I thought I was doing the right thing. When I moved into my house 11 years ago, I found a purple barberry shrub (Berberis thunbergii) planted in the backyard by the previous owner. I thought right away, it had to go. I knew Japanese barberries, so commonly planted in landscapes, were escaping (by seed dispersal) into natural woodland areas and creating dense thickets in some areas to the exclusion of native plants. These thickets have been shown to make suitable habitats for Blacklegged Ticks. I didn’t want to contribute to that situation, so I donned my work gloves and set out to remove that prickly beast of a shrub.
In the barberry’s place, I planted a “native” purple-leaf ninebark, Physocarpus opulifolius ‘Diabolo’. It had deep burgundy foliage that made a nice replacement for the burgundy-toned barberry. And, I was selecting a Maryland native plant. I thought it was a perfect choice.
I sang praises about my purple ninebark for years when people asked me for native plant recommendations. It has great spring blooms and beautiful foliage color, and I never had to prune it. And native plants support native insects so I was doing a good thing to help wildlife. I was doing the right thing!
Or so I thought.
It was just this year that I learned from my colleague, native plants specialist Dr. Sara Tangren, that this particular cultivar of the native ninebark was actually detrimental to a specific native insect, the Ninebark Leaf Beetle. My heart just sank when I heard this. I am first and foremost a plant enthusiast, but I also appreciate insects—the essential roles they play in our world as well as their often stunning beauty. And when I looked up the Ninebark Leaf Beetle, I discovered that it is indeed a beauty. And then my heart sunk even further. My purple ‘Diabolo’ ninebark, it turned out, was no good for this native beetle. The alteration in the leaf color – changing the green of the native species to the burgundy of the cultivar – makes it distasteful to the beetle.
The effects of altered leaf color on plant-feeding insects were noted in a new study published in HortTechnology magazine last month. The authors (Baisden et al.) conducted experiments on several native woody plant cultivars compared to the straight natural species. They looked at whether six altered traits in the cultivated varieties – leaf color, variegation, fall color, growth habit, disease resistance, and fruit size – had any effect on insect feeding, development, and abundance.
In all three experiments they conducted, the researchers found that the cultivars with leaves that were altered from green to red, blue, or purple deterred insect feeding. Results were not consistent for the other cultivar traits they tested.
There are a couple of hypotheses as to how leaf color affects insect feeding. Most plant pigments are compounds that do not contribute to the growth of a plant. They may instead provide a defense mechanism. Anthocyanin pigments make a red coloration that may warn insects that the plant has defensive, distasteful chemicals – and they stay away.
The question of whether cultivars of native plants – nativars – have positive or negative effects on native wildlife is an active and ongoing area of research. Results in past studies have been mixed. (For more on this topic, see Mt. Cuba Center research).
I know how appealing it is to choose plants with special characteristics – the colored foliage, bigger flowers, the more compact form, or general appearance that suits my personal taste. But my choices may not, in some cases, be to the taste of other things in our environment and the things that depend on them for food. It is a dilemma. I do like specific non-native plants (I’ll never give up my dahlias), but I also adore the lilting songs of chickadees in the springtime. Many birds like Carolina chickadees need caterpillars to feed their young and many caterpillars can only feed on wild, native plants. See New Smithsonian Study Links Declines in Suburban Backyard Birds to Presence of Nonnative Plants. I should note that a different study (Craves) found that native birds, including chickadees, were able to find insect food on non-native, invasive Amur honeysuckles. (Which makes the issue even more confusing, right?)
“When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” – John Muir
For an ecologically-minded gardener, it feels complicated to sort this all out and do the right thing. I felt disappointed that my ‘Diabolo’ ninebark was a missed opportunity to support a particular native insect, but, as Dr. Trangren explained to me, it becomes more of a problem when cultivated nativars cross-pollinate with the wild species and change the genetics of the native populations, making them less capable of supporting insects on a broader scale. For this reason, she recommends choosing cultivars that are sterile, if possible.
It was a lesson learned and one that makes me more thoughtful about my plant choices and their broader impacts.
By Christa K. Carignan, Maryland Certified Professional Horticulturist; Coordinator, University of Maryland Extension Home & Garden Information Center.
Craves, Julie A. 2017. Native birds exploit leaf-mining moth larvae using a new North American host, non-native Lonicera maackii. Écoscience, 24:3-4, 81-90.
Baisden, Emily C., Douglas W. Tallamy, Desirée L. Narango, and Eileen Boyle. 2018. Do cultivars of native plants support insect herbivores? HortTechnology 28(5) 596-606.
Narango, Desirée L., Douglas W. Tallamy, and Peter P. Marra. Desirée. 2018. Nonnative plants reduce population growth of an insectivorous bird. Proceedings of the National Academy of Sciences of the United States of America.
Learn more about native plants on the University of Maryland Extension Home & Garden Information Center website.
Thank you for raising this issue with ninebarks (and other purple-leaved cultivar species). It always seems like a good idea to trust in nature first. With sterile cultivars, there’s nothing a plant wants to do more than reproduce, so over time it will find a way. And even if it’s sterile, but has the aesthetic attributes you like, is it still part of the local ecosystem, playing it’s co-evolved role that we’re hoping to revive? How much privilege should we give ourselves in a time of mass extinction? Those complex topics and more are in my book A New Garden Ethic.
Thank you for commenting. Good questions. I would like to check out your book.
I share your concerns. For people who don’t like landscaping with locally native plants, but also don’t want to harm biodiversity, sterile cultivars would offer a way forward. By sterile I mean seed and pollen sterile. The University of Georgia recently released a seed-and-pollen sterile Pennisetum.
Thanks for this important information that will reach a broad area of the population. Dr Tangren gave a fascinating talk to a group who gathered at the Crofton Community Library a couple of years ago. The Library System is the venue & collaborator with the Anne Arundel County Master Gardener Project “Library Workshops” which my co chair & I lead. It was disappointing to me too that my beautiful purple ninebark was not an active contributor to the ecosystem. I am determined, especially now reading your article, to dig it up & plant the species. Your well written article helps us realize we all make mistakes but we keep working at making it better! Happy Thanksgiving!
Thank you for your comments. I really appreciate hearing from others who are thinking about these issues too. I continue to add more plant diversity in my small suburban yard. I have removed the worst offender plants (I finally took out the butterfly bush this year) and I have added a variety of straight native plants that are known to be beneficial for a lot of insects — Pycnanthemum, Solidago, Symphyotrichum, Monarda, etc. I have put in three native trees and would like to add a fourth. I have also been thinking about ways to support broader conservation efforts in my county. Happy Thanksgiving to you too!
First let me thank you for monitoring and replying. Legitimate questions and issues have been raised to past articles but alas ignored. As for the suggestion to only buy sterile cultivars, isn’t that an impossible task (unless horticultural testing has changed)? As the Bradford Pear scourge continues, it was marketed as a sterile cultivar as established with testing. Unfortunately the tests were only testing self pollination, whereas we learned they did pollinate with other varieties, hence the proliferation onslaught throughout woodlands. So isn’t sterility illusory?
There are university plant breeders working to develop sterile forms of barberries. Their research has involved several years of screening for sterility, but I do not know the details of the testing process. As far as barberry is concerned, I hope they get it right, but you raise a good point about the case of the ‘Bradford’ pears. For anyone who is not familiar with how the “sterile” variety of ‘Bradford’ became terribly invasive, take time to read the following article. https://www.washingtonpost.com/lifestyle/magazine/how-we-turned-the-bradford-pear-into-a-monster/2018/09/14/f29c8f68-91b6-11e8-b769-e3fff17f0689_story.html?utm_term=.5485c3e6da6a