|Updated : 21/9/2014|
Tree and Shrub Selection
Choosing the Perfect Trees and Shrubs
When you choose new trees and shrubs, think about what you want these plants to do in your landscape. Do you need shade or shelter from the wind? Does a corner of the yard need a spark of colour? Consider all the seasons of the year when you make your decision; the best shrubs and trees have practical or decorative value in several seasons, not just one. A Japanese maple, for example, may have ornamental leaves in spring and summer, colourful fall foliage, and interesting branching patterns and bark in the winter. I've chosen some of the best and most versatile landscape trees and shrubs for this section.
If you want to know more about flowering and shade trees, shrubs, and conifers, and how to care for them, pick up a copy of Trees & Shrubs For Dummies (Wiley). The book also lists many of the most attractive and disease- and pest-resistant varieties of each tree and shrub.
Shade trees frame the landscape, cool the space around them, and provide a backdrop for colourful flowering trees and shrubs. Most of these trees grow too tall for you to treat for pests or diseases, so if you're planting new trees, your best bet is to choose species that suffer few problems in the first place. If you already have large shade trees with chronic pests, or if you suspect disease, call an arborist who can diagnose the trouble and offer advice and treatment.
✓ Maple (Acer species): A large and diverse group of valuable landscape trees, maples range in mature size from 20 to 100 feet, depending on species. The larger species serve as shade trees; smaller ones make good specimens and street trees. Many maples are renowned for their brilliant autumn foliage; some also have attractive bark.
Maples that grow 25 to 30 feet high include trident (A. buergeranum), hedge (A. campestre), fullmoon (A. japonicum), Japanese (A. palmatum), Amur (A. tataricum), and Shantung (A. truncatum). All have yellow, orange, or crimson fall foliage. Larger species for shade include paperbark (A. griseum), red (A. rubrum; note this tree's bad habits in "Avoiding troublemakers," earlier in this chapter), and sugar (A. saccharum) maples.
Most maples prefer well-drained, fertile soil and regular moisture. Some maples, including hedge and trident, tolerate drier and less fertile soil, such as occurs near roads. For damp soil, try red or Amur maples. Prune only to remove dead, damaged, or diseased limbs, or any that rub or hang over buildings. Prune in early summer, avoiding late winter through spring, when pruning cuts will bleed sap profusely.
Leaf spots, cankers, caterpillars, borers, leafhoppers, and scale insects injure maples, especially those growing in stressed conditions or in unsuitable soils. Some species are also more prone to insects, disease, and structural damage than others, including box elder (A. negundo), silver (A. saccharinum), and sycamore (A. pseudoplatanus) maples. In hot, dry climates, look for brown leaf edges that signal leaf scorch. Give additional water to moisten the soil to a depth of at least 12 inches.
✓ Birch (Betula species): When you think of birches, you likely picture a clump of white-barked trees. Not all birches have white bark, however. Many actually make better landscape specimens, especially for organic gardeners, because they resist the most common and devastating pest — the bronze birch borer — and tolerate a wider range of growing conditions.
Birches cast light shade, making them good choices for cooling a shady seat. Most birches thrive in moist, well-drained, acidic soils, but try gray birch in drier situations and river birch in damp soil. Prune birches in late spring to early summer after the leaves have fully emerged. If pruned too early or late in the season, they bleed sap excessively. Avoid disturbing the soil around their shallow roots.
Birches are subject to attack from many insects and several diseases, including leaf miners, canker, and leaf diseases. The bronze birch borer decimates the white-barked birches, especially the European white birch throughout the eastern and midwestern United States, targeting stressed trees and older specimens first. The first symptoms of infestation are dead branches near the top of the tree.
Choose species that resist the bronze birch borer, such as Asian white (B. platyphylla japonica), gray (B. populifolia), and river (B. nigra) birches. If the pest is not a problem in your area, try white-barked paper (B. papyrifera) and European white (B. pendula) birches. All these birches are hardy in Zones 3 to 6 except Asian white birch, which grows north to Zone 4, and river birch, which grows south to Zone 9.
✓ Hackberry (Celtis species): These native North American species thrive in adverse conditions, tolerating wet to dry soil and urban to open prairie situations. They grow 40 to 60 feet tall and form an elmlike silhouette. Common hackberry (C. occidentalis) grows in Zones 3 to 9, tolerates midwestern wind and dry soil, and has wildlife-attracting berries. Sugar hackberry (C. laevigata) grows in Zones 5 to 9, prefers low wet areas, has smooth gray bark, and resists some common hackberry diseases.
Common hackberry is prone to a disfiguring disease called witches' broom that makes the twigs grow abnormally. The cultivar Prairie Pride resists the disease. Other problems include leaf spots and galls. Mourning cloak butterfly larvae enjoy its foliage.
✓ Ginkgo (Gingko biloba): Growing 50 to 80 feet tall and 30 to 40 feet wide, ginkgo is best suited to large yards. Young trees have a pyramid shape, but older trees become widely spreading. Varieties also differ significantly in width and shape. Ginkgo's pollution tolerance and neat habits make it attractive for planting along streets, in parks, and in other urban and suburban areas.
Not fussy about soil, the gingko thrives in Zones 4 to 8. It needs little care and has no significant pests or diseases. Choose a male cultivar such as Autumn Gold, Fairmount, or Princeton Sentry to avoid the unpleasant-smelling fruits that drop from female trees.
✓ Katsura (Cercidiphyllum japonicus): This tree has so many good things going for it that's it hard to go wrong by choosing it for your yard — as long as you have the space to accommodate it. It grows 40 to 60 feet high and spreads just as wide from several branched trunks. The nearly round leaves are attractive from spring to their final fall blaze of apricot-orange.
Katsura prefers moist, slightly acidic, well-drained, fertile soil and full sun. Keep the soil moist but not saturated for the first few years after transplanting. Mulch to hold soil moisture. The species has no significant pests or diseases. For something different, look for the cultivar Pendula, which grows cascading limbs that reach 15 to 25 feet high with a wider spread.
Plant dormant, balled and burlapped, or container-grown specimens in well-drained soil. Pin oak (Q. palustris) and swamp white oak (Q. bicolour) can grow in wet soils. Preferred soil pH varies widely. Pin oak is especially sensitive to high pH soils, and the leaves will turn yellow if the soil isn't acidic.
Oaks suffer from many insect pests that eat their leaves — including gypsy moths, oak moths, mites, and borers — plus various fungus diseases. Few pests prove fatal to otherwise-healthy trees.
Other shade trees to consider are ash (Fraxinus spp.), beech (Fagus spp.), black gum (Nyssa sylvatica), Chinese pistache (Pistacia chinensis), hickory (Carya spp.), Japanese zelkova (Zelkova serrata), Kentucky coffeetree (Gymnocladus dioicus), lacebark elm (Ulmus parvifolia), linden (Tilia spp.), and yellowwood (Cladrastis kentukea).
Flowering and ornamental trees
Flowering trees give your yard a colourful exclamation point whenever they bloom. The anticipation of cherry or crabapple blossoms marks the changing of the seasons, often highlighting the start of the gardening season.
Varieties with unusual foliage include Forest Pansy, which grows in Zones 6 to 9 and has dark purple new leaves that mature to deep burgundy; and Silver Cloud, which has creamy-white, variegated leaves. Other varieties have flower colours ranging from white to reddish purple.
✓ Dogwood (Cornus species): Distinctive horizontal branching, clouds of spring flowers, and fiery autumn foliage make this group of landscape trees well loved. The most commonly planted species is flowering dogwood (C. florida), which is hardy in Zones 5 to 9.
Many good varieties exist, including Cherokee Princess, which is very cold-hardy and disease-resistant. Another under-appreciated species is kousa dogwood (C. kousa), which grows in similar conditions but stays smaller and blooms later than flowering dogwood, and is more resistant to anthracnose disease.
Plant dogwoods in well-drained, moist, humus-rich soil. In hot climates, give midday shade. Canker, twig blights, anthracnose, and wood-boring insects frequently damage these trees. Prune out diseased wood when you see it. Dogwood is also prone to rot diseases if the bark is damaged — take care with the mower and string trimmer!
5 and even Zone 4. Magnolias prefer moist, well-drained, slightly acidic soil with plenty of organic matter and full sun to light shade. Protect the shallow roots from drought and weed competition with a layer of organic mulch. Prune only to shape the tree and remove undesirable limbs. Magnolias as a group suffer from few pests or diseases except for saucer magnolia (M. x soulangiana), which is prone to several leaf diseases.
For shrub to small tree-size magnolias, look for lily magnolia (M. liliiflora); any of the Kosar-DeVos hybrids, such as Ann or Betty; sweetbay (M. virginiana); and star magnolia (M. stellata). Species that grow up to 30 feet tall include Yulan (M. denudata), Loebner (M. x loebneri), and many hybrids.
✓ Flowering crabapple (Malus species): One of the most widely grown flowering trees in Zones 4 to 7 for home and public landscapes, flowering crabapples have a lot to offer. Spring bloom, persistent and colourful fruit, and attractive branching and bark make them justifiably popular. Hundreds of varieties are available, and many of the newest ones have built-in disease resistance. Plant in nearly any well-drained, moderately fertile soil and full sun. Prevent weed and grass competition with organic mulch. Prune while dormant, in late winter to early spring.
Choose the best varieties by first looking at disease resistance. Crabapples suffer from many serious diseases, including leaf scab, cedar-apple rust, powdery mildew, and fire blight. Expect aphids, mites, caterpillars, rodents, and deer to take a bite too.
After selecting disease-resistant cultivars, select for flower colour (white to deep pink), height and growth shape (column to wide and low), leaf colour (green to reddish), and fruit size and colour (red to yellow).
✓ Stewartia (Stewartia species): If you have moist, well-drained, acidic soil, consider stewartia for its flaky, mottled bark; late-summer bloom; and striking fall foliage. Trees stay 20 to 40 feet tall, making them ideal for most home landscapes. They suffer from few pests or diseases and need little pruning.
Other small, ornamental trees include white fringetree (Chionanthus virginicus), goldenrain tree (Koelreuteria paniculata), purple-leaf plum (Prunus cerasifera), red buckeye (Aesculus pavia), sassafras (Sassafras albidum), and silverbell (Halesia carolina).
Flowering and ornamental shrubs
The seasonal stars of the show, flowering shrubs light up the landscape with drifts and spots of bloom. The best shrubs, however, offer more than a week or two of flowers; attractive foliage, stems, and colourful berries can carry the show through the rest of the year.
Plant cotoneasters in nearly any well-drained soil, including sandy, heavy clay, drought-prone, salty, and high- or low-pH soil. Prune to shape or remove damaged limbs. Pest and disease problems include fire blight, leaf spots, canker, and spider mites.
Hollies in general enjoy moist, well-drained soil and full sun, although Chinese holly withstands drought and flooding with aplomb. In windy areas and climates where the soil freezes, protect the foliage from drying out by covering with burlap or other windbreak material. Hollies suffer from many pests and diseases, including scales, spider mites, nematodes, leaf miners, and various bugs and caterpillars, as well as mildew and leaf spots. The most problem-free species include Yaupon (I. vomitoria), inkberry, and the Foster hybrids.
Winterberry (I. Verticillata), one of my favorite hollies, grows in Zones 3 to 9. It loses its leaves in winter but retains the characteristic masses of red berries. Winterberry has few pests or disease problems.
✓ Spirea (Spirea species): These easygoing shrubs are useful for informal hedges, foundation plantings, and mixed shrub borders. Most have flat clusters of little white to pink flowers; some, such as Magic Carpet and Goldflame, even offer three seasons of interest with their brilliant yellow foliage, floral display, and autumn colour. Plant in any well-drained soil, and give them full sun and regular watering in dry spells.
Spireas come under attack from many pests and diseases, but these attacks rarely prove to be fatal. After flowering, cut the weakest or oldest one-fourth of the shoots to the ground each year to keep shrubs vigorous and tidy.
Species that grow in dry soils include wayfaring tree (V. lantana), arrow-wood viburnum (V. dentatum), and blackhaw viburnum (V. prunifolium). Few pests or diseases cause serious damage.
Other shrubs to consider include azaleas and rhododendrons (Rhododendron spp.species), mountain laurel (Kalmia latifolia), red-osier dogwood (Cornus sericea), and Virginia sweetspire (Itea virginica).
Needle- and cone-bearing trees and shrubs, called conifers, provide the backdrop for more colourful garden elements and serve as hedges, screens, and windbreaks. Their imposing size and stiff formality make many evergreen trees difficult to integrate into small home landscapes. Think twice before planting a potentially 50-foot-tall Colorado blue spruce in your front yard! In larger settings, where the trees can attain full size, evergreen trees add grandeur and provide refuge for wildlife.
Shrub-size conifers are invaluable for the year-round colour and texture they add to the landscape. Some creep over the ground and drape over walls; others grow into neat cones, pyramids, and rounded cushions. Foliage colours range from gold through a wide range of greens to silver and even purplish. Some plants appear fuzzy and soft; others are stiff and bristly.
Although you can shape many conifers into geometric and fanciful forms, most don't require pruning at all except to remove dead, diseased, or damaged limbs and undesirable growth that detracts from the plant's appearance. In fact, pine, spruce, and fir trees that grow in whorls (with layers of branches around the trunk) will not sprout new limbs in response to pruning. To control their growth, pinch or prune their new, soft growth in late spring before it hardens, cutting into only the new tissue. Arborvitae, false cypress, cypress, juniper, and yew plants with random branching can tolerate more pruning, however, and usually will sprout new limbs to replace the ones that you remove. Conifers that usually grow into a pyramid shape with one central trunk sometimes develop additional leaders (competing main trunks) at the top of the tree. Remove all but one leader to retain the tree shape.
As a group, conifers suffer from their share of pests and diseases. The most troublesome pests include spruce budworm, bagworms, and various caterpillars. Bacterial and fungal diseases cause blights, cankers, and root rots. See Part III for more information about these pests and diseases. The best defence against disease is to plant your trees and shrubs in the soil and sun conditions they prefer, and keep them growing strong. Protect evergreens from drying winter winds wherever the ground freezes.
Hundreds of varieties are available, including low-growing shrubs to stately trees. Leyland cypress is used widely for hedges because it grows up to 3 feet per year and tolerates salt spray and any soil except poorly drained. Hinoki (C. obtusa) and threadleaf (C. pisifera) false cypresses have many popular varieties used in home and commercial landscapes. Cypresses require no pruning except to shape the plant or remove damaged limbs. Bagworms are the only troublesome pests. Twig blight occurs in some areas but isn't prevalent.
✓ Juniper (Juniperus species): Versatile and tough as nails, junipers are justifiably among the most popular landscape shrubs. They tolerate poor, dry soil, as well as urban and roadside conditions; they come in a seemingly infinite number of shapes, sizes, colours, and textures. Depending on the species and variety, you can find a juniper to grow in any climate from coastal Florida to the Canadian plains. Ground-hugging forms make excellent carpets for slopes and lawn substitutes. Taller varieties serve as shrubs for hedges and planting around buildings.
In cold-winter Zone 3 or 4, and warmer, look for Chinese (J. chinensis, J. x media), creeping (J. horizontalis), savin (J. sabina), and Rocky Mountain (J. scopulorum) juniper varieties. Shore juniper (J. conferta) enjoys the heat in Zones 6 to 9 and tolerates coastal conditions.
Junipers can suffer from several insect pests and diseases, including bagworms, scales, webworms, borers, twig blight, and cedar-apple rust. Creeping juniper is more disease-prone than others, but savin juniper varieties Calgary Carpet, Arcadia, Scandia, Blue Danube, and Broadmoor resist juniper blight. In wet, poorly drained soils, junipers are prone to root rot.
✓ Spruce (Picea species): Give spruce trees plenty of room if you plant them in your landscape, because they tend to spread widely at the base as they mature, often measuring 20 feet or more across at the ground. Most spruce have a stiff, formal pyramid shape, which looks best in large landscapes or when the trees grow in groups. Use for windbreaks or large screens.
A few dwarf varieties are available; they grow into small mounds or weeping specimens suitable for planting in home landscapes. For dwarf varieties, look for Norway (Picea abies), Little Gem, Pumila, Nidiformis, or Bird's Nest Spruce; black (P. mariana) Nana; or white spruce (P. glauca) Conica, also known as Dwarf Alberta Spruce.
Spruces prefer cool climates and well-drained but moderately moist soil. Avoid planting them in dry soil and polluted urban locations. They often suffer from aphids, spruce budworms, bagworms, and other pests, as well as canker and twig blight.
✓ Pine (Pinus species): Most pines grow into large, picturesque trees up to 100 feet tall, but a few dwarf varieties stay small enough to serve in home landscapes. For small pines, seek out Japanese red pine (Pinus densiflora) Umbraculifera, Japanese white pine (P. parviflora) Glauca, Japanese black pine (P. thunbergii), and mugo pine (P. mugo).
Pines have long needles that give them a softer texture than most other conifers. The needles occur in bundles of two, three, or five. Pines with the same number of needles in a bundle often have other common characteristics, such as growth habits and cultural requirements. Two-needled pines, such as Scotch pine (P. sylvestris), for example, tolerate drier soil and more heat than the five-needled species, such as white pine (P. strobus).
Pines don't tolerate air pollution or road-salt spray. Common diseases and pests include white pine blister rust, spruce budworms, and white pine weevils.
✓ Yew (Taxus species): Yews, among the most widely grown conifers for hedges and shearing into fanciful shapes, respond to pruning by sprouting ever-denser growth. Keep in mind that most yews will grow into
30- to 60-foot trees if allowed to do so. They grow best in well-drained, fertile, moist soil and full sun to part shade, and may need protection from the winter wind in cold climates.
In hot, muggy climates, look for the variety Tauntonii, which tolerates the summer heat better than most other yews. Pests include deer, weevils, and mealybugs, as well as blights and root rot.
✓ Arborvitae and white cedar (Thuja and Platycladus species): These tough trees and shrubs grow in a wide range of soils and climates, from soggy to well drained, in Zones 3 to 11. They need full sun to grow lush and full. Few pests or diseases cause them serious trouble, although bagworms, spider mites, blight, and canker can show up when plants are stressed. Protect them from road-salt spray and drying winter winds.
As trees, arborvitae grow up to 50 feet high, but many varieties stay shrub-sized and come in many shapes, including globe, cone, column, pyramid, and weeping. Some have yellowish foliage; others have deep green foliage. Choose a variety that matches your climate, soil, and specific landscape needs. Arborvitae make a classic tall hedge without shearing, and smaller varieties are suitable for planting around buildings