AURORA - Butterfly bush, also known as summer lilac, Buddleia or Buddleja, is a very popular garden plant in the Pacific Northwest and other temperate regions of the world. Photos of its beautiful blossoms grace the pages of slick gardening magazines and catalogs, television programs and garden center displays. Garden writers laud the butterfly bush as a fast-growing, robust, easy-to-grow shrub that attracts a wide variety of butterflies.
But there's a dark side to this popular plant. Butterfly bush can be a very aggressive, or invasive non-native shrub that, in certain situations, can overtake native vegetation, according to horticulturists with the Oregon State University Extension Service and weed biologists with Oregon's Department of Agriculture (ODA).
In Oregon, the ODA officially classified it as a class "B" noxious weed in 2004. Its strategic plan includes efforts to eradicate butterfly bush in the wild, but not from people's yards.
James Altland, nursery crops researcher at OSU's North Willamette Research and Extension Center in Aurora, and his student Julie Ream, are studying the relative invasiveness of cultivated species of butterfly bush in Oregon.
Butterfly bush is extremely invasive in natural areas. There are serious infestations on the North Fork of the Willamette River near Oakridge and along the Coquille River near the coast. It has spread to most of the counties in western Oregon and Washington. It has been a huge problem in England, where it is one of the top 20 weeds, having overtaken large tracts of disturbed land 50 years after it was introduced from China. It is a terrible problem in New Zealand as well, especially in areas prone to frequent flooding.
"We want to know if there are types of Buddleia that can be grown safely without escaping and threatening the natural environment," said Altland.
In reviewing the scientific literature about the invasiveness of butterfly bush in the United Kingdom, the OSU researchers found that seed there requires a long time to develop and release from the plant. British researchers have discovered that flower heads from a previous summer do not release seed until dry weather occurs the following spring, said Altland. Practically applied, this means that if nurseries and home gardeners prune all the spent blossoms off their butterfly bushes in the fall, it is a way of controlling the release of seed from the plant.
Altland and Ream are conducting trials on five cultivars of butterfly bush commonly produced in Oregon to see exactly when seeds are released in the Willamette Valley, as seed releases may be slightly different from England. They also conducted research on the soils, and habitat of butterfly bush-infested sites to determine and describe what factors favor these infestations in Oregon.
So far, they have found butterfly bush infestations in a wide variety of sites, from floodplains to mountain slopes, said Altland. They found the densest infestations in burned sites in reforestation areas and in frequently disturbed floodplains and riparian areas. They found few escaped seedlings from nurseries, as production nurseries often cut back plants at the end of the year to encourage branching.
Both OSU and ODA scientists are encouraging home gardeners to pay close attention to choosing butterfly bushes that are cultivated varieties, not the straight wild species Buddleia davidii. Only this species Buddleia davidii, not specially bred cultivars are subject to Oregon's noxious weed listing. It is most commonly seen growing wild along roadsides, in riparian areas and in forest openings.
Some cultivars have been found to produce much less seed than others. For example, a study at Longwood Gardens in southern Pennsylvania found large differences in the amount of viable seed produced by B. davidii varieties. For example, cultivars 'Summer Rose' and 'Orchid Beauty' produced 20 times fewer viable seeds than 'Potter's Purple' and 'Border Beauty'. The study also found that a single flower cluster of 'Potter's Purple' was found to produce more than 40,000 seeds. In the Longwood study, some Buddleia species and hybrids produced fewer viable seeds than B. davidii and likely have lower potential for escaping gardens and colonizing natural areas.
Altland would like to conduct similar studies in Oregon.
If you already have butterfly bush on your property or are planning on planting some soon, there are ways to keep it in control.
Don't let Buddleia go to seed. Deadhead, or clip off all flower heads in the fall. Do not wait until spring.
Do not leave the clippings on the ground, as they can easily take root and create a new plant. Dispose of plants by sending away in your yard debris pickup service, where it will be ground up and composted. Or burn the branches. Whatever you do, don't dump the clippings along a roadside or along a creek or river, as these are preferred habitats for escaped butterfly bush.
Watch your property for new seedlings. Dig up and get rid of any volunteer bushes. Don't give them away to friends.
Buddleia or Buddleja are both considered correct spellings of the Latin name of the genus of the butterfly bush, according to the gardeners' encyclopedia of ornamental plants called Flora: A Gardeners Encyclopedia, published by Timber Press. The genus has about 100 species in the wild. Most grown in our region are native to Asia. Others originate from South Africa. And some are from South America. Only a few species in the genus are domesticated and garden-grown in our region.
In addition to ODA's "B" noxious weed listing, Buddleja davidii appears on the "Most Invasive" species list of the Pacific Northwest Exotic Pest Plant Council and the Native Plant Societies of Oregon and Washington. The OSU Extension Service Master Gardener Program no longer recommends it for butterfly gardens because of its invasiveness.