New and Improved
Whether it’s the latest model F150, next generation iPhone or newest fusion restaurant opening in town, most people like new. Landscapers are no exception – we enjoy learning about and working with the latest plant varieties as it can set apart our work from the competitors whose list of options for perennials extend no further than Yellow Flag Iris, Lilyturf and Black-Eyed Susans. In addition, it is often our customers that are demanding the latest and greatest ornamentals showcased on HGTV and want their property to be the jewel of the neighborhood.
The Five Percent Rule
When it comes to tree cultivars however, one genus tends to become very popular for a variety of reasons (growth rate, color, pest resistance, etc) and then an influx of slightly varied cultivars flood the market. Right now, Freeman Maples are all the rage with their bright red foliage and enticing monikors: Autumn Blaze ® , Autumn Fantasy ® , Celebration ® , Firefall (™), etc. The problem is that when there is a high concentration of one species, genus or family of any plant, the entire population of that plant is more susceptible to great devastation from pests such as insects or disease. Dr. John Ball of South Dakota State University has proposed that no more than five percent of a community’s trees be in one genus.
A Pandemic Past
Unfortunately there have been several catastrophic tree losses in the United States in recent history including:
the Emerald Ash Borer, an exotic beetle, has killed tens of millions of ash trees in North America and threatens to kill the remaining population of over 8 billion
Chestnut Blight, a root rot disease, wiped out nearly every chestnut tree in the U.S. in the first half of the 20th century
Dutch Elm Disease, a fungal disease, has killed over 25,000 elms in the northeastern U.S.
- Oak Wilt, a fast acting fungal disease that is killing oak trees in eastern, central and midwestern states.
In each of the aforementioned cases, the pests originated from a foreign continent and without the existence of specialized predators and parasitoids that suppress populations of the offender, they were able to spread quickly and in devastating fashion. These massive losses disrupt the balance of ecosystems and have tremendous economic impact as well (destroying trees which were to be sold as timber, decimating city budgets as they treat or replace infected trees, etc).
The five percent rule is a meant to improve upon Dr. Frank Santamour Jr.’s “10-20-30 rule for diversity” which suggested a less comprehensive model, limiting plantings in a community to 10 percent in a species, 20 percent in a genus, and 30 percent within a family. Dr. Ball points out that in cases such as the Emerald ash borer or Dutch elm disease, not just one species of tree is affected but rather the entire genus and therefore following the 10-20-30 rule can still result in the loss of up to 20 percent of the population.
Landscape Management & Responsibility
New and dangerous diseases and insects capable of causing tree pandemics are bound to make their way stateside again and the best defense is biodiversity. The environment, though resilient, is fragile and as landscape managers, it is our job to protect it, manage natural resources and help create beautiful and useful spaces for people to enjoy. It will require a team effort in order to introduce the amount of new genera needed to abide by the five percent rule. As Dr. Ball states, “Consumers will not buy a tree they have not heard about, garden centers will not carry a tree they cannot sell, and nurseries will not grow what garden centers will not carry.” We must continually educate ourselves, our customers and our suppliers in order to protect the landscapes we work so hard to manage. Part of that education includes proper substitutions for plants described in landscape plans that may be unavailable or vetoed by customers; Dr. Ball wrote a great article on this topic as well.