Building green lists of ornamental plants
Ornamental horticulture is the most important pathway for plant invasion worldwide. Legislative measures are often limited to the listing of a few high-risk species banned from sale or planting. Such an approach may however give the impression that non-listed species are not considered to be a risk and are therefore safe to use.
A green list of non-native ornamental species assessed as having a low risk of escaping from cultivation could contribute to the prevention of plant invasions. The criteria identified for non-invading species to be listed in a green list include:
- sufficiently long residence time, which should be a minimum of 125 years for Britain (as it exceeds the average lag times for species introduced from the 18th century onwards);
- high propagule pressure, which can be assessed for a species through marketing time, planting frequency in a sample of gardens, volume, market frequency and plant and seed prices;
- no records of invasive behaviour elsewhere, assessed by consulting various existing invasive alien plant databases such as DAISIE;
- some robustness to climate change, assessed through the hardiness information for ornamental species in the gardening literature.
Such criteria have been applied to a random sample of 534 non-native ornamental species in Britain. It appears that 13 species represent good candidates for a British green list, these species are the following: Amsonia tabernaemontana var. salicifolia (Apocynaceae), Baptisia australis (Fabaceae), Campanula thyrsoides (Campanulaceae), Campsis grandiflora (Bignoniaceae), Clematis integrifolia (Ranunculaceae), Dictamnus albus (Rutaceae), Eryngium alpinum (Apiaceae), Francoa sonchifolia (Francoaceae), Globularia cordifolia (Plantaginaceae), Helleborus lividus (Ranunculaceae), Lobelia cardinalis (Campanulaceae), Scabiosa graminifolia (Caprifoliaceae) and Tricyrtis hirta (Asparagaceae).
Although only a small percentage of species met the criteria for a green list, the case study was based on a random sample of species offered in 19th century horticultural catalogues, many of which are not very popular today. Most species assessed then did not pass the high propagule pressure criterium as they had only been sold in small quantities in the past. Starting with species popular today, which could be proposed by the horticultural industry, would probably result in more species meeting the criteria.
This list could be particularly useful in large-scale planting and landscaping projects. The approach is seen as a possible complementary policy to existing invasive species policies.
Dehnen-Schmutz K (2011) Determining non-invasiveness in ornamental plants to build green lists. Journal of Applied Ecology 48, 1374-1380.
Contact: Katharina Dehnen-Schmutz, University of Warwick, UK, E-mail: K.Dehnen-Schmutz@warwick.ac.uk