Home & Garden

Selecting trees and shrubs: dwarves versus giants

gardening by Mary Robson, WSU Extension Agent
During a short walk through almost any area, it's possible to see many examples of good plants stuffed into the wrong places, growing too tall under eaves or gobbling all the morning sun with deep evergreen branches.
   In a one-block survey of my own Seattle neighborhood, I spotted a Douglas fir planted out as a live Christmas tree for a 4-year old girl. She's now 28 years old, and the tree has grown to over 35 feet tall.
   That wouldn't be so bad, except that the sweep of lower branches completely fills the small backyard it's planted in. On a lot only 40 feet wide, this Douglas fir with its rapid and vigorous growth has consumed most of the outdoor living space.
   Another neighbor has just completed the installation of a handsome brick retaining wall along the streetside of the house. He's chosen to put an 8-foot evergreen magnolia tree directly in front of the wall with the trunk so close that it touches the wall.
   This tree, Magnolia grandiflora, can grow to 80 feet tall with a 40-foot spread and a trunk about two feet around.
   The little tree looks fine now but will continue to dominate the space, send roots under the wall and ultimately have to be relocated or removed.
   In my own garden, I've just had to remove a fine, attractive cherry tree, the ornamental cherry "Shirofugen," which was planted between two houses that are only 16 feet apart.
   In only 10 years of steady growth, this tree touched both houses and produced shallow roots that pried up a brick terrace and dislocated the neighbor's fence. Taking out trees that are in the wrong spots is an aggravating--and expensive--garden chore.
   Obviously, the moral of the story is to consider the eventual size of a plant and measure the space available before setting out to the nursery.
   Just as it's hard to imagine that an 8-pound baby will, in only 15 years, be a 6-foot 3-inch tall adolescent, it's difficult to see the growth potential in small plants.
   Conifers in particular, often selected as adorable small living Christmas trees, will shoot forward with astonishing speed. Not just the Douglas fir (Pseudotsuga menziesii) mentioned above, which can get to 200 feet tall and taller, but all the others.
   Noble fir (Aibes procera) tops out at 90 to 200 feet. Grand fir (Aibes grandis) can reach 300 feet. Choose live holiday trees with this understanding: Given enough space the tree can become a family heirloom. In too little space, it can rapidly become a liability.
   One landscape planner uses the "rule of 25," meaning that it's necessary to allow at least 25 feet between trees no matter what the tree chosen.
   Even shrubs need to be watched for eventual size butterfly bush (Buddleia alternifolia and many others), can start out as a 1-foot shrub and reach 12 feet with a spread of 8 feet quite rapidly.
   Many rhododendrons often become tree-size, as a walk in the Washington Park Arboretum in Seattle will prove. Planting the larger sorts under a house window will lead to problems.
   Laurels of all sorts are famous for their tree-like intentions, and indeed they make handsome tall evergreen trees in the maritime Pacific Northwest.
   The Portugal laurel, Prunus lustitancia, becomes a 30-foot or taller multi-branched tree. The common English laurel, Prumus laurocerasus, frequently tortured into a hedge that requires constant clipping, grows to 30 feet or more with amazing speed. Planting this as a hedge is a guarantee of extra maintenance problems.
   Plant hybridizers, growers, and nursery personnel understand the shrinking spaces available for planting, as people colonize on smaller and small pieces of land.
   Check with nurseries for cultivars that grow more slowly and will behave in urban and suburban settings. Many trees are desirable garden additions but remain small even at full growth.
   One that's particularly good for year-round interest is the Japanese dogwood, Cormus kousa, a fine specimen planting for a smaller garden. It can grow to 30 feet, with shining white flower bracts in June and brilliant fall color.
   The Japanese dogwood also resists attack by the common fungal problem, dogwood anthracnose, which often devastates the foliage of western dogwood (Cornus nuttalii) and eastern dogwood (Cornus florida). Choosing plants with some disease or insect resistance can reduce garden maintenance problems.
   Another useful small deciduous tree is the Japanese snowbell, Styrax japonica, growing relatively slowly to about 25 feet tall.
   The snowbell has pendulous white flowers along the branches in June and clear yellow fall color after a deep summer green.
   For stunning fall color, look for the parrotia, Persica parrotia. Slow-growing to about 15 feet, parrotia's glory is leaf color in fall when leaves change from gold to orange, pink, and red.
   Conifers are also bred for smaller sizes--the dwarf white pine, Pinus strobus 'Nana', grows very slowly to 3 to 7 feet tall. It provides the wonderful texture of the soft, two-needled pine without the soaring and nearly uncontrollable growth.
   Look also for attractive smaller hemlocks, such as Sargent's weeping hemlock, Tsuga canadensis 'Pendula.' This one will maintain a low, broad form about 3 feet high and 6 feet wide, beautiful in a rockery or at the top of a wall. Nurseries display dozens of "dwarf" conifers suited to smaller gardens.
   In a season of Thanksgiving, also among the best seasons for planting and transplanting trees and shrubs in landscapes, thankfulness can be felt for the "right plant in the right place." Select plants to complement rather than overwhelm the garden.