Home » Gardening Information » Great Plants
Selecting Conifers for Your Garden
by by Kim Tripp
We gardeners have the best chance for success if we choose plants suited to our climates and the particular conditions in our gardens. Fortunately, there are plenty of delightful conifers for every garden from coast to coast. The following are some important factors to keep in mind when selecting conifers.
Temperature
Gardeners tend to focus on cold hardiness as the single determinant of whether a plant is an appropriate choice for their properties. However, referring to the widely used USDA Plant Hardiness Zone Map, which is based on average annual minimum temperatures, is only a first step. In order to thrive, a long-lived plant must be well-adapted to seasonal extremes of heat as well as cold, and also to other patterns of temperature variation throughout the year, such as the duration of the extremes, and the accompanying moisture and other conditions.
Although most conifers are evergreen, there are dormant or quiescent periods when there is little or no active growth. In general, plants are far more tolerant of temperature extremes during a dormant or quiescent period than during active growth. For example, a prolonged period of extreme winter cold is much less likely to damage northern populations of balsam firs than an extended period of extreme summer heat because they are not actively growing in winter.
Keep in mind, too, that individuals of a species with a wide natural distribution may vary in hardiness. For instance, Douglas-fir is native from the mild Pacific Coast to the harsher Dakotas. Plants grown from seed collected in the western part of the range will likely be less cold- and drought-tolerant than those grown from more eastern sources. If possible, gardeners should be aware of the origin, or provenance, of these species, although unfortunately this information is rarely available.
Because roots are far more susceptible to temperature extremes than above-ground plant parts, below-ground temperatures are another consideration, especially when choosing conifers for containers, where the roots will be subjected to greater temperature fluctuations than they would be in the ground.
Macroclimate and Microclimate
Macroclimate describes the general characteristics of a region, such as annual precipitation, temperature range, daylength and regional trends in soil types. Microclimate describes the specific characteristics found in smaller areas of a region, or in a garden—a poorly drained corner, a sudden slope, a pocket of sandy soil in a clay basin, an especially windy inner city corridor, or the south side of a building, for example. When you choose conifers, keep in mind that cold air flows downhill and pools in low places, while hot air tends to rise and radiate away from buildings and stone walls. Make sure your conifer selection is suited to both your macroclimate and the particular microclimate you have in mind.
Soil
Conifers have an undeserved reputation for being very sensitive to soil type. While this is true to a certain extent for a few conifers (for example, most true firs are generally intolerant of poorly drained clay soils), beautiful conifers are available that will thrive in almost any soil. It is extraordinarily difficult to generalize about soil types throughout the country. The best way to determine the kind of soil in your garden is to submit samples to your Cooperative Extension office for analysis and then select conifers accordingly.
Soil pH is an especially important consideration when choosing conifers. Neutral soils have a pH from around 6 to 7, acid soils from 1 to 5 and basic or limy soils from 7 to 10. Some conifers—junipers, for example—perform better in relatively basic soils. Many southeastern pines, on the other hand, thrive in acid soils with pHs as low as 3.5 to 4.0.
Moisture
Moisture in the garden exists in many forms throughout the year: rain, fog, dew, relative humidity, snow, sleet, overhead and/or drip irrigation. The total annual precipitation in your area, the kinds (rain, snow, sleet) and the seasonal distribution (evenly throughout the year, for example, or only in one season) are all factors to consider when choosing conifers for your garden. Many tolerate periodic flooding in the winter, or quiescent season, but may be stressed or die if flooding occurs during the active growing season. Others, such as bald-cypress, thrive on permanently wet, swampy sites. The key is to choose conifers suited to the conditions in your area and your garden. If, for example, you garden where there is an extended dry period in one season, it is best to use conifers such as some junipers and cypresses that will tolerate or even prefer such an extended dry period.
Light
Light is essential to higher plants, but can also be a source of stress. Different plants are best adapted to different ranges of light intensity; some plants incur tissue damage when directly exposed to intense sunlight. Most garden books categorize the varying tolerances as "full sun," "partial shade" or "full shade." The gardener's challenge is to make sure plants receive enough light for growth without exposing them to overly intense light.
Too much sun can be especially damaging in winter. A break of bright sunlight after weeks of dark, cloudy weather can result in both leaf and trunk sunscald. One way to avoid this problem, particularly if plants are class="border"rline hardy for your area, is to wrap trunks of newly planted trees for the winter or to avoid siting plants where they get full southwesterly exposure.
Light intensity is key for variegated and colored selections. Some require intense light to develop full color, while others need protection to avoid leaf scorch.
Air
Air is the source of oxygen for respiration and carbon dioxide for photosynthesis. It is also the source of potential pollutants, and the flow of air can create pockets or sustained blasts of excessive cold or heat. Winds near the coast can cause damage to salt-sensitive plants.
It's difficult to control air quality, but if it is a problem in your area you can choose a conifer that is relatively pollution tolerant. It is possible to modify air-flow patterns with buildings and wind breaks, or to select conifers that can stand up to strong prevailing winds.
Pest Resistance
When selecting conifers, be sure to find out about diseases and pests that are problems in your region. If you're thinking of planting a Canadian hemlock in the eastern states, for example, you should know that it will require preventive treatment for hemlock woolly adelgid, probably for the lifetime of the plant. Your Cooperative Extension agent or local nursery professionals can advise you about pest- and disease-resistant selections.
Kim Tripp is the Director of The Botanic Garden of Smith College, Northampton, Massachusetts, and Associate for Research at the Arnold Arboretum of Harvard University in Jamaica Plain, Massachusetts. Previously, she was Curator of Conifers at the North Carolina State University Arboretum. She is co-author of The Year in Trees (Timber Press, 1995).