UC Nursery and Floriculture Alliance
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UC Nursery and Floriculture Alliance

Science to the Grower: How much would you pay for a plant out of place?

by Richard Evans

My worst performance in all my years of schooling, except perhaps for the time I played Ilderim in a kindergarten production of Ben-Hur, was in Weed Science. Maybe that's why I went astray after I was asked to write an article summarizing recent weed research.

I began writing about a study at the University of Utah, where researchers compared the carbon and nitrogen isotope ratios of leaves and flowers of marijuana to see if they could distinguish between indoor- and outdoor-grown crops, and to see if they could identify the geographic location of production (West et al. 2009). It turns out that's not what I was supposed to write about at all. By the way, the Utah group did find that the isotope ratio of outdoor-grown plants differed from that of indoor-grown plants, but they were less successful at figuring out where the plants were grown. Perhaps the Emerald Triangle growers can keep their business secrets for a while, after all.

A weed is a plant out of place. That’s almost all I learned in my Weed Science class. So let’s examine some plants out of place — ornamental plants that have become invasive weeds. An article about horticulture as a vector for invasive species stated that over half of the invasive plant species in North America were introduced as ornamentals (Reichard and White 2001). As ornamental crop production increases its share of the total value of agriculture, the potential for introduction of invasive species grows. The number of cultivars in North America has increased nearly four-fold in the last 20 years, and commercial cultivars now greatly outnumber native plant species.

The authors of a review of the horticulture industry’s response to invasive species introductions noted that many of the desired characteristics in horticultural plants — large flowers, attractive seeds and fruits, long or repeat blooming seasons, low maintenance requirements and adaptability to diverse climates, stress tolerance, and novelty—are also characteristics that can promote invasiveness (Drew et al. 2010). Much of the review describes how characteristics of the horticultural crop distribution chain may contribute to invasive plant introduction. In addition, the authors discuss the relative ineffectiveness of legislation and regulatory efforts to stop or slow the spread of invasive species. They conclude that the industry needs to fund more breeding and research trials to combat invasive plant introductions, and enlist consumers in the fight by educating them and offering them alternatives to invasive species.

Prevention is the key to control of invasive plants. It is much cheaper to keep them out than it is to eradicate or control them. Legislated control of invasive plant introductions hasn’t been effective, so people have turned to the idea of industry self-regulation. To this end, a large group of horticulturists and scientists met in 2001 and developed the St. Louis Declaration and Voluntary Codes of Conduct (http://www.centerforplantconservation.org/invasives/CodesN.asp). The Codes of Conduct for Nursery Professionals focused on assessing invasive potential before introduction, identifying invasive species within the nursery’s region and promoting alternatives to invasive plants.

But how effective has self-regulation been? A group of graduate students at UC Davis conducted a survey of Northern California nurseries (growers, wholesalers and retailers) to find out if they followed the Codes of Conduct (Burt et al. 2007). Only 7% of respondents had heard of the Codes of Conduct. Most had engaged in at least two preventive measures, but few had participated in a majority of the measures. They identified lack of information, limited personnel, and the cost and labor required as the major obstacles to adopting the preventive measures.

Self-regulation might be more attractive if it were profitable. One important study found that 98% of consumers would not buy plants labeled as invasive (Reichard and White 2001). A University of Minnesota group recently explored such consumer preferences by testing whether labeling plants as invasive decreases consumer demand, and whether labeling native plants decreases demand for invasive species (Yu et al. 2010). I have to warn you that the study was conducted in April, by which time Minnesotans can’t remember what living plants look like. Nevertheless, the researchers found that consumers would pay an extra $0.35 for plants labeled native and non-invasive. Their willingness to pay for invasive species decreased by up to $1.66 when the plants were labeled as such. The authors conclude that this labeling could be a useful strategy for combatting the introduction of invasive species.

These studies offer some hope for better control of invasive plant introductions. Information and outreach are keys to success. Growers can be pointed to the Codes of Conduct, but they still need sufficient information about species invasiveness. Consumers may respond to labeling about invasiveness, but they need to know what alternative species are non-invasive. It looks like I’d better get back to studying weed science.

Richard Evans is Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.


West JB, Hurley JM, Ehleringer JR. 2009. Stable isotope ratios of marijuana. I. Carbon an nitrogen stable isotopes describe growth conditions. Journal of Forensic Science 54: 1-5.

Reichard SH, White P. 2001. Horticulture as a pathway of invasive plant introductions in the United States. BioScience 51: 103-113.

Drew J, Anderson N, Andow D. 2010. Conundrums of a complex vector for invasive species control: a detailed examination of the horticultural industry. Biological Invasions 12: 2837-2851.

Burt JW, Muir AA, Piovia-Scott J, Veblen KE, Chang AL, Grossman JD, Weiskel HW. 2007. Preventing horticultural introductions of invasive plants: potential efficacy of voluntary initiatives. Biological Invasions 9: 909-923.

Yue C, Hurley TM, Anderson N. 2011. Do native and invasive labels affect consumer willingness to pay for plants? Evidence from experimental auctions. Agricultural Economics 42: 195-205.

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