Columnar trees

Columnar accent trees have been popular in recent years. It could be because of the strong impression that a tall columnar form makes in the flat prairie landscape. For many residents, it may reflect the longing for the statuesque, pillarlike forms of the Lombardy poplar, a tree which was so widely grown in Europe, eastern Canadian provinces and adjoining states with similar Zone 4 and up climatic zones but rarely survives in colder Zone 2 and 3 conditions.

For colder climates, the columnar Swedish aspen (Populus tremula ‘Erecta’) became the popular choice of northern gardeners as an accent tree for the past 35 years and nothing could remove it from its dominant position until the fungal bronze leaf disease (BLD) established itself as a major disease in the past 15 years. Gardeners are now asking: “What alternative columnar trees are available for northern zones?” Here is what you can grow, where you should plant it, and how to maintain several of the newer alternatives of columnar trees.

Guardian aspen. Tree breeders have been searching for disease resistance in the aspen family and the newly released Guardian aspen, from Jeffries Nurseries in Manitoba, has shown good resistance. It is a complex hybrid of the columnar Swedish aspen and shows many of the characteristics of its parent including dark green foliage, columnar growth form, male seedless character and superb hardiness. It is becoming known and available as the alternative choice for gardeners who have a wish to have a tree similar to columnar Swedish aspen. It is available in a range of sizes at prairie nurseries and garden centres and there is no substitute for Guardian and its proven disease resistance.

‘Sundancer’ poplar. A recent introduction from the former PFRA Tree Nursery in Saskatchewan, ‘Sundancer’ has been growing in popularity as its features have become known. It is more closely related to the cottonwood poplars but has a totally different crown structure. It propagates well from hardwood stem cuttings, has rapid growth, excellent leaf rust

story and photos by Wilbert ronald

‘Guardian’ aspen. ‘Sundancer’ poplar.

resistance, stem canker resistance and is not susceptible to the BLD affecting the columnar Swedish aspen. It is capable of growth up to 6 feet per year as a young established tree and trees have reached 70 feet at 18 years of age in nursery plantings. The rapid growth rate and ultimate height dictates that this tree is most suited to larger acreages, parks and farms. It is widely available as smaller bare root or smaller container grown plants and the generally lower price point makes it suited for shelter and screen plantings. For a shelterbelt, trees should be planted 6 to 10 feet apart in the row.

‘Parkland Pillar’ birch. A new columnar birch tree known as ‘Parkland Pillar’, selected by Alberta’s Parkland Nursery and Landscaping in Red Deer, has become a popular accent tree in the last 5 years. It has grown to a height of 30 feet and width of 6 feet in 8 years in prairie landscapes. It is a mid-size tree with striking golden fall colour, white bark and healthy green foliage. Since it is in the birch family, it requires adequate moisture to grow well. Use this Asiatic birch with care in suitable planting sites and mulch it well to keep the roots cool and moist. Birch trees are generally susceptible to bronze birch borer and leaf miners and require moisture in periods of drought. Good gardening practises will help you to enjoy this striking columnar tree and, while it may not be regarded as an exceptionally longlived tree, there is no reason why it cannot prove a valued accent tree.

Spire crabapple. Recently, dramatic progress has been made in the breeding of a new group of columnar flowering crab apples and gardeners now have several choices to draw from in various forms and foliage colours in the Spire series. These columnar trees trace to early breeding work by Dr. David Lane of the Summerland Research Centre who crossed ‘Thunderchild’ crabs with columnar apples. Three cultivars are named and avail-

able with a fourth to follow in the 2020 growing season. They include the oldest cultivar Purple Spire, with purple foliage and the green-silver foliaged Emerald Spire, developed from Purple Spire. Both are dense columnar trees and have moderate flowering with fruit about 1 ½ inches in diameter. The ‘Thunderchild’ parentage imparts excellent fire blight resistance and hardiness to this group of trees.

The most recent columnar Spire introduction is the Green Wall cultivar, with its dense dark green foliage, showy white spring flowers and smaller bright red fruit which is held on the branches, a great advancement for the gardener who is worried about fruit drop. The superior flowering and smaller fruit size are a step up from earlier cultivars and as the name implies, this tree will make an excellent accent or barrier in the landscape.

Next year the shiny dark purple foliaged Indigo Spire is set for introduction and it likewise has smaller fruit and excellent disease resistance and hardiness. These trees have reached a spread of 6 feet and height of 20 feet at 20 years of age. The only downside to the columnar crab apples has been browsing deer which may eat low hanging foliage and fruit. This would not be a problem in most city landscapes but could be a problem in the country.

Green Spires and Sutherland pea

shrubs. One other columnar tree that deserves mention is the columnar form of pea shrubs (Caragana hybrids). Green Spires is the newest pea shrub and trees have matured at 6 feet wide and 12 feet tall at 12 years of age. It has beautiful golden yellow flowers which are sterile, and the tree has dense green foliage. An older pea shrub, developed in Saskatchewan, is Sutherland. These are both tough, hardy trees and have a place in the prairie landscape as a unique specialty plant.

Columnar trees are generally low maintenance trees that have an important role as accent and screening plants for the prairie landscape. They may require pruning to keep a central leader if the tree leader was damaged in handling or construction. Like any tree or shrub, watering and mulching are helpful in establishment. Although the day may be past for the columnar Swedish aspen, newer alternatives offer good choices for height, flowering and foliage colour. Choose wisely to suit the tree to the specific location and you will not be disappointed. Remember the

‘Parkland Pillar’ birch.

‘Green Spire’ aspen. ‘Green Wall’ crabapple.

‘Purple Spire’ crabapple.

ultimate growing size of the tree you are buying and plan accordingly so that you have years of enjoyment in your landscape. d