(Editor's Note: This article was originally published on December 23, 2007. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Evergreens are plants that retain their foliage for the winter, and are generally categorized as either needle or broadleaf. Needle evergreens include familiar 'Christmas Tree' varieties such as firs and spruces, as well as pines, arborvitae and junipers. These needle bearing plants grow their seeds within cones, so they are also referred to as conifers. Some examples of broadleaf evergreens are rhodendrons, azaleas, camellias and mountain laurels, and they are generally slower growing than the conifers. Evergreens are generally low-maintenance, easy care plants.
USDA Hardiness zone and winter severity are factors affecting whether a specific plant actually remains evergreen. A viburnum, for example, may be reliably evergreen in zone 8, but may drop its leaves in zone 6. Similarly, a particularly cold winter can trigger leaf drop in a shrub or tree that usually retains its leaves. The individual leaves or needles however, do have a finite lifetime, and are regularly replaced. Anyone with a white pine certainly knows this!
Within each category, there is a tremendous variety of evergreens that will suit growing conditions that run the gamut. For example, pines generally require good drainage and will grow well in very sandy soil. Conversely, I have arborvitae growing very happily in a low, wet area in my clay soil. Many broadleaf varieties, such as rhododendron, prefer shadier conditions and acidic soil, whereas junipers thrive in full sun and can tolerate more alkaline conditions.
In addition to these important cultural considerations, gardeners must also factor in size, shape, texture, color and how the plant will be integrated into the garden. Will it be used as a specimen, as part of a mixed bed or perhaps as a screen?
As lot sizes continue to shrink, gardeners are looking to evergreen screens as living fences to shield prying eyes and secure one of the last bits of privacy left in our lives. Unfortunately, many make the mistake of planting a straight row of one particular type of evergreen. Inevitably, as this hedge matures, one or more of the individuals will succumb to disease, pests or a brutal winter. Then the screen is ruined and finding full-sized replacements will be impossible or very expensive. It is also unnatural looking (it looks like a fence!) and calls undue attention to itself. A much better option is to use a number of different shrubs, perhaps including some natives, and stagger them from front to back, in a much more naturalistic fashion. Not only will it more closely resemble Mother Nature's layout, but if you lose one, it can be replaced without ruining the entire effect. This does not require much more room to accomplish.
In this search for privacy, gardeners have recently flocked to faster growing evergreens. One of the most popular is Thuja 'Green Giant'. It resembles American arborvitae (Thuja occidentalis) in appearance and form, but reportedly grows three to five feet per year once established. Unpruned, it can eventually reach about 30 feet high in 30 years. I have one, but it has not grown nearly that fast, adding about two feet per year. That might be a result of the few hours of shade it receives in the afternoon. In fact, my two Norway spruces (Picea abies) have grown just as fast, and I find it a more attractive tree. It is, however, a much wider tree. The final size can be more than 70 feet high and 30 feet wide. Leyland cypress is falling out of favor as a choice due to disease and pest problems, as well as requiring regular pruning.
Dwarf evergreens have also become more popular since our gardens are getting smaller and people are looking for lower maintenance plants. There are many to choose from, in both conifer and broadleaf. Breeders have been hard at work at producing dwarf specimens, such as Monrovia's rhododendron 'Scarlet Wonder Dwarf', which sports bright red flowers and grows only two feet tall. One of the things I love about rhodos is how they tell me just how cold it is outside! When temperatures drop to the twenties, their leaves begin to curl up. They do this to reduce moisture loss. As the temperature drops further, this effect gets more pronounced, and in the low teens the leaves are completely closed, and they droop down as if trying to hide from Jack Frost. So in January and February, I always glance outside at my rhodos before heading out so I know what I'm in for.
I grow a number of dwarf conifers, such as Cryptomeria japonica 'Black Dragon', which grows slowly to six to ten feet, with a pyramidal shape. The needles grow in branchlets which give it an attractive 'whirled' appearance. It turns a nice dark green in winter. I also grow dwarf alberta and blue spruces. All these are small enough to easily fit into the mixed border, as I use them. One of my absolute favorites is a dwarf deodar cedar called 'Silver Mist'. It has bluish-white needles that sparkle in the sun, though the plant should be shaded in the hottest part of the day. One of the delights of conifers in general is the range of colors and textures available - from near white to blue, gold and many shades of green, and textures ranging from the finest feathery needles, to the stiffest weapon-like, scaly armor!
A cautionary word about dwarf evergreens - they tend to be expensive, primarily because it takes so long to propagate them. And they do grow very slowly, so be prepared to wait up to ten years or more for that one foot tall plant to reach three or four feet!
Pruning of evergreens is more complicated than with deciduous plants. Different approaches and times are required for the various plants, and they are far less forgiving of errors than the deciduous are. This can be the subject of a future article.
Evergreens are indispensable for anyone wishing to attract wildlife, especially birds. Two of the four requirements for a 'backyard habitat' are cover or shelter, and a place to nest. Conifers - large trees in particular, are ideal for this. I have had the pleasure of watching so many birds nest and take cover in my spruces and cedars. And my Norway spruces are the overnight home to swarms of mourning doves. I had a genuine Alfred Hitchcock experience the first time I saw hundreds of them 'coming home' at dusk! They first land on nearby trees and shrubs before sorting out their 'sleeping arrangements'. I am awestruck each time I witness it.
There is also a very practical use to evergreen trees in the garden - reducing the heating bill! By planting tall conifers between your home and the prevailing winds, you can significantly cut the wind effect on your house. In fact, this was the main reason I planted my two Norways my first year in my house. Now that they are around 20 feet high, we get much less wind hitting the house.
So whether for beauty, diversity, privacy, energy efficiency or wildlife, evergreens deserve a place in your garden. Oblige them!
Look for an upcoming article from Dea that will profile conifers in greater depth!
Photos: 1. Dragon's Eye Pine 2. Rhododendron 3. Nandina domestica
4. Deodar Cedar 'Silver Mist' 5. Mockingbird taking refuge in Norway Maple
6. Backyard scene - l - r: Blue Spruce, Arborvitae and Eastern Red Cedar
Sources: More information on evergreens
Thuja 'Green Giant'
Another fast growing conifer
'Scarlet Wonder Dwarf' Rhododendron
Source for Broadleaf Evergreens and Conifers Watchdog Entry
Source for Conifers Watchdog Entry
Source for Rhododendrons Watchdog Entry