Butterfly bush: in Ontario, stay in control

It’s beautiful, fragrant and butterflies love it. So do many gardeners.

“It got every single butterfly you could think of,” a friend tells me. “The clearwing moth lived in it.” Although she gave her Butterfly Bush (Buddleja davidii) to a friend when it was losing a territorial dispute to a Winterberry (Ilex verticillata), she still lights up at the memory of the diversity and number of visitors it attracted

But there are two key strikes against this alien plant from China, leading to the Great Butterfly Bush Debate. 

- It does not support future generations of butterflies because it is not a host plant, meaning that its foliage cannot be consumed by any lepidoptera (butterflies and moths) that are native to North America. 

- It’s extremely invasive in many parts of the world, each plant's hundreds of thousands of dust-like seeds borne on the wind into disturbed areas like roadsides, railroads, abandoned lots, and riparian areas where it smothers native plant communities. 

I got drawn into the Great Butterfly Bush Debate at a recent talk I gave on pollinators, organized by Tiny Township at the Perkinsfield Community Gardens.

I mentioned what I thought to be a third important concern - that the nectar of alien plants does not provide the same nutritional benefit that lepidopterans get from the native plants with which they have evolved. The alien nectar from some plants is the equivalent of junk food, I suggested (erroneously, it turned out). 

I got pushback “That’s so unfair!” said one, clearly reviewing the pleasure she was getting from the crowds at her Butterfly Bush. I had to qualify and explain I had read about the issue of the nutrient value of non-native plant nectar, but I didn't know whether this applied to Butterfly Bush specifically. And then I felt duty-bound to chase down the truth of the matter. 

Who better to turn to than native plant ambassador and lepidopteran expert Doug Tallamy, professor of Entomology and Wildlife Ecology at the University of Delaware? Is non-native nectar like a Big Mac for butterflies, I asked him.

“That’s nonsense,” he replied. “All nectars are fast food. It’s sugar water - there’s no nutritional difference that I know of.” It's a myth that originated in a newspaper article some 15 years ago, he said. 

I pointed to a reference in the recently published A Garden for the Rusty-Patched Bumblebee by Lorraine Johnson and Sheila Colla to research that compared the energy in nectar from the native Cardinal Flower (Lobelia cardinalis) to that produced by hybrid varieties of the plant; one hybrid was found to provide less than 20 per cent of the energy that came from the straight species.

Tallamy agreed, yes, nectar can differ in the amount of sugar content. But don’t confuse it with the other resource flowers provide for insects - pollen, which is used by bees to provision their larvae. Pollens do differ and bees can be very specific in their nutritional requirements, with more than a third of North America’s 4,000 native bee species being specialists that visit only a specific species, genus or family of plants with which they have evolved (which therefore are native plants).

In contrast, adult butterflies and moths aren’t in the business of passing on nutrients to the next generation - the eggs they carry were created at the larval stage. They’re looking for the energy from nectar, from whatever plant, to take them to the host plants, where they will be laying those eggs, which again are likely to be of a specific species, genus or family that has foliage that the caterpillars, when they hatch, have evolved to be able to consume. 

Hopefully there will be many of those plants. “The best source of nectar is a diversity of nectar,” Tallamy says, from plants that also play host to future generations of butterflies and moths. 

New York Ironweed (Vernonia noveboracensis), for instance, which is coming into flower now. “Not a single bush," Tallamy advises - instead, "a field of them, making it easy for pollinators to get what they need, both pollen and nectar.” Blissful images of a field of New York Ironweed floated before my eyes.

Having disposed of the Big Mac myth, Tallamy emphasized the danger Butterfly Bush represents as an invasive species. I said that I was not aware of any escapes here in Central Ontario. “I hear that all the time,” he said, adding that a 10-year horizon is probably what we’re looking at. On his property in Delaware, he’s continually pulling out seedlings that have spread from his neighbour’s bushes. 

A map of butterfly bush distribution in North America is included in an 2009 Botanical Review article by Nita Tallent and Michael Watt on The Invasive Buddleja davidii (Butterfly Bush).  It shows that the shrub has spread in the coastal states, both east and west, while the middle of the continent has been spared - but Ontario is on the map. Cold is a limiting factor.

“Alas, the evidence from around the globe informs us that this plant species is likely to be a problem in parts of Ontario where winter soil temperatures do not regularly reach -29°C,” says Cathy Kavassalis of the Canadian Coalition for Invasive Plant Regulation. It is reported as having escaped cultivation in the Haldimand-Norfolk area, she said, and with climate change the Carolinian Floristic zone along the shore of Lake Erie is at greatest risk. (It’s in the wild on the US side).

So, should we plant butterfly bush? 

No, says Matthew Shepherd of the Xerxes Society for Invertebrate Conservation. “If we want to bring back into our communities the birds, butterflies, bees, and so much else that has declined, we should be planting the highest-value species that we can: beautiful flowers for us; nectar or pollen source for bees, butterflies, and more; food for caterpillars and other plant-eating insects that are the food for songbirds… Overall, it's a species that is best not to be planted! The benefit is outweighed by the potential damage.”

No, says Tallamy. “I don’t like it because there’s so many other good alternatives,” like New Jersey Tea (Ceanothus americanus), Buttonbush (Cephalanthus occidentalis) and Summersweet (Clethra alnifolia), all mid-summer bloomers that fill the flowering gap between spring and fall. 

But then he reconsiders. “I don’t like to be dogmatic… Perhaps it’s a mistake not to have allowed any to remain," he muses, contemplating his efforts to eradicate the interlopers from his neighbour’s property. “I know I'm not home enough to religiously deadhead any on my property but it's not like I'm overloaded with nectar plants either. Maybe I'll let the next one stay for a while.”

Bottom line: in Ontario, you may enjoy your butterfly bush if you are alert and exercise control, although there are areas in North America where it should not be sold or planted. There’s a simple way to stop the spread - clip off the spent flowers. But if and when seedlings proliferate, chase them down and pull them out - and be aware that the wind may take the seeds far off your property, perhaps to productive natural environments. In any case, ensure that you have provided plenty of host plants for butterflies drawn in by the butterfly bush.

These are two good internet sources to help identify the host plants that will work for you: 

-The National Wildlife Federation’s Keystone Plants by Ecoregion. https://www.nwf.org/Garden-for-Wildlife/About/Native-Plants/keystone-plants-by-ecoregion. Based on Tallamy’s research which has found that 14 per cent of native plants support 90 per cent of lepidoptera, the lists drill down to the best plants to provide maximum benefit for your area.

-The Pollinator Partnership’s Ecoregion Planting Guides. https://www.pollinator.org/guides Enter your postal code and you’re directed to one of 31 ecotype-specific guides.

Diane Greenfield event a huge success

Tiny Township gardener Diane Greenfield held a foodbank fundraiser over a few days in August that some of my readers attended. An amazing $2,130 was raised! “I was absolutely over-the-moon impressed with the generosity of so many people who came to the walk-and-talk in my garden," Diane says.

"It was a good exchange of ideas about gardening more effectively and wisely …and we all learned to think about a sustainable environment and the very many critters who share our garden space. Plus the funds are in the process of being distributed to different organizations who work with food banks and food insecurity for many people in our communities. So it was good work all around!"