1 - A sweet new magnolia
Urban Forestry has added a second American magnolia to the list of evergreen choices, the northern sweetbay Magnolia virginiana ‘Jim Wilson.’ Trademarked as Moonglow, this cultivar was introduced commercially only in 2001. It was selected for being more evergreen than other sweetbays from the northern part of this tree’s range and for its upright growth to 35 feet. Spread is 18-20 feet. The semi-elliptic leaves are silvery white underneath, and unlike southern magnolia are thin, delicate looking and not leathery. The cream-colored, cup-shaped flowers are lemon-scented. They appear in late spring and sporadically through the summer. Seeds are glossy red and appear in clusters resembling a cone. Birds find these seeds attractive. Bark is smooth and gray. The trees can take a few years to fill in, but may never be quite as densely branched nor make as deep a shade as the southern magnolia.
2 - A southern belle that endures northern winters
Another southerner that has made itself at home in the north is Magnolia grandiflora. The official state flower of Mississippi, this evergreen tree with its large, white flowers (up to 8” across) scented of lemons started a craze for North American garden trees in Britain in the mid-1700s, and was a prize tree in the garden of the Duchess of Beaufort as early as 1730. Although eclipsed later when the colored flowers of Asian species arrived, Southern magnolia remains a popular park, garden and street tree the world over.
Growing natively in the South at elevations below 500 feet, not all cultivars are hardy enough to survive Portland’s cold winters or snow and ice storms. One that has endured is M. grandiflora ‘Edith Bogue,’ which although originally from Florida withstands snow and ice better than the species and most cultivars.
Edith Bogue grows at a moderate pace 30’ to possibly 40’ tall and 15’ to 20’ wide. Its lustrous leaves are narrower than the species, with only a hint of brown indumentum underneath. Flowering is from late spring and sporadically through the summer. Like most magnolias, the orange-pink seeds are attractive to birds, which will pick them from the 3” to 5” long cucumber-like aggregate containing them.
3 - Bambooleaf oak
Many members of the oak family are actually evergreen. Perhaps none makes a more attractive street tree than Quercus myrsinifolia.
Widespread across China, Korea and Japan, it is one of the few trees also native to Laos, Vietnam and northern Thailand that are hardy enough to grow in Portland.
Called bambooleaf because of the long, unlobed leaves, this oak will grow rapidly to 30-35 feet, perhaps more with time. It forms a densely branched, round headed tree with smooth, gray bark. Although there is no fall color change, new leaves emerge an attractive silvery purplish red during the growing season. Specimens planted within the past 20 years are already a respectable size and have proven tough as nails, sailing untroubled through ice storms and snowfalls.
4 - Silverleaf oak
The southwestern U.S. and northern Mexico share a common flora rich in evergreen oaks. One that has proven hardy in Portland during the past two decades is the silverleaf oak Quercus hypoleucoides.
The silvery undersides of the leaf give this tree its common name. Silverleaf will astonish with its fast growth, forming a densely branched tree to 40 feet or more in no time. The leathery leaves are gray-green above and make the tree incredibly drought tolerant once established. Two silverleaf oaks planted near the Visitors’ Center at Hoyt Arboretum already show how equally well the tree withstands cold and heat if given the sun they crave.
5, 6, and 7 - Oregon’s own broadleaved evergreens
Most Portlanders are familiar with the native madrone Arbutus menziesii. It has spectacularly sensual exfoliating bark that emerges with a snakelike olive color and ripens to a rich ocher. Related to rhododendrons, the trees boast big clusters of bell-shaped white flowers in spring, followed by orange fruits relished by local bird. Far rarer than when frequent fires set by Native Americans kept encroaching conifers at bay, madrone does not transplant easily and requires excellent drainage and a sunny location. Watering it in summer once it’s established can actually kill it. Introduced diseases have also taken a heavy toll of trees around the city. But if you have the right conditions, particularly in a yard, it can be a valued wildlife tree that’s pretty to look at year round.
Drive just a few hours south of Portland and a whole new world of broadleaved evergreen trees begins to appear. In addition to more madrone, you’ll see trees like canyon live oak (Quercus chrysolepis), and Oregon myrtle (Umbellularia californica). Known in California as California bay, the latter tree is a relative of the true bay laurel from the Mediterranean. It makes a rather large shade tree 60 to 90 feet tall. Tough and tolerant of drought and shade, Oregon myrtle has leaves that are heavily spicy when crushed. This has given rise to the nickname the “headache tree,” bestowed by people overcome by the pungent aroma. One key drawback to the tree is that suckers sprout regularly from the lower trunk and require removal to avoid creating a multi-stemmed monster. The tree also produces olive-sized fruits, beloved by birds but less so by gardeners.
Far better behaved in cultivation is the canyon live oak. Although on dry ridges it can be a little shrub, on good soil it becomes a stately, even majestic tree 60 to 80 feet tall. Its young leaves somewhat resemble those of holly, although generally smaller. Older trees seem to produce leaves that are less armed than those of young trees. The 1 to 2-inch long acorns of this tree were a staple food of many Native American people, who leached out the tannins before boiling or roasting them. Band-tailed pigeons gobble up the acorns whole, as will any resident squirrels. A deep tap root means this tree will endure the driest summers unruffled while maples and other eastern trees wilt and defoliate early.
8 & 9 - Welcome Californians
California native interior live oak (Quercus wislizeni) is noted for its exceptional drought and heat tolerance, being able to survive on as little as 15 inches of rain a year. Interior live oak can reach 70’ but is usually shorter. Often as broad as it is tall and densely branched, the tree shelters many birds and animals in inclement weather, and feeds them with its narrow, cone-shaped acorns. These sit deeply in their cup, taking up to two years to ripen. The leathery leaves can be smooth, toothed or spiny like a holly. On young trees, bark is smooth and light gray, becoming fissured and darker with age.
A cousin of interior live oak, also widespread in California, is the blue oak (Quercus douglasii). The scientific name of blue oak honors the same David Douglas recognized in the name of Oregon’s state tree, the Douglas-fir. The common name refers to the pretty powder blue or blue-green color of the leaves. In Portland’s colder winters the tree will defoliate but put out a flush of new leaves in spring. When winters are mild, the leaves persist.
Native Americans had many uses for blue oak, eating the acorns and using the wood for everything from firewood to bait, medicine, dyes, utensils, games, toys and construction materials.
10 - Southern live oaks
There are Quercus virginiana trees that survived the Civil War still standing today, as this is a long-lived tree often lasting centuries. In that time, these gracious trees can attain heights of 60’ with an even broader spread, casting welcome shade on hot summer days. They have well-anchored roots, allowing them to survive frequent hurricanes. Dark green and unlobed, Southern live oak leaves have smooth margins, are leathery, and are lighter-colored underneath. They range from 1 inch to 6 inches in length. Like blue oak, the leaves may drop in especially cold winters, with the tree leafing out again in spring.
11 - The noble laurel
The leaf spicing up your next pasta probably came from the true bay laurel Laurus nobilis. Just be sure to remove any whole leaves before serving!
A native of lands around the Mediterranean, this tree’s aromatic leaves were used to make wreaths to crown victors in the Olympic Games. In ancient Athens, retired Olympic champions were fed at public expense for life, giving rise to the phrase, “to rest on one’s laurels” rather than achieving anything new. If you grow this tree you will have a ready supply of leaves to cook with, as the tree is densely clothed in foliage. Bay laurel is capable of reaching 30 feet or more with a spread of 20 feet. The small yellowish flowers along the branch stems are followed by a small, one-seeded shiny black fruit eaten by birds.
12 - A long-lived African and European tree
Few trees from Africa can survive in Portland but most of those that can hail from the mountains of Algeria and Morocco, where winter snow and freezing temperatures are not uncommon. The holly or holm oak (Quercus ilex) is from that region as well as the Iberian Peninsula, Italy and Malta, where it is the national tree. The Latin name for the species means “holly” because leaves on young trees can be somewhat spiny, resembling those of a holly although not approaching it in prickliness. However, as trees mature leaf spininess becomes less pronounced. The leaves are pale white underneath. Acorns are long and quite pointed at the tip. Bark is dark brown to almost black, with many cracks and fissures forming a maze of small plates and ridges.
In Morocco, holly oak grows alongside Atlas cedar. It can reach 60 to 80 feet in height with a spread up to 60 feet or more. Those forests are home to an endangered primate, the Barbary macaque. Holly oaks are very long lived. The first ones planted in England in the 1500s are still living. There are reports of the trees reseeding in England and elsewhere, so it might be best to avoid planting this tree if you live near a natural area.
Why Do Evergreen Trees Matter?
In any city east of the Cascades as far as New England, winter can be depressing. Skeletal, bare-limbed, deciduous trees dominate northern cities, relieved only by a few hardy evergreen conifers. Given Portland’s mild climate, our urban forest could look a lot greener even in winter than those dreary northern cityscapes. But it doesn’t. Why is that?
The reason is a heavy reliance on deciduous trees that lose their leaves each fall and don’t green up till spring warmth returns. Tree inventories done by Urban Forestry show that in built up parts of the city, 90% to 98% or more of the street trees become leafless each fall. This lifeless landscape makes winter more challenging – emotionally and environmentally – than it has to be.
A legacy of leafless winters
Planting deciduous trees is to be expected in places where polar vortexes can send temperatures plummeting well below zero. Many of our most common urban trees – elms, ashes, maples and crabapples to name a few - come from mid-latitude forests with much colder weather than we get. Trees from those areas are well-suited to survive bitter winters and maximize their growth in warmer months, irrigated by frequent summer rains.
The pioneers who founded Portland quite naturally planted their new parks and streets with deciduous trees familiar to them from their home states. For example, as early as the 1870s the downtown park blocks were planted with stately elms native to the eastern U.S. Even when Oregon natives were tried as street trees, the one most commonly experimented with was the deciduous bigleaf maple (Acer macrophyllum).
Although Portland is actually north of such icebox cities as Minneapolis, Chicago, Detroit, and Toronto, Canada, we experience much warmer winters. As importantly, rain all but ceases here from July to September. When rains do return in autumn, deciduous trees are starting to shut down. Just when we need them most, deciduous trees lose the very leaves we require to help intercept moisture and slow flooding.
The obvious solution to our winter-rainfall woes is to plant more evergreen trees. A lot more. Currently, evergreen trees make up the thinnest slice of the city’s street tree pie. You can count the percentage they represent on one hand. While our parklands have respectable amounts of native evergreen conifers (mostly Douglas-fir, western red-cedar and western hemlock), the streets where most Portlanders live do not.
To encourage change, Urban Forestry recently increased the number of evergreen choices on its Approved Street Tree Planting Lists, available for download at portlandoregon.gov/trees. In particular, the number of broadleaved evergreens has increased to an even dozen. In addition to the stalwart Magnolia grandiflora ‘Edith Bogue’ from the eastern U.S., it’s now possible to select from several exciting new trees.