Tuesday, November 20, 2012

Boxwoods For Ontario Landscapes

 
Buxus or Boxwoods are a genus of evergreen shrubs and trees. They are known worldwide for their usually compact size and small, leathery leaves. Steeped in a rich tradition and heritage these versatile shrubs have graced the homes of royalty and peasants alike.

According to the American Boxwood Society there are about 90 species of boxwood world-wide and over 365 cultivars (148 of which are currently available commercially). Most of these species are tropical or sub-tropical but there are a few species and several notable varieties and cultivars that are suitable for our Ontario climate.


Choosing a Boxwood Variety That is Right For Your Landscaping Needs


With so many cultivars to choose from how does one determine what would be a good boxwood choice for their landscape? To begin with it helps to know a little bit about the growth habit of various cultivars and their growing preferences. North American selections of boxwoods can range in height anywhere's from 10' (for American boxwood) to 2' (for some of the dwarf cultivars like Boxus microphylla 'Koreana' and Buxus microphylla 'Winter Gem'). Some cultivars are more cold torerant than others, some bronze in the winter and are thus not as good of chooses for more northerly gardens especially in exposed sites. Following are a few notable varieties and cultivars, but there are many others available.

English Boxwood: (Buxus semperviren ‘Suffruticosa’)
Often referred to as common boxwood this species is native to Europe, North Africa and Western Asia. Most notable characteristics of this dwarf cultivar include a very dense growth habit and slow growth rate (averaging only 1” of growth per year). English boxwoods have a tendency to bronze in the winter and mature shrubs are susceptible to boxwood decline, especially when grown in full sun. Performs best in part sun in well drained sites.
Height: 60-90cm (2'-3')
Width: 90-120cm (3'-4')
Zone: 5-8
Landscape uses: This boxwood makes an excellent edging plant, (perhaps outlining a formal garden or along unsalted pathways and driveways). Also a good choice for knot gardens, topiary and bosai.

American Boxwood: (Buxus semperviren ‘Arbvorescens’)
This is a faster growing cultivar with a looser growth habit. Average height 6’-10’ (but some can reach up to 20'). Growth habit is upright; conical to irregular form with evergreen leaves that are more pointed at the ends rather than rounded. They will tolerate sun or shade and a variety of soil conditions.
Height: 180-300cm (6'-10')
Width: 90-120cm (3'-4')
Zone: 5-8
Landscape uses: Use for an informal large evergreen shrub or shear into a cone, pyramid or rounded globe shape. They can be used for hedges (in areas sheltered from cold winter winds) and topiary.

Japanese Boxwood: (Buxus microphylla var. japonica 'Green Beauty")
Japanese boxwoods are typically more heat and drought tolerant than their English counterparts, making them better suited to areas with hot summers. They will retain their dark green foliage even in the hottest summers but tend to bronze in cold weather. 'Green Beauty' has a moderate growth habit with a naturally rounded shape and smaller leaves that give it a more delicate texture. Also known as Boxus harlandii
Height: 120-180cm (4'-6')
Width: 120-180cm (4'-6')
Zone: 5-9
Landscape uses: Excellent for specimen plantings, small hedges and topiary.

Boxus microphylla var. koreana 'Winter Green'
(renamed to Buxus sinica var. insularis 'Wintergreen', but still sold under both names)
Owing to its Japanese parentage this selection features smaller leaves. Growth habit is mounded, slightly wider than taller, with usually a slightly loose and open habit. Slow annual growth (to 2" in height per year).
Height: 60-120cm (2'-4')
Width: 60-120cm (2'-4)
Zone: 4-9
Landscape uses: Small hedges or for edging formal beds or unsalted pathways, specimen plant in a mixed shrub or perennial bed, bonsai and containers.

Popular Hybrids: The green series introduced in Ontario by Sheridan Nurseries are a cross between Buxus microphylla var. koreana and Buxus sempervirens. These boxwoods are more cold hardy with less winter bronzing and tolerate full sun better than many varieties. Their growth habit is compact and fairly dense and tend to require little pruning, although they can be easily sheared to desired shapes. 'Green Velvet', 'Green Mountain', 'Green Gem' and 'Green Mound' are four 4 popular cultivars from this collection and well suited for Ontario landscapes.

Boxus 'Green Velvet':
'Green Velvet' is full bodied, semi-spherical (slightly wider than taller with a flatter top) and lends itself well to pruning and shaping into globe shapes. Green Velvet is a particularly hardy variety and holds its colour well into winter.
Height: 100cm (3')
Width: 120cm (4')
Zone: 4-9
Landscape uses: Makes and excellent low hedge, edging plant or specimen plant in mixed shrub or perennial bed. Also does well in containers. Mixes well in both formal and informal landscapes.

Boxus 'Green Mountain':
Has an upright, pyramid shaped growth habit that holds its shape well with very little pruning. 'Green Mountain' is a faster growing selection that holds its colour well over the winter months making it an excellent choice for hedging. This Hybrid does well in full sun when given cool moist soil conditions, but it will also do well in part-shade.
Height: 150cm (5’)
Width: 100cm (3’)
Zone: 4b-9
Landscape uses: Perfect for cone shaped specimen plantings, as mid-sized hedging, topiary, spirals and containers.

Boxus "Green Gem®':
A slow growing, dwarf cultivar with small narrow leaves. It is naturally globe-shaped with a dense branching habit that requires very little pruning. 'Green Gem' has excellent winter hardiness and holds its colour well over the winter months (although it can bronze a bit in exposed sites). This hybrid performs best when planted in a more sheltered location which protects it from strong winds and full sun. 'Green Gem' tolerates some drought.
Height: 50 -75cm (1½' - 2 ½')
Width: 50 -75cm (1½' - 2 ½')
Zone: 4b-9
Landscape uses: An excellent specimen planting for the front of a mixed shrub or perennial bed, bonsai, containers, topiary balls and edging.

Boxus 'Green Mound':
Features medium sized leaves which are oval and grow outward to a point with tight spacing along narrowly branched stems. Green foliage holds it colour well throughout the year, protection from winter winds will help to protect against winter bronzing.
Height: 100cm (3')
Width: 100cm (3')
zone: 4b-9
Landscape uses: suitable for low hedging as well as specimen planting.


How To Care For Your Boxwood Shrubs


Light requirements: Boxwood will grow in full sun but prefer part shade. (Some cultivars tolerate more sun than others. i.e. the green series hybrids prefer more sun.)

Soil pH: Boxwood prefer a soil pH between 6.5 and 7

Moisture: Moist well drained soil is preferred. Boxwood hate wet feet and are highly susceptible to root rot when grown in standing water.

Propagation: Stem cuttings are the most common technique but layering is also an option. When selecting stems to cut choose smaller 1-year old branches, they are said to have a higher concentration of growth hormones. Non hybrids can be grown from seed, to do this scarify the seeds and plant them at a depth of about 1”. Germination will take anywhere from 30 to 190 days.

Mulching: Boxwoods are shallow rooters and benefit from a shallow layer of mulch to protect the roots. Deep mulching should be avoided as it may encourage roots to grow above the soil surface putting them at high risk of damage when dried out.

Salt spray: Most boxwood are very sensitive to the salt spray from roadways, plant them far enough away from these areas to avoid injury.

Poisonous leaves: The leaves of boxwood contain steroidal alkaloids making them mildly toxic to both humans and animals.

Pruning: Boxwood can take repeated close shearing making them a popular choice for topiary, bonsai, and formally clipped hedges and shrubs. Species and cultivars (such as Buxus sempervirens) with a dense growth habit benefit from thinning to open up their centers to better air and light circulation. This encourages growth of leaves on the interior and may help to reduce disease.



Potential Problems


Boxwood blight: a fungal disease causing rapid and severe defoliation. Caused by the pathogen, Cylindrocladium buxicola. Symptoms first appear as leaf spots that are light to dark brown, with or without a dark border, as well as stem lesions or cankers. After infection has taken place white spore masses develop on the undersides of leaves. Wounding is not required

Volutella blight: a fungal disease caused by the fungus volutella buxi. Symptoms include salmon coloured spots on lower leaf surfaces and some stems, followed by yellowing and then leaf and stem death. Black steaks are sometime visible on petioles and stems. Unlike boxwood blight wounding is required for this infection to take place. Volutella buxi requires 18 hours of wet conditions in order to germinate.

Spider mites: leaves of injured plants appear stippled.

Phytophthora root rot: Caused by the fungi, Phytophthora. Plant boxes in well drained soil to help prevent this disease. Buxus sempervirens ‘Suffruticosa’ and Buxus sempervirens ‘Arborescens’ are most susceptible.

Winter Bronzing: leaves turn a bronze colour as the result of exposure to cold, dry winter winds. Plant boxes in a sheltered spot or place physical barriers (about 18” away from the plant) on the windward side to protect them.

Nematodes: these microscopic, worm-like organisms feed on the roots. Plants appear to be in decline and appear smaller than normal.

Leaf Miner: damage appears as blotch mines between the upper and lower leaf surfaces. American boxwood tends to be the most susceptible.

Boxwood decline: older English boxwoods, particularly those grown in full sun and drier soils, show a slow progressive decline.

Psyllid: Feeding of this insect causes the outer leaves to curl and form a cup which encloses the nymphs.


In spite of these potential problems boxwood remain a popular and easy to grow shrub. Whether you are looking for an evergreen to naturalize, a small tidy tree or a highly ornate shrub, boxwood has a species or a cultivar to suit almost any landscaping need.


Resources:
 
Books
  • Diseases and Pests of Ornamental Plants (fifth edition); Pascal P. Pirone; The New York Botanical Garden 1978
  • Lois Hole’s Favorite trees & Shrubs; by Lois Hole with Jill Fallis; 1997 Lois Hole and Lone Pine Publishing
  • Ever Green Shrubs; Alexandria, Va.; Time Life Books 1989

Articles
  • Growing Boxwoods in the Landscape; Erv Evans, extension Associate; Richard E. Bir, Extension Specialist; Stephen Bambara, Extension Entomology Specialist; Department of horticulture Science, North Carolina Cooperative Extension Service (revised 2/99)
  • IPM Series: Boxwood; University of Maryland Extension; Mary Kay Malinoski, David L. Clement, Raymond V. Bosmans; Regional Specialists, Home and garden Information center; Rev.5/2009; www.hgic.umd.edu
  • Major Diseases of Boxwood; Mary Ann Hansen, Extension Plant Pathologist, department of plant pathology, Physiology and Weed Science, Virginia Cooperative Extension, Virginia tech, and Virginia State University, May 2009
  • Volutella blight of boxwood; Fang (Amy) Shi and Tom Hsiang; School of Environmental Sciences, University of Guelph
Websites

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