Saturday, November 27, 2021

The Vexing Problem of Invasive Ornamental Grasses

Cortaderia  selloana in Virginia 

The use of ornamental grasses has been growing rapidly since the 1970s, when Cortaderia selloana (pampas grass) and Festuca glauca (blue fescue) were the only well known ornamental grasses sold in West Coast retail stores.

It's a different situation today, with ornamental grasses gracing many residential and commercial areas, and even having their own sections in gardening stores.  

But look closely at the selection in many stores and you'll come to realize that many (if not most) of the ornamental grasses are not native to the region in which they are being sold.

Huge Cortaderia selloana on sidewalks in Virginia 

Unfortunately, many people select ornamental  grasses without any consideration about their potential for harming the environment, and  the horticultural industry itself may exacerbate  the problem by continuing to sell plants that have been proven to be invasive. This lack of awareness extends to even reputable botanical gardens and arboretum, and I have repeatedly seen some institutions make heavy use of ornamental grasses like Miscanthus sinensis. Granted, these non-native grasses are often beautiful, but this dependence on such cultivars creates problems due to the potential invasiveness of the species. 

Escaped Cortaderia selloana in Florida

It's not all black and white though. What may be invasive in one region, might be relatively benign in another clime. For example, Imperata cylindrica is one of the most invasive grass in many subtropical and tropical areas in the world, but a red  cultivar (called Japanese Blood Grass)  is relatively harmless in colder areas, even though it can allegedly revert to an all-green invasive form when grown in a more suitable climate.


Large Miscanthus sinensis in parking lot of suburban strip mall in NJ

However, a species that may at first seem non-invasive can become invasive over time. This might have been the case with the popular  Cortaderia selloana (pampas grass) from South America.

Escaped Miscanthus sinensis at forest edge in NJ

This species became very popular in the late 19th century, with society becoming enamored of the large pure white inflorescence and using it almost everywhere. At one point, half a million of the plumes were being sold yearly. But for decades, the invasive nature of C. selloana seems to have been overshadowed by a close relative. C. jubata. In fact, even in the 1970s, researchers reported most of the infestations they found were of C. jubata, and non-cultivated C. selloana did not exist far from human habitation. This seems to have changed by the 1990s, when new surveys showed that C. selloana had become the more invasive of the two, with outcrossing populations found in many different native habitats. Even the morphology of these populations had changed, with the pure white plumes becoming darker.

Arundo donax in an arboretum in NJ

There are sterile options when it comes to non-native ornamental  grasses, including hybrids of M. sinensis. These can be a way out of the trap of desiring particular grasses that have the potential to be invasive, although consumers have to do their research  before buying such species.

In the end, there are several possible solutions to the problem of invasive  ornamental  grasses:

  1. Make it illegal for the retail industry to sell invasive species, and enforce the law.

  2. If possible, sell sterile cultivars of any non-native species.

  3. Continue to inform people about the problems associated with non-native ornamentals, even those that do not seem to be currently invasive.

  4. Promote the use of native ornamental grasses, such as Panicum virgatum, Schizachyrium scoparium, and Andropogon gerardii cultivars. Emphasize not only the environmental reasons for using these, but also their beauty and adaptability.
Only through the rigorous implementation of all these will we be able to make sure we continue to protect the integrity and beauty of our native habitats.

Cortaderia selloana in North Carolina parking lots

References

Lambrinos, JG (2004). A tale of two invaders. The dynamic history of pampas grass and jubata grass in California. Cal-IPC News. Vol. 12, Nos. 3/4


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