A century of ornamental plant trade reveals consistently lower prices for naturalised species

Abstract

The trade in ornamental plants has resulted in the frequent and sustained cultivation of non-native species worldwide, often resulting in subsequent garden escapes and plant invasions. Species that are widely sold generally have a higher likelihood of naturalisation, but the market factors that drive increased sales of species remain poorly known. Simple market economics would suggest that the lower the retail price of a species, the higher the sales volume. Thus, those non-native species that can be propagated and marketed more cheaply may be more likely to become naturalised. We collated retail price data of species offered for sale in New Zealand nursery catalogues between 1861 and 1992 and compared temporal trends in the sale price of three groups of species: species with no history of naturalisation, species naturalised in New Zealand or species that naturalised and have been classed as invasive species. Because woody ornamental species are likely to be sold at a higher price than herbaceous species, our analyses also controlled for plant habit. We used robust linear mixed-effects models (RLMMs) to show that when controlling for plant habit, naturalised ornamental plant species were sold for a lower retail price in nursery catalogues than species with no history of naturalisation. The price differential across all years was on average NZ$4.72 but varied markedly between years and genera. In some genera, such as Ilex, Pinus, Acer and Lonicera, species that had naturalised were offered for substantially lower prices. However, there was no difference in retail price between naturalised species that had been classed as invasive and those that had not. Synthesis and applications. By examining the ornamental market over more than a century, we present robust evidence that non-native species that are less costly to propagate and market, and thus sold more cheaply, are more likely to naturalise. The lower prices may subsequently result in higher sales and greater propagule pressure. Given that purchasers of ornamental plants are sensitive to retail prices, differential pricing through an additional tax on high-risk species could be a tool to reduce their risk of naturalisation