Versatile Viburnums for a Vibrant Landscape

Gardeners in the temperate zones of North America can select from hundreds of small trees and shrubs which will readily flourish in our climate. However, among all the choices there are only a handful of “must-have” species, with viburnum at the very top of that fairly short list.

There are approximately 200 species of viburnum worldwide, with more than 120 species in North America alone. Locally, in the Mid Atlantic region, our native landscape offers several absolutely dazzling specimens of viburnum that are suitable for almost every yard and situation, including Mapleleaf, Arrowwood, Possumhaw, and Blackhaw Viburnum (Viburnum acerifolium, V. dentatum, V. nudum, and V. prunifolium).

Michael Dirr at the University of Georgia writes in his landmark Manual of Woody Landscape Plants: “A garden without a viburnum is like life without music and art.” Dirr’s sentiment simply reflects the fact that it is difficult not to fall in love with viburnum. After all, we are talking about a species which is drought-tolerant and capable of handling dry soils, while it also thrives in moist, sometimes wet conditions.

The viburnum cited above will shine in either full sun of partial shade, making them ideal as either specimen plantings, or tucked in among other shrubs in a woodland garden.

Like most members of the Caprifoliaceae (honeysuckle) family, they are virtually pest-free and tolerant of pollution and harsh urban conditions. They can be trimmed and shaped to form an elegant privacy screen or hedge, or left alone to spread or “naturalize” across a wild, informal landscape.

But these noteworthy considerations are only horticultural nuts and bolts. Viburnums are easy to love because they are simply lovely.

Our native species all feature small white or creamy-white flowers appearing in flat-topped clusters or cymes roughly three or more inches wide. Actually, yellow stamens glowing against the white petals create the creamy color in both the Mapleleaf and Arrowwood shrubs. Some closely related viburnum species are even credited with pink or rosy blooms. The flowers typically appear in April through May, with each inflorescence lasting about two weeks.

But the show does not end in spring. While the flowering activity is somewhat reminiscent of our native dogwoods, the autumn foliage encore puts Cornus florida to shame. Depending on the species and growing conditions, fall leaf colors range from chartreuse and yellow, to orange, pink, red, burgundy and deepest purple. Moreover, the flowers will form drupes or berry-like fruits in attractive clusters from late summer through fall, which might last into winter, depending on avian appetites. The drupes range in color from a delicate porcelain-blue to purple and black.

Naturally, each of our featured viburnum have unique characteristics worth noting. For example, Mapleleaf Viburnum is decidedly a small multi-stemmed shrub, generally growing at a medium rate to between three and six feet high, with about the same spread or width. The species is known for its suckering ability, whereby shoots sprouting from the shrub’s base help it spread into a dense clump.

In fact, you might well consider planting a Mapleleaf for a hedge rather than turning to invasive species like privet, burning bush, or thorny barberry. Mapleleaf handles pruning easily, and can be kept in-bounds by mowing over tender suckers which might pop up in turf areas. This species can be easily propagated from cuttings taken between June and July. Buy just a couple of containerized plants and you can have a nursery-full in about a year.

Arrowwood is much like a big brother to Mapleleaf, sharing many of the former’s growth habits, although it can reach between eight and 13-feet in height, with branches arching out up between ten to 15-feet. Arrowwood also tends to grow in masses, making it another contender for a native hedgerow or screen.

Interestingly, many gardeners like to prune Arrowwood back to a single leader, encouraging it to behave more like a small specimen tree. Certainly its flowers, fruits, and autumn color establish it as a substitute for dogwoods, which are now under attack by fungal foes and insect-borers, and a fit companion for other woodland natives like Amelanchier and redbud.

Like all viburnum, Arrowwood is a phenomenal addition to any backyard wildlife habitat. However, as the shrub is “self-incompatible,” meaning that it cannot pollinate itself, you should plant at least several different specimens to ensure a dramatic and abundant display of fruit. The cedar waxwings, cardinals, and brown thrashers will thank you with a chorus of songs and whistles.

Arrowwood is also easily rooted, allowing you to propagate an entire shrub border from its ample supply of stout suckers. Indeed, the shoots sent up from the base of the shrub were so prized by Native Americans that they used them to make arrow shafts, from which this viburnum’s common name arises.

Possumhaw, also called Smooth Witherod, grows somewhat larger than Arrowwood, up to 20 feet, and can be decidedly more tree-like, featuring a rounded crown with a considerable spread. It also features long-stemmed, flat clusters of small, creamy white flowers which first start appearing in May.

The common name for this species denotes the popularity of its long, dangling clusters of fruits among opossums, although other small mammals such as squirrels, rabbits and chipmunks, as well as songbirds, make quite a feast from the half-inch long oval drupes, which start out green, quickly turning pink, and finally ripening to a dark blue or purplish-black and lasting throughout winter.

The species other common name, “withe-rod,” reflects that the tree’s smooth grayish-brown twigs are strong and flexible, and, in fact, were often twisted together to form a rope to gather together sheaves of wheat.

One of the more lovely features of Possumhaw are its waxy, dark green leaves, perhaps the glossiest of any viburnum, which turn to a rich cinnabar (cinnamon to scarlet) in autumn. ‘Winterthur’ is probably the most popular cultivar currently and commonly found, having a more compact form, growing to about six feet, with aromatic flowers in spring and brilliant red fall color.

The last selection is the largest and most versatile. Blackhaw is the most like a small tree, growing up to 21-feet or somewhat more. Although it also produces suckers and can be multi-stemmed, for the most part it is a slow-growing tree with a rounded crown and a thirst for moist and wet soils.

Blackhaw is the best choice for a stand-alone ornamental, and is a superior substitute for invasive and troublesome species such as Russian and Autumn Olive, Bradford Pear, and many of the other popular Asian pears and ornamental cherries.

Blackhaw can be also be used as a tall deciduous screen or hedge, much like hawthorns, to which this species is often compared. It takes well to transplanting, which is helpful should you need to thin out a too-dense hedgerow.

From dry upland slopes to soggy floodplains, viburnum can provide privacy, habitat, and year-round beauty, adapting to most every type of soil and exposure.

Copyright 2009, Joseph M. Keyser

Smart Solutions for Steep, Sunny Slopes

A steep, sunny slope in your landscape can be a real challenge, especially if you’re trying to manage that area as a lawn. The slope itself makes mowing difficult -- if not outright dangerous -- and it is difficult to keep the turf green, as most water simply flows downhill, leaving the hillsides brown and barren. A proper solution should really turn this landscaping liability into an attractive asset.

There are a number of suitable solutions, but most are fairly expensive, such as designing and installing retaining walls, along with extensive replanting. Moreover, if the angle of your slope exceeds 20 degrees, you may need to engage a soils engineer or landscape architect. There might even be heavy equipment in your future. Ka-ching!

A simpler fix, ideal for the do-it-yourselfer, would simply call for replacing turf grass with a low maintenance ground cover. Keep in mind that many of the most popular ground covers are actually invasive weeds, and can often create management headaches in their own right. These include Five-leaf Akebia, Japanese Barberry, Crown Vetch, Wintercreeper (Euonymus fortunei), and English Ivy, which can often bake in full sun. Moreover, the “wild” nature of such plants makes mulching difficult, and can lead to even more weeds taking hold. So much for low maintenance!

For my money – or yours – one of the best solutions is an evergreen groundcover: something with an obedient and predictable growth habit, and an attractive year-round appearance. Think juniper.

There are literally scores of procumbent juniper species, and similar conifers, which will happily bask in the hottest sun, stretching across your hillside over the course of several years, all providing excellent erosion control and weed suppression, especially when combined with diligent annual mulching, and saving you the need to combine mowing with mountaineering. And these hardy plants are almost always drought-tolerant, helping to conserve water, while requiring no fertilizing.

Among the most common and popular junipers used as groundcovers are the creeping junipers (Juniperus horizontalis), slow to medium-growing shrubs, which form dense, aromatic mats between 12 and 18 inches in height, extending trailing branches from four to eight feet across, depending on the cultivar. Most feature either soft or prickly scale-like bluish-green foliage, which often “bronzes” once hit by cold winter temperatures. But don’t worry, they will “green up” again in the spring as air and soil temperatures warm up.

Blue Rug or ‘Wiltoni’ is probably the most popular juniper sold. It hugs the ground at four to six inches, and provides coverage up to eight feet in diameter. Blue Rug is drought-tolerant and thrives in conditions ranging from coastal Maine to the mountains of Georgia.

Other popular cultivars offer a Mardi Gras parade of colors and shades, from gold to green to purple, in a range of heights. Favorites include ‘Bar Harbor,’ which is similar to Blue Rug, although more colorful in winter, the bright green ‘Emerald Spreader,’ four-to-six inch tall ‘Prince of Wales,’ gold-variegated ‘Mother Lode,’ and ‘Pancake,’ lowest of the low at two-to-three inches.

‘Blue Star’(J. squamata) is a taller, different species, featuring cool blue foliage, and a pyramidal growth habit. This selection will generally stay below two feet in height, but can reach three feet, with only a two to four foot spread. It would be especially useful under a retaining wall for terraces, or against a rock outcropping.

Not all junipers are groundcovers: some are actually full-grown trees, like our native Eastern Red Cedar, which reaches an impressive 30 feet. Others tend to be shrubs, useful as hedges, such as the Chinese Junipers (J. chinensis), which reach about six feet in height, although some can become small trees up to 20 feet. One notable exception is Sargent Juniper, a cultivar that hovers at about 18 to 24 inches with an eight to ten foot spread. Many landscapers turned off by a juniper’s tendency to turn bronze in winter prefer Sargent, as it keeps its grayish-blue color year-round.

Dwarf Japanese Garden Juniper (J. procumbens ‘Nana’), provides bluish-green foliage which turns a stunning purple-bronze in late fall and winter. Horticulturist Mitch Baker (my go-to guy) at American Plant in Bethesda, Maryland is enthusiastic about ‘Nana’s’ spray-like mounding habit, which provides an exciting departure from the flat mat-like appearance of many other species, and allowing for a level of dimension to your hillside.

Mitch also noted options beyond junipers, such as the confusingly-named Siberian Juniper (Microbiata decussata), which is also called either Russian Arborvitae or Siberian Carpet Cypress, due to the shape of its foliage. Siberian Juniper is fairly new on the market, but is quite versatile thanks to its tolerance for Siberian winters and partial (not full) shade, as well as average soil and dry conditions. This specimen’s feathery foliage stays within eight to 12 inches in height, with an above-average ten foot spread.

And “spread” is the name of the game with junipers and other ground covers. Obviously, the wider the area covered by the plant, the fewer plants you will need. Most junipers can be planted approximately six feet apart, although you should check – and do the math – before purchasing and planting. Larger container stock, about three gallons, will provide a vigorous root system, ideal for early season planting, and will get your hillside off to a fine start.

Costs can range from 15 to 35 dollars, depending on the cultivar, size, and source. Smaller containers will be less expensive, but might take years to fill in. Don’t make the mistake of buying a lot of smaller containers and planting them too close together to create a finished look. Overcrowding can lead to disease problems and might require substantial transplanting in just a few years. Remember, you want a low-maintenance landscape: plan and plant properly, according to your budget, and let time and patience do the rest.

Decide in advance if you want to address the entire hillside at once, or just tackle a portion at a time. For best results, smother the grass or weeds on the site with landscape fabric or cardboard and a three to six inch layer of shredded wood mulch. Shredded mulch will lock together in place, where nuggets or chips might be washed off. Be sure to stagger your planting holes so that plants can diffuse flow down the slope and prevent erosive gullies. Check your soil to see if it needs amending with compost – although most junipers are tolerant of relatively poor soils as long as they receive good drainage.

Copyright 2009, Joseph M. Keyser