A subject close to our hearts at the Horticultural Taxonomy department of the Royal Horticultural Society is the vastness of the UK cultivated flora – in fact, the latest RHS Plant Finder 2018 lists over 76,000 plants grown in the UK. Stroll through any village, town or city and it is clear that the botanical life of our urban places is dominated by cultivated plants. However, cultivated plants appear only rarely in Floras, the scientific work that catalogues the plant life of a given area. Recording introduced plants is essential if the ecosystems of our towns and cities are to be fully understood.

London street trees providing welcome shade for pedestrians on a sunny day. © Yvette Harvey.

Why Does this Matter?

There is increasing evidence that plants grown for ornament serve more than just an aesthetic function. The flexibility of fauna in adapting to available vegetation has been documented in a 30-year study of a suburban domestic garden (Owen, 2010). The four-year RHS experiment known as Plants for Bugs found that to encourage pollinating insects in gardens the best strategy is to plant a mixture of native and exotic flowering plants (Salisbury et al., 2015). There is also a greater understanding that the human environment can be managed by an informed use of cultivated plants. Examples include the value of street trees and green walls in mitigating heat island effect and the role of green spaces in reducing water runoff.

Pollinators visiting an ornamental flower bed. © Yvette Harvey.

The Concept of a Flora

Floras may range greatly in size and format but generally what they have in common is that they concentrate on the native species that make up the vegetation of an area. Some now include those species that have become established in the wild (eg Stace, 2010), but entirely cultivated plants are generally excluded. This approach is no longer adequate for areas where the importance of cultivated plants in ecosystems is greater than that of native plants.

Non-indigenous flower mix planted at Bridport’s municipal tip. © Yvette Harvey.

Cultivated Plant Communities in Cities

There is a high degree of similarity in the habitat composition of large cities, with features such as parks, residential areas, streets and squares common to all. The homogenisation of conditions between widely dispersed urban areas has led not only to uniform assemblages of alien plants, but also to a cultivated flora that is highly predictable. This is observable in a general sense and within defined habitats. Picture in your mind municipal park flower beds filled with red Salvia, blue Lobelia, marigolds and tall gingers, and green walls festooned with ferns.

The selection of cultivated plants is influenced strongly by durability, ease of propagation, availability and aesthetic considerations. Consequently, the same assemblages of plants are found repeatedly, meeting the criteria of recognisable plant communities.

Quintessential British promenade flower bed (Weymouth). © Yvette Harvey.

Addressing the Flora Issue

In a recent conference paper (Armitage & Zhang, 2017), proposals were made to facilitate the recording of cultivated plants as part of a holistic view of plant life in urban spaces and these included: developing a protocol for how to record cultivated plants; establishing an agreed set of names (generic or species if cultivar name proves too difficult to establish); clarifying the terminology for the status of plants; providing an online means of identification; and creating a virtual herbarium of cultivated plants. The RHS is already taking steps to address some of the above through its UK Garden Flora project, which will provide an online resource for cultivated plants in addition to herbarium vouchers. And we welcome specimens of named UK cultivars from you.

Cultivars from the RHS herbarium. © The Royal Horticultural Society 2018.

References

• Armitage, J. D. and Zhang, L. (2017). The case for recording cultivated plants in floras of urban areas and some recommendations regarding their treatment. Proceedings of the VI International Conference on Landscape and Urban Horticulture. Acta Horticulturae. 1189: 383-388.
• Cubey, J. (Ed.). (2018). RHS Plant Finder 2018. London: Royal Horticultural Society.
• Owen, J. (2010). Wildlife of a Garden. A Thirty Year Study. London: Royal Horticultural Society.
• Salisbury, A., Armitage, J., Bostock, H., Perry, J., Tatchell, M. and Thompson, K. (2015). Enhancing gardens as habitats for flower-visiting aerial insects (pollinators): should we plant native or exotic species?  Appl. Ecol. 52: 1156-1164.
• Stace, C. (2010). New Flora of the British Isles. Cambridge University Press.

Written by Yvette Harvey, Keeper of the Herbarium, and James Armitage, Senior Botanist, at at the Royal Horticultural Society.

Please note the 76,000 quoted in the first paragraph has been revised from 400,000.