Just a bit of background first. Boxwood, or Buxus is a genus of about 70 species. They are native to parts of Europe, Asia, Africa, Madagascar, South and Central America, Mexico and the Caribbean. Only the European and some Asian species are frost-tolerant; the remainder are tropical and sub-tropical. These shrubs are slow-growing and evergreen.
I purchased a group of Buxus 'Green Velvet' to plant spaced around my pool fence. My requirements were that it be hardy in my zone (6a) and not over 2' or so high. This is what my landscaper recommended. The photo at the left is one as it was delivered from the nursery. I did some reading after they were planted, in PlantFiles here at Dave's Garden and on the web. This is a hybrid boxwood, Buxus sempervirens x B. microphylla var. koreana, a cross between B. sempervirens, or American boxwood and B. microphylla var. koreana, Littleleaf Korean boxwood. What I found is that it is hardy to zone 5a. I am in 6a, so this sounded good until I read further. In USDA Zones 5 and 6, this boxwood is best sited in a sheltered location which protects it in winter from strong winds and full sun. In my yard it is planted in full sun with no protection from strong winds (and, believe me, we get strong winds). The listed size is 3' by 3'... not too bad. I have no intention of shearing these shrubs into 'bowling balls,' but trimming enough to keep them in bounds and keep the center open so light can get to the inside branches is routine maintenance that I can manage. At right, you can see 3 of the 9 plants I purchased in their permanent homes. This was their first winter in my garden. I don't find the 'bronzing' of the leaves (next photo) unattractive, but it seems it isn't good for the plant. It means that the leaves have gotten dessicated (dried out) from the harsh winter winds. The last photo is one of my shrubs beginning to recover from the winter. At this point I'm not going to replace them, but I may try spraying them with an anti-dessicant, such as Wilt-Pruf, next fall. I am pleased that all 9 of them made it through the winter.
In truth, this was not my introduction to boxwood. There was one cone-shaped shrub in my original landscaping that was simply labelled 'boxwood' on the plan. It never occurred to me to ask for more details. Unfortunately, it declined rather quickly and this soured me on boxwood in general for a while. That is until I realized that I had voles in my front garden and it was highly likely that these rodents had eaten the roots of the boxwood in question. I was blaming the flora when it should have been the fauna. I've learned quite a bit since then and I have Dave's Garden to thank for most of it. The point to all of this personal interaction with boxwoods is that you need to know the plant, it's proposed use and the location where you intend to plant it before you buy it. Now let's look at some others.
Boxwood is an excellent hedge shrub. It takes very well to pruning. Pictured at left is a hedge of Harland boxwood (Buxus harlandii). It grows to 4' to 6' in height and is hardy from zone 6a to 8b. In mid-to-late spring there are small cream or light yellow blossoms which are fragrant, but have no petals. Most boxwoods have similar flowers. It will take some shade and prefers slightly acidic soil. Harland is also more resistant to pests than most other boxwoods.
Green Mountain boxwood (Buxus microphylla var. koreana x B. sempervirens 'Green Mountain'), seen at right, is an upright, pyramidal form, with foliage very much like 'Green Velvet' discussed earlier. It can grow from 3' to 5' high and is easily maintained in the pyramid shape or can be used in a hedge. Coming out of Sheridan Nursery in Canada, it is known for it's superior hardiness, zones 4 through 8. It is less likely to yellow in the winter than other boxwoods and will tolerate wind and air pollution.
Another excellent use of boxwood is in privacy screens. A nice example is in the photo at left. American boxwood 'Green Tower' (B. sempervirens) is well suited for this task. You can expect it to reach a height of 8' to 10' by about 2' wide. (Note: the plants at the right of the photo were purchased about 6 months before those at the left.) 'Green Tower' will also make a lovely specimen planting, perhaps on both sides of an entryway, as it will keep it's upright, narrow shape without pruning. It is hardy in zones 5a to 9b, does best in full sun and a limestone soil with a PH of 6 or higher.
If you are fond of variegated foliage, you may like American boxwood 'Variegata' (B. sempervirens), pictured at right. It is hardy from zone 6a to 8b and can get as tall as 6', although average would be more like 4' to 5'. A more compact variegated cultivar is 'Elegantissima', at 2' to 3' in height.
Here's an interesting one, Rosemary-leaf boxwood (B. sempervirens 'Rosmarinifolia', at far left). If you saw it, you might not recognize it as a boxwood because of the narrow leaf. Other than that, the plant has the same properties as the other boxwoods. It is 4' to 6' tall and hardy zones 6 through 8. And at near left, thinking out of the box (forgive me, I had to get that awful pun in here somewhere), is the Giant Box Tree (Buxus balearica). It is only hardy in zones 8a through 9b and will reach the astronomical height of 15'. It takes it's name from it's home in the Balearic Islands of Southwestern Spain. The Mediterranean background explains the lack of hardiness.
Boxwood are often used for garden topiary, which is a shrub trimmed into a decorative shape. Some cultivars lend themselves to particular shapes more than others. At Langley Boxwood Nursery in Hampshire, UK, Buxus sempervirens is commonly used for all types of topiary. B. 'Rotundifolia' (as seen in the opening thumbnail) is suitable for large ball shapes and standards (a ball at the top of a trunk). The natural shapes of the shrubs are used to suggest the appropriate topiary. 'Green Tower' is used for obelisks, cones and related shapes. Topiary are also sheared into spirals, double ball standards and fanciful animals. There are very formal gardens where the boxwood hedges are used for fancy knots, mazes, and other intricate designs. A site you might like to visit if you are interested in boxwood topiary and sculpture is Boxwood Gardens. Also, there is an excellent article at About.com on How to Plant Boxwood Hedges if you should be interested in planting one of your own.
Just a couple of other notes about boxwood in case they have grabbed your interest. Good news... deer will only eat them in a very harsh winter when there is no other food available. All boxwood are shallow rooted. You must take care not to cultivate too closely around them. They should be mulched with 2-3" of organic mulch to keep the roots cool and prevent them from drying out. The best defense against disease is to keep the plant pruned so that light can reach the inner branches, be sure it is planted in a well-drained area with plenty of organic matter and that it is properly fertilized (too much can be as harmful as too little). For additional information on specific cutivars, check out PlantFiles, the huge plant database here at Dave's Garden.
Good luck with your new boxwood planting! Let me know how it goes
Additional photo credits: mystic with permission from Lazy S'S Farm: Harland boxwood; mgarr: Green Mountain boxwood; plantmover: American boxwood 'Green Tower'; growin: American boxwood 'Variegata', Rosemary-leaf boxwood and Giant Box Tree.
 Buxus. (2008, April 3). In Wikipedia, The Free Encyclopedia.
 NC State University Plant Fact Sheets, Shrubs
 Langley Boxwood Nursery, Hampshire, UK