The Yin and Yang of Cultivars

We horticulturists like plants that are different, but we like these different plants to all look the same. Nursery people are always on the lookout for mutations and chance seedlings in their fields that display unusual characteristics like variegation, dwarf stature, large flower size, pest resistance or outstanding fall color. In some cases, these oddities are vegetatively propagated (cloned), given a "cultivar" name and introduced to the nursery trade and the gardening public. While a cultivar of a woody ornamental plant is easily recognized as belonging to its species, the characteristics for which it was selected are not typical of the species and therefore will usually not be maintained if the plant is propagated by seed. The cultivar must be propagated by some vegetative method such as budding, grafting, rooting of cuttings or micro-propagation (tissue culture).

The obvious advantage of vegetatively propagating our cultivars is that all individuals in the clone are very similar in appearance and each plant displays the characteristics for which the clone was selected (yang). However, the not-so-obvious disadvantage is the risk associated with this extreme form of monoculture. The genetic diversity found within a seedling population generally provides some measure of safety in that some plants in the population will probably be resistant to any given disease, insect or stress problem (yin). Two notable exceptions to this rule are American elm and American chestnut. Since there is zero genetic diversity within a clone, all members of that clone will be equally susceptible to any given pest or stress problem. If a new or more virulent pest comes along, it may have devastating effects on a popular cultivar that has been widely planted in urban landscapes. And, given our global economy, it is increasingly common to find exotic pests such as Japanese Beetle, Asian Longhorn Beetle and Emerald Ash Borer being introduced into the U.S.

Perhaps the most often cited example of a widely planted clone demonstrating the dangers associated with monoculture is ‘Bradford’ pear (Pyrus calleryana ‘Bradford’). Soon after it was introduced, ‘Bradford’ became extremely popular. The reasons for its popularity are obvious. ‘Bradford’ is a medium sized tree that has a uniform, oval shape, glossy leaves, spectacular bloom and sometimes a brilliant fall color. It also transplants fairly easily and is reasonably tolerant of clay soil, drought and pollution. Many municipalities planted long rows of Bradfords, lining streets and boulevards. Unfortunately, it soon became apparent that this cultivar has some problems that limit its life span in most landscape situations. The primary problem is that this cultivar tends to produce many closely spaced branches which form narrow angles with the trunk. After about 15 years, most Bradfords lose the upright oval shape for which they are known, becoming more wide-spreading and open. At this point they become highly susceptible to damage from wind, ice or vandalism. It is a common sight after a severe ice storm to see numerous Bradfords split in half with half of each tree lying on the ground. There have also been many reports of trees snapping off near the soil surface, due either to delayed graft incompatibility or to the presence of stem girdling roots. Another problem in Missouri is damage from fall and spring freeze events.

Until recently, ‘Bradford’ was considered by most people to have few pest problems. However, over the past several years, there has been a substantial incidence of fire blight on this and some of the newer cultivars of Pyrus calleryana planted in Midwest landscapes. Interestingly, some of the cultivars such as ‘Cleveland Select,’ ‘Aristocrat’ and ‘Redspire,’ which are not as prone to storm damage, appear to be more susceptible to fire blight than ‘Bradford.’ Hopefully, the increased incidence of fire blight on ornamental pears in the Midwest over the past few years is primarily due to favorable weather conditions. However, another unsettling possibility is that a new, more virulent strain of the disease is attacking Pyrus calleryana cultivars.

There are other cultivars of trees that are considered by some to be "over-planted." Examples might include ‘Red Sunset’ maple, ‘Autumn Purple’ ash and ‘Greenspire’ linden. In each case, there are good reasons why the cultivar has gained such popularity. They are all uniform, easy to grow and have some outstanding ornamental features (yang). The intent of this article is not to discourage the planting of cultivars, but rather to stimulate thought about the risks associated with their indiscriminate use. There is a wide palette of plants from which to choose for landscape planting, including hundreds of under-utilized cultivars of both "exotic" and native species as well as seed-grown natives (yin). Common sense suggests that it would be wise to diversify our landscapes by using a much wider range of species and cultivars than we currently employ. In the long run, creative energy invested in landscape diversification will have been well invested.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630