Plant lovers are often asked to name their favorite plant.  A common response is, “The next one I discover!”  While new discoveries keep life exciting, the unknown can also bring threats of danger.  A large number of invasive plant species are the result of horticultural introductions.  These include bush honeysuckle (Lonicera maackii and L. tatarica); Callery pears (Pyrus calleryana); autumn olive (Elaeagnus umbellata); Japanese knotweed (Fallopia japonica) and numerous others.  Their commonality is that with the best of intentions they were all intentionally introduced as ornamentals.  The challenge of green industry professionals is to recognize common traits that increase the potential to be invasive and apply these to the selection process.

Autumn Olive (Elaeagnus umbellata), Missouri Botanical Garden

Even in horticultural circles, Idesia polycarpa (Iigiri tree, Chinese wonder tree) is relatively unknown.  From its description it sounds like a dream plant.  It has single or multiple trunks with smooth, light-colored bark that is a traffic stopper.  Its fragrant flowers ripen forming copious quantities of small, showy, persistent, bright red berries on 8-inch panicles.  It grows fast and thrives on tough sites.  Idesia has no known significant insect or disease problems.  Ready to rush out and buy several?  First let’s look at lessons learned from some invasive species that initially seemed to good to be true.

Genetics and place of origin are two significant indicators for potential to be invasive. Idesia, formerly in the now defunct family Flacourtiaceae has now been placed in Salicaceae (willow/poplar family).  Willows and poplars are known to regenerate and spread rapidly over large areas by root suckers.  This is also true for Idesia.  Idesia is from eastern Asia (e.g. China, Japan, Korea and Taiwan).  This region has a similar climate to eastern North America and has been the source of many of our invasive species.  It is likely kept under control in Asia by insects and diseases not found in North America.

Fruit and seed are other factors important in the rate of reproduction and spread.  Species like Idesia that flower at a young age have a reproductive advantage over species that flower later.  Species that produce large numbers of seeds per fruit and seeds which germinate easily also have an environmental advantage.  Idesia seed germinate at rates close to 100% without the need for special treatment (chilling, etc.), yet another advantage.  Plants with small fruit are often viewed as being advantageous in the garden because they are not messy.  The small berry-like fruit of Idesia are in the size range and color (red) that make it highly attractive to birds, a major agent in the spread of plants.

Still excited about introducing this beautiful, tough durable plant into your landscape?  The ability to learn from our mistakes and apply this knowledge to future decisions is an important human characteristic.  By comparing the characteristics of known invasive species we can often anticipate the potential for new species to become invasive.

Definitions:

Native – a species that is recognized to have been growing and reproducing naturally in an area for a long time. For North America this is often recognized as pre-European settlement.  Provenance refers to where a specific individual evolved.  Acer rubrum (red maple) is native to eastern North America. However the provenance of the southern forms do not allow it to thrive at more northern latitudes.

Nonnative – (a.k.a. alien species) a species that has not been growing in an area for an extended period of time.  Certain species (i.e. camel, horse, ginkgo, dawn redwood, tree of heaven, etc.) evolved in North America but became extinct here after spreading naturally to other regions.

Naturalized – a nonnative species that is growing and reproducing in an area without human assistance.  Naturalized species may or may not be invasive.

Invasive – a native or nonnative species that has become dominant in an area at the expense of survival of other species.

Exotic – a horticultural selection cultivated for the aesthetic and functional traits it offers to the landscape.  Exotic species may or may not be able to survive without human assistance.

Submitted by William M. Fountain, PhD, Extension Professor in Arboriculture, University of Kentucky