if the other crab-apples are really crab-apples or crab-pears. Taxonomists usually determine the genera of the crab-apples by their ability to hybridise. Malus and Pyrus species generally cannot hybridise with each other.

Crab-apples grow all over the temperate zones of North America and Eurasia. Today crab-apples have been planted in temperate southern Africa, Australia, New Zealand and South America as well. There is great variation in fruiting time, frost hardiness and size between varieties within each species. Crab-apples seldom exceed 7 metres in height, and grow best in open areas, not enclosed forests. Crab-apples require a winter dormant season to 'set their fruit'. This setting period typically varies from 700 hours to 1500 hours per year of extended near freezing temperature.

Flowering occurs in early spring just before leaf break. Male and female elements are combined into single flowers. The showy petals vary from white to pink, the colour depending very much on environmental factors as well as genetics. Insect pollination is mostly accomplished by bees. Apple fruits develop only if their ovules are pollinated, and even then the majority of fruits on each tree are aborted. An apple has a multi-seed fruit called a poma or a 'pome'. Apple flowers tend to be self-infertile, they are said to be 'self-unfruitful'. Since orchards can consist of vast tracts of cloned trees, this means that at least one other variety should be planted nearby to ensure pollination and full fruit development. Strangely enough, some crab-apples are able to pollinate apple trees and allow them to fruit fully.

Since ancient times cultivated apples have become progressively less tart, sweeter, and larger. Not only were varieties developed, but individuals with unique traits were cloned by grafting twigs onto rootstocks. These cloned cultivars are the 'varieties' that one encounters in the market: 'Annurca', 'Delicious', 'Fuji', 'Granny Smith', 'Gravenstein', 'Jonamac', 'McIntosh', 'Mutsu', 'Spigold', 'Spartan', 'Wealthy', and many others. Some varieties are best suited are for cooking, others for eating raw, and still others for making cider. Each variety is a heterozygous gene mix. Since the seeds have undergone meiosis followed by out-breeding, the offspring are unlikely to match either parent. Which is why trees grown from seed do not exactly match either parent tree. Natural outbreeding was the original source of apple varieties in the early history of apple culture. Many varieties originated from feral seed-grown hybrids. Observant people then noticed the feral apples, and liked what they tasted. They then commenced clonal propagation of their new-found 'variety'. Several such 'accidental' discoveries have been recorded in history. Nowadays, most new varieties are developed on purpose by trained horticulturalists.

Apple growing became ever more sophisticated over the centuries. The root stock varieties eventually became as specialised as the scions. These root stocks were selected not only for hardiness, but they also influence the ultimate size of the grafted scions. 'Standard' rootstock allows the tree to grow up to 7 metres tall, while 'Malling 27' rootstock dwarfs the tree to under 3 metres. There are many other rootstock varieties, with many other salient features. Today, almost all commercially grown apples are scions of one variety grafted on to the root stock of another variety.

Many of apple varieties produce so much fruit that the branches become structurally unstable. Hence orchardists must prune the trees before the fruiting season to ensure that the number of fruits per branch is just right. This pruning requires skill, as one must recognise fruiting spurs from leaf buds, and one must neither under-prune nor over-prune. Pruning is best done during the dormant season, late autumn or early winter. As stated before, more than one variety must be planted nearby to ensure fertilisation, or the pollination must be accomplished manually. Natural pollination is easiest, and orchards usually contain at least two varieties to ensure cross-pollination. When pruning, pollination, grafting and rootstock selection are all considered, it becomes evident that apple cultivation is one of the most complex branches of horticulture.

Note: Apples can be infected with a bizarre species of fungus. The cedar-apple rust fungus has alternate hosts. One host is the redcedar, and the other host is the crab-apple - and it relatives. This is why redcedars and apple orchards should, ideally, be planted far from each other.

Note: The science of apples, crab-apples and pears is known as pomology.

The ISA Species Rating: 52-62% (as an ornamental).

Shadbushes or Amelanchiers

Serviceberries, juneberries, shadblows or shadbushes (Amelanchier spp.) are members of the rose family. The tree, or shrub, resembles a flowering-cherry in overall form. Though, the flowers and fruit are more like those of crab-apples. They have rounded leaves with small teeth, they are thorn-less, with pointed buds, and rose-like flowers. The bark is smooth with distinctive stripes on the smaller branches and twigs. The flowers are bisexual and insect pollinated. The flowers bloom in the spring, before or during the leaf break, and the pomes mature by the summertime. The little edible pomes are relished by birds. Most species seldom exceed four metres in height. Shadbushes are usually found in clearings, along river banks or in open floodplains.

The Saskatoon berry, or serviceberry (A. alnifolia), is common fairly northern species, it is widespread from the aspen parklands to the boreal forest. The tree serviceberry (A. arborea) is somewhat more southern in its native range. It reaches up to ten metres in height. One common garden species is the juneberry (A. lamarckii). Lamarck's juneberry is perhaps the mostly widely cultivated of the American amelanchiers. It has showy flowers, and it is fairly hardy in urban settings.

Amelanchiers are popular shrubs, because of their small size, bright flowers, colourful fruit, and their hardiness. They also attract birds. They are excellent garden choices for places where large trees would not be a propos.

Stone Fruits

The stone fruits or drupes (Prunus spp.) are a group of trees in the rose family. Drupes normally have only one seed per fruit. The actual seeds or ‘nuts’ of most species are slightly poisonous. Like apples, they require at least a mild winter season. The stone fruits include: the plums, prunes, almonds, apricots, peaches, cherries, sato trees and the mazzard. They flower in the early spring, just as the leaves being to unfurl. Like apples, these fruits are ripe by late summer or early autumn.

Cherries are basically small plums. Sand cherries (P. cistena) is a small shrub with of early succession. It often sprouts up in the early stages after a wildfire. The fruit is rather small and tart. Cultivars have been selected that have persistently reddish-purple leaves. Ornamental cherries, such as P. serrulata, are often of Asiatic origin. They are grown more for their flowers than for their fruits. Black cherry (P. serotina) is a valuable lumber tree in Ontario. The fruit is rather tiny, dark and very tart and distasteful. Bird cherry (P. avium) is the domestic cherry of Eurasian origin. There are many varieties and cultivars, for sundry uses. The fruits vary from those eaten raw to those best used in baking.

Plums are basically large cherries. Canada plum (P. nigra) has rather small fruits, just a few centimetres wide. The plum is greenish, even when ripe. There are many similar species just south of the Great Lakes. The cultivated plums are often of Eurasian origin. The Eurasian ‘plum’ (P. domestica) is widely cultivated. Many varieties have been selected for their large esculent fruits. When dehydrated they are called ‘prunes’. Apricot (P. armeniaca) has a fruit that is often dehydrated before it is eaten. Peach (P. persica) has a fruit much larger and sweeter than most other plums. The almond (P. amygdalus) has a fruit for which the seed is the edible part. Its seed is less toxic than most other drupe nuts.

Stone fruits in general are not excellent yard trees. They are prone to bleeding resin from wounds, and sporadic dieback. The fallen seeds can be quite messy. They also can require a continuous and proper pruning regimen, if one wants to harvest their fruit. If not trained, the fruits often grow far out-of-reach. Except for the flowering varieties, most are best grown by serious orchardists.

The ISA Species Rating varies enormously from species to species, and even by cultivars: 53-73%.

Tamarack and Larch

Tamarack, hakmatack, or eastern larch (Larix laricina) is a conifer in the pine family. This larch grows in the Canadian boreal forest, even somewhat north of the tree-line in places. It has leaves (needles) a few centimetres long which are arranged spirally on long leading shoots. On the lateral and non-leading shoots, the twigs are very stout, they sprout bundle-like tufts of needles. (Its leaf pattern is very much like that of the deodar and the cedar of Lebanon.) Tamarack is deciduous, the needles fall in the autumn. Its crown form is similar to a spruce, but it is less dense, and it has daintier foliage. Spring growth is a brilliant luminous green, and the autumn needles golden-brown. Male and female cones are separate on the same tree. The seed-cones are less than two centimetres long. The southern subspecies of tamarack can reach 30 metres in height.

Tamarack is the most northern plant that qualifies as a tree. The early British settlers were not so familiar with the larch of Europe, so they called this 'new' tree by various Amerindian names, such as hakmatack or tamarack. Tamarack does not grow well in shaded areas, and it prefers boggy to moist soils, and it is common in muskegs near the tree-line. Tamarack can grow quite well in southern Ontario, even in fairly dry soils, if planted.

In its extreme northern limits tamarack is much more stunted than further south. The far north-western version, with its stunted squat crown, is a subspecies called 'Alaska larch'. Other larch species live in the Americas. The western larch (Larix occidentalis) lives in the montane forests of the central Rockies. The alpine larch (L. lyallii) grows near the alpine tree-line in the mid-Rockies. Eurasia has many boreal and alpine species of larch. Siberian larch (L. siberica) is common in Eurasia's boreal forests. In the far north-east of the Siberian taiga almost pure stands of Dahurian larch (L. dahurica) can occur. The European larch (L. decidua) is both alpine and boreal, and is very similar to tamarack in appearance.

Both tamarack and European larch are fine looking conifers in the spring and summer. Neither can tolerate extemes of drought, nor compact soil. They are better as yard trees than street-side trees. Some people object to tamarack's winter appearance, it can look like a dead spruce. However, tamarack's bright spring green more than compensates for its winter dreariness.

The ISA Species Rating: 62-72%.

Blue Spruce

Blue spruce or Colorado spruce (Picea pungens) is a gymnosperm in the pine family (Pinaceae). This spruce grows very slowly, requiring decades to reach a height of 30 metres or more. The crown is thin relative to the height, a fully grown tree may have a crown of only 3 to 4 metres wide. It is called 'blue' spruce because the pale waxy coat on the leaves (needles) gives them a distinct bluish tinge. These leaves are about 2 or 3 centimetres long. Like all spruces its needle-like leaves are not bundled and are arranged spirally along the twig like a bottle-brush. The female sporophylls (cones) are up to a decimetre long, which is long for a spruce-cone. Male sporophylls are produced separately, but on the same tree. Spring pollen production can be quite prodigious. Like all of the spruces, the pollen is transported by wind.

Blue spruce originated in high altitude mountain forests of Utah, Wyoming, Colorado and New Mexico. Typically they grow in forests above 3500 metres. Often they are the dominant pine on these mountains. The snow in these regions can be many metres deep by late winter. The downward sloping branches do not collect snow, and the tree seldom breaks with snow load. Glaze storms do not seem to phase the tree either, and blow-down is rare. The spruce is very well adapted to winter weather.

Blue spruce and the Engelmann spruce (P. engelmanni) often form natural hybrids, where their ranges overlap. Engelmann spruce could be considered a northern race of blue spruce, or vice versa. Engelmann spruce itself intergrades with white spruce (P. glauca) to the east, which in turn intergrades with the west coast Sitka spruce (P. sitchensis). All four spruce form a complex of hybridisable species.

Blue spruce's elegant form, bluish colour and all-round toughness have made it a favourite urban ornamental. Blue spruce is very tolerant of urban environments, and it grows well throughout the temperate zone. Like most spruce the crown droops to ground level, even on old trees. If lower the branches are pruned up, the tree generally looks less attractive. It is better to allow the spruce to droop, at most trimming the lower branches just slightly.

Mostly it is private landowners that plant blue spruce. Urban foresters seldom choose the spruce as a city tree, because it does not suit everyone's fancy. Many people object that the spruce is aesthetically 'out of place' in smaller front yards. Most people prefer to see the tree in wide open yards, in golf courses or in parks. Blue spruce is a beautiful tree, but it does not suit every setting.

The ISA Species Rating: 72-80%.

Norway & White Spruce

White spruce and Norway spruce are two splendid spruce with similar appearance. Both are hardier than black spruce in the urban milieu. They are less colourful than blue spruce. Both grow quite well in eastern North America, except in the deep south. Both reach up to 30 metres tall, and have splendid pointed tapering crowns. Both species have much genetic variation. Experts can often tell by appearance alone the geographic race of a spruce tree. The Norway spruce, currently, has more named cultivars than does white spruce.

White spruce (Picea glauca) is a native to most of North America's Boreal Forest Region. It has relatively small cones (2-4 cm), shorter than Norway spruce's. The twigs when bruised give off a disagreeable odour. White spruce grades into the blue, Engelmann and Sitka spruce species.

The ISA Species Rating: 72%.

Norway spruce (Picea abies) is native to boreal Europe west of the Urals, with some outliers in the southern Alps. The cones are longer (8-14 cm) than those of white spruce. The smaller branches on most varieties are somewhat pendulous.

The ISA Species Rating: 71%.

Aesthetically spruce trees do not suit everyone's fancy. They are not generally planted along city streets. However, they are suitable for windbreaks and highway margins. A recurring error of private gardeners, is to plant spruce trees too close together, too close to walkways, or too close to buildings. Most varieties have crowns over five metres wide at maturity.

Douglas Fir

Douglas fir or Pseudotsuga menziesii is a large conifer in the Pinaceae family. It is not actually a 'fir', but has flat needles (leaves) 2 to 3 centimetres long that splay out laterally like those of a fir. The female cones, 5 to 9 centimetres long are unusual in having bracts that extend out like 'adder tongues' from under each scale. At close range it looks more like a fir. It does not have the glossy needles of fir, and it generally has a less compact crown. Overall the tree looks like a spruce in aspect. It is an evergreen and grows at a moderate rate, after a few centuries it can obtain 80 metres or more in height. Those trees planted outside of the natural range have not yet had time to grow to the maximum height. In the wild Douglas fir occurs in the Rockies and in the Cascade Range of North America. Its natural range is from California to about Prince George British Columbia in the north. The hard reddish wood of the Douglas fir is highly valued for lumber.

There are several other Pseudotsuga species in eastern Asia, only two Douglas firs grow wild in North America. (There are still unsettled questions in Pseudotsuga taxonomy.) Several races of Douglas fir have been recognised. Generally, only the coastal and interior subspecies are considered recognisable races. Many regional variations exist within each subspecies. The bigcone Douglas fir (P. macrocarpa), which may be just a subspecies, grows wild only in the cross-ranges of southern California and Baja California.

During the twentieth century it was discovered that Douglas fir made a hardy urban tree. It grows well in most of the temperate region and tolerates urban environments. However, Douglas fir eventually grows into a giant of a tree. It should be given plenty of growing space. Douglas fir is an excellent choice for an urban tree, as long as the growing space is available.

The ISA Species Rating: 70%.

Silver Maple

Silver maple (Acer saccharinum) is a member of the Aceraceae family. It is a 'soft maple' and has much softer wood than the hard maples. The leaves are palmate with very deep sinuses between the lobes. Leaf margins are 'toothed', unlike the hard maples. Male and female flowers occur in separate clusters on the same tree, and are small with green petals. Flowers bloom and seeds are almost fully developed before the leaves unfurl in the spring. The paired seeds each have a 'wing', together they form the classic 'maple key'. The leaves are silvery in the summer, and turn pale yellow in the autumn before they fall. The maple grows to a moderate height, of 30 metres. It has slightly drooping branches and a wide crown. The tree is prone to damage during snow and glaze storms. It ranges throughout the Carolinian deciduous forest region. It is quite common in the Ohio Valley. But it is not very common as a wild tree in southern Ontario and Quebec.

Where there are spring thaws with sharp cold spells, silver maple can be use as a source of 'maple sugar'. In the late winter and early spring the sap flows copiously. This can be tapped, and after most of the water is boiled off it can be rendered into maple sugar. Nowadays silver maple is not often exploited as a sugarbush tree. In southern Quebec and Ontario the sugar maple (Acer saccharum), a hard maple, is far more productive as a sugarbush tree.

Note: The species names of both silver and sugar maple contain the common rootword sacchar- or sugar. Both maples are potential sources of maple sugar.

Silver maples can hybridise with the native red maple (Acer rubrum) to form Acer x freemanii or 'swamp maple'. Red maple maple has less deeply cleft leaf sinuses than the silver maple. The red maple is a somewhat more northern species which prefers moist forests. Red maple has a rich red-orange autumn coloration. This soft maple was the original model for the ‘Canadian maple leaf’ insignia.

Silver maples is considered a fast growing tree. It is hardy, to a point, but it does drop a lot of branches during and after the winter season. At one time it was the favourite urban tree throughout North America. Now that many silver maples are approaching the century mark, people have become annoyed with its dropping branches. Furthermore, some of the older trees are dying and have become a liability. Nevertheless, one can fully appreciate why urban foresters often chose the maple. Silver maple grows rapidly and delivers a handsome full sized tree within a human lifetime. It is a good choice for an urban tree, but its popularity is waning.

The ISA Species Rating: 56-65%.

Horse-Chestnut & Buckeyes

horse-chestnut epicormic shoots - in Dublin Ireland

Horse-chestnut, European buckeye or marronier (Aesculus hippocastanum) is not actually a 'chestnut' but a member of the Hippocastanaceae family. It is a medium sized tree, 25 metres tall at most. Its large compound palmate leaf has 5 to 7 leaflets. These are spirally arranged on very thick twigs with very large buds. In late spring, after leave have unfurled, the tree flowers. Panicles of showy flowers, mostly white with some orange, bloom for a long period in the spring. These insect pollinated flowers are male and female combined. The seeds that develop by late summer are globular shiny nuts, somewhat like a chestnut. Each seed is surrounded in a husk. The husks are covered in short broad spikes. These spikes are not as close-packed as are the spikes on real chestnuts.

buckeye flower

Horse-chestnut is a Eurasian species of Aesculus. There are native American species which are called 'buckeyes'. Most buckeyes are smaller and more southern in their natural range than horse-chestnut. Ohio buckeye (A. glabra) is planted in Ontario as a flowering ornamental. It has more slender leaflets than horse-chestnut, and it has rather showy flowers. Buckeyes have bitter nuts that red and grey squirrels do not like to eat. Like all buckeyes the nuts can be boiled to render them edible, but few people bother. Supposedly, horses don't mind the flavour of the raw nuts, hence the name 'horse chestnuts'.

Buckeye nuts are largely ignored by the grey squirrel (Sciurus carolinensis). However, the fox-squirrel (Sciurus niger) does cache buckeye nuts. The large red-grey fox-squirrel plants buckeyes by leaving a few of its 'surplus' nuts in the ground. The fox-squirrel is far more common south of the Great Lakes. It is rare in Ontario's deep south. This could explain why even the winter hardy Ohio buckeye is not native in Ontario.

Horse-chestnut is widely planted in the northern part parts of the Carolinian deciduous forest region. It can thrive further north than most native buckeyes. Horse-chestnut is popular because of its vivid floral disply. Horse-chestnut also fairly hardy in an urban environment. It sprouts new stems from branches that are cut-off. Even if all greenery is cut off, it has a remarkable ability to recuperate. It is one of the species that can be trained into a 'mop-top' by pollarding it back each autumn. If allowed to grow fully, the crown can be quite broad. The tree should be given plenty of growing space. It does drop a heavy leaf load, along with plenty of nuts, in the autumn. This does make for extra work in the 'fall clean-up'.

The Species Rating: 57-67%.


Sweetgum, star-leaved gum, alligatorwood or liquidamber (Liquidamber styraciflua) is a native of the central and southern Carolinian forests. It is a member of the witch-hazel family (Hamameliaceae). Sweetgum has plane-tree like palmate leaves. These star-shaped leaves can turn a beautiful deep red, orange or mixture of hues, before they fall in the autumn. The crowns can be broad, but also tall. It is an hermaphrodite, but the male and female flowers occur on separate catkins. The female flower racemes expand into spiky round ‘buttonballs’ of radiating seed capsules. These buttonballs are smaller and firmer than those of the plane trees. The mature bark has a deeply crevassed ‘alligator hide’ texture. The stately forest tree can reach 40 metres in height.

Sweetgum has two close relatives in Asia. In fact, the Liquidamber orientalis of Asia Minor was known to Europeans before they found a similar tree in the Americas. The resin or ‘amber gum’ of this tree was used as an ingredient in incense. Various native American peoples used the resin of the native sweetgum for very similar purposes.

Sweetgum grows wild as far north as New York State. The gum is most common in lowland forests. It is less common in the higher elevations, and nearly absent in the high Appalachians. Introduced specimens grow very well on Vancouver Island, and on the mainland of southern British Columbia. The gum is not commonly cultivated in Ontario. Sweetgum trees can grown without apparent problem as far north as Burlington Ontario. Probably unseasonal frosts would eventually kill any sweetgum planted much further north in Ontario than Toronto. Although it is a beautiful tree, sweetgum is perhaps better appreciated growing in its southern habitat.

Prickly Pear Cactus

prickly pears

Prickly pear is the cactus that is depicted on the Mexican flag - with an eagle and rattlesnake. Prickly pears or Opuntia cacti are native to Canada. Mostly they occur ‘out west’. Opuntias all have scale-like leaves subtending their flowers. But they are otherwise leafless. The opuntias rely mostly on the photosynthetic tissue in the stems. These stems are divided into oval pads. This cactus has both spines and very tiny bristles. The spines and bristles occur on little zones (areolae) that are neatly arrayed on the pads. Opuntia flowers in early summer. The flowers have many pointed yellow petals. The fruits are encased in an envalope of leafy scales - and stem tissue with spines and bristles. These fruits are widely harvested and eaten, especially in Mexico. The fleshy ‘pears’ or ‘Indian figs’ have many rather large pits.

In Ontario there are two native species, the western prickly pear (Opuntia humifusa) and the brittle prickly pear (Opuntia fragilis). The brittle prickly pear is probably a recent migrant from the prairies. Western prickly pear has been in Ontario for a longer time. This cactus is definitely more of a shrub than a tree. This species occurs in the wild from Mexico to as far north as southern Canada. Western prickly pear is fairly well established on Pelee Point, and a few other locales near the Lake Erie shore. Though, it is suspected that only the cacti near Pelee are truly wild. Many of the other patches were almost certainly planted. It requires open sandy areas where other weeds do not out-compete it. Even near Pelee, human effort has been necessary to keep the other weeds at bay. Prickly pears of various species occur in the Canadian prairies. Fewer species are to be found in eastern North America. Many more species are endemic to the New World tropics. The large Opuntia ficus-indica is the most widely planted of these species. Indian figs, as garden escapees, have become naturalised in Eurasia, Africa and Australia.

Prickly pear is increasingly being planted as an ornamental in southern Canada. This is a good thing, as it is declining in the wild in Ontario. Mostly human disturbance or destruction of habitat is to blame for the decline. The only draw back of the prickly pear is its tiny bristles. These can easily become embedded in one’s skin – and that can be extremely irritating.

Cacti in general -

The Cactaceae family is a group of plants related to the Portulacaceae. Almost all species are endemic to the Americas. Most are nearly leafless spiny succulent plants that live in deserts. Cactus spines are modified leaves. Spines arise from highly foreshortened twigs called ‘areolae’. Cactus flowers mostly have ‘inferior ovaries’ - the flower-stem tissue engulfs the ovaries. Thus, the fruit can be covered in spines or leaves. Most cacti utilise crassulacean metabolism (CAM). That is, they can close their stomata by day to conserve water. At night the stomata open to collect carbon dioxide. The stored-up carbon dioxide is then available for daytime photosynthesis.

Not all cacti are strictly desert plants. The ‘Christmas cacti’, such as Rhipsalis spp., root as epiphytes on the branches of trees. Oddly, the Christmas cacti occur also in the Old World tropics. Not all cacti are succulents. The primitive cacti of the Pereskia genus are woody, spiny and leafy. They are usually said to be non-succulent. Though, some botanists argue that pereskias are semi-succulents. These pereskia ‘cacti’ live in the New World tropics.

Cacti can be actual trees. (1) From the Sonora Desert through to New Mexico there are tree sized relatives of the opuntia. The jumping cholla (Cylindropuntia fulgida) and staghorn cholla (C. versicolor) are both considered to be small trees. These chollas are fairly winter hardy, and they can tolerate snow. Chollas have more tubular stem-segments than do regular ‘prickly pears’. (2) In the Senora Desert there grows the famous saguaro (Cereus giganteus). This tall ‘organ pipe’ cactus slowly grows to about twenty metres tall. It has fairly strait rows of spines, and showy white flowers that mostly open at night. (3) If one wants to see a truly enormous cactus, go to Cuba or some other islands in the Greater Antilles. The Flor de Copa (Dendrocereus nudiflorus) can grow to the height of a tall apple tree, with a squat broad crown, and a trunk up to a metre wide at the base. It has relatively few rows of spines, and its flowers are white.

In an amazing display of convergent evolution, the Flor de Copa cactus (Dendrocereus nudiflorus) looks very much like the Euphorbia candelabra of east Africa. Though, of course, the African euphorbia is not actually a cactus. This is not an isolated example. There are spurges that look like barrel cacti, or chollas, and even saguaro cacti. The cacti-spurge similarity is due to convergence.


Edwards, E.J. and Donoghue, M.J. 2006. Pereskia and the Origin of the Cactus Life-Form. The American Naturalist. 167 (6): 777-793.

Griffith, M. Patrick. 2004. The origins of an important cactus crop, Opuntia ficus-indica (Cactaceae): new molecular evidence. American Journal of Botany. 91: 1915-1921.

Juniper, Barrie E. 2007. The Mysterious Origins of the Sweet Apple. American Scientist. 95(1): 44-51.

Morency, Pierre. 1989, L'Oeil Américain - histoires naturelles du Nouveau Monde. Boréal / Seuil. Québec. 276-288.

Otto, Stella. 1993. The Backyard Orchardist. OttoGraphics. Maple City.

Peattie, Donald Culross. 1991. Trees of Eastern and Central North America. Houghton Mifflin Company. Boston.

Spiller, Mary. 1980. Growing Fruit. Allen Lane. London.

Species Rating, Ontario ISA Standards

Species Rating is the estimated value of a particular species within a particular geographic region. These Species Rating values (0-100%) have been standardised by the various regional chapters of the International Society of Arboriculture (ISA). In Ontario the Species Ratings are based on the ranges outlined by the Ontario Chapter of the ISA.

Reference: International Society of Arboriculture of Ontario. 1998. Ontario Supplement to Guide for Plant Appraisal 8th Edition. Ontario Chapter, International Society of Arboriculture.

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