By Leslie Cameron, Extension Master Gardener
Butterfly bush (Buddleia davidii) is native to China. It was introduced as an ornamental in Europe in the 1890s and then in North and South America and other parts of the world. It has naturalized in temperate and sub-Mediterranean zones outside its native range. Butterfly bush tolerates a variety of environmental conditions, grows and matures quickly, and is a prolific seed producer. It can quickly fill nutrient-poor sites and overwhelm native vegetation. It is listed as invasive in Arlington and Alexandria, VA LINK and in other parts of the United States.
Because butterfly bush has become increasingly problematic or invasive in many parts of the world, and its sale has been limited or discouraged, interest has grown in more environmentally friendly alternatives. Selective breeding programs have produced reduced fertility cultivars identified and sold as sterile or almost sterile, or as environmentally safe, environmentally friendly, or non-invasive. In 2019, the Ticino Society of Natural Sciences in Switzerland reviewed the available research on these B. davidii cultivars to assess how environmentally safe they are.
“Reduced fertility” is usually defined as reduced or no production of viable seed, which has been seen as a safer option. As one example, the Oregon Department of Agriculture permits the sale of certain cultivars, which have been determined to produce 2% or less viable seed. However, butterfly bush is a prolific producer and disperser of seeds. The Brandywine Conservancy cited a study at Longwood Gardens in Pennsylvania that found a single flower spike produced over 40,000 seeds. The Ticino Society review found that butterfly bush can produce up to 3 million seeds per plant. A reduced fertility cultivar that produces 2% of viable seeds of the potential 3 million could therefore produce up to 60,000 viable seeds. Butterfly bush seeds are light and easily and widely wind dispersed, increasing the reach of these viable seeds.
The number of viable seeds is not the only problem. What is not included in standard definitions of “reduced fertility,” which is typically limited to viable seed production, is that stamens of these plants can still release viable pollen that can reach other plants, including in fertile wild (escaped) populations, and the plants in the wild can deliver pollen to the stamens of these cultivars. This interbreeding with nearby and wild populations can modify the wild populations and inversely can modify the reduced fertility cultivars. Gene recombination from this interbreeding could lead to restored fertility in the cultivars seen as safer and could also pass on characteristics from the “safer” cultivar to wild populations, potentially contributing to more resilient lineages able to colonize a greater variety of habitats. There is evidence of this interbreeding and reversion to fertility. Researchers found examples of wild B. davidii populations with white flowers, most likely derived from a cultivar. One hybrid (B. davidii ‘Lochinch’) thought to be sterile reproduced abundantly by seed in the field and showed invasive qualities – it may have reverted to fertility.
The Ticino Society review raised serious questions about how “environmentally safe” B. davidii cultivars are. It concluded that:
- Reduced fertility is still fertility;
- cultivated reduced-fertile or sterile cultivars can cross with invasive individuals found nearby in the wild;
- gene transfer from reduced-fertile or no-seed cultivars can modify wild individuals and, conversely,
- gene transfer from fertile wild individuals can modify reduced-fertile or no-seed cultivars; and
- sterile plants can revert to fertile plants in time (by gene transfer or spontaneously) (Marazzi, B., & De Micheli, 2019).
We know that while butterfly bush attracts adult butterflies for nectar, it is not a host plant for a single native butterfly. If adults lay eggs on the plant, the larvae will die because they can’t eat it. Because it is so prolific, butterfly bush outcompetes the native plants that native butterflies DO need for their eggs and caterpillars. Despite appearances, because adult butterflies flock to butterfly bush, the plant harms our native butterfly populations. And, butterfly bush is a host for the brown marmorated stink bug – a pest that is native to eastern Asia.
So what can gardeners do? There are beautiful native alternatives that do not pose these potential problems. While gardeners can cut off all the blooms well before butterfly bush seeds set, this means removing the blooms that make the plant attractive and requires a certain amount of vigilance in terms of timing. This does not apply to native shrubs and perennials.
For ideas on how to create a truly welcoming butterfly garden, see The Hospitable Gardener: Welcoming Butterflies to Your Garden.
Marazzi, B., & De Micheli, A. 2019. Are sterile Buddleja cultivars really sterile and “environmentally safe”? Bollettino della Società ticinese di scienze naturali, 107, pp. 55-60 (ISSN 0379-1254). Available from https://extranet.kvu.ch/files/documentdownload/200907162038_2019_Marazzi_DeMicheli_ISSN_03791254.pdf