What Granny loves to have flowering colourfully in her front-yard, might drive conservation biologists into despair. But why should they be so concerned? Isn’t a plant a plant, a flower a flower, the more colourful, the better? Conservation biologists put plants into two different categories: native and non-native. The non-native species are then pigeonholed into harmless and invasive or, more frankly speaking, pests!

These pests are often introduced into an area as crops or ornaments. They become pest species as they are not as sessile as they might have seemed and manage to escape and establish viable populations in the wild. So what? These plants make the world more colourful, don’t they? Actually they do not! Often they outcompete the native vegetation and cause damage to the economy or ecosystem of the new habitat.

In New Zealand is the innocent sounding non-native Wandering Willie (Tradescantia fluminensis) forms dense carpets under native forest remnants hindering the natural regeneration of native trees. Wandering Willie and other invasive plant colleagues hanging around in New Zealand are summed up in the National Pest Plant Accord.

In 2008 the total economic cost for New Zealand’s regional governments for plant biosecurity and pest control operations was estimated at NZ$ 21 million. Besides that the central government spends another NZ$ 337 million dollars per year for control of invasive plants and animals. This amount does not even include the cost of protecting native biodiversity.

In how far can we put botanic gardens in the dock for this?

Philip Hulme, Professor of Plant Biosecurity at Lincoln University, examined patterns of how these floral pests came to places where they should not be. On the list of suspects are, among others, botanic gardens, which most of the human population might regard as beneficial for nature conservation. In his study “Resolving whether botanic gardens are on the road to conservation or a pathway for plant invasions” he found several lines of evidence pointing to these institutions as prime suspects encouraging pest plants. Philip browsed through databases including “Plant Search” and “the IUCN Red lists”. He checked backgrounds, programmes and agendas of over 3000 botanical gardens worldwide. Philip compared trends within different times and global areas concerning the species composition of botanic gardens and their handling with invasive species.

Philip divided the plants into three categories: ornamental, endangered, and potentially invasive. He counted the species in each category and compared them with the numbers found in botanic gardens worldwide. He found that seven of the eight most invasive plants globally, including wild ginger (Hedychium gardnerianum) and Lantana (Lantana camara), that cause damage in New Zealand are known to have escaped at least once from a botanic garden.

Craving for sensation

Philip discovered that 99 % of approximately 450 plant species rated as major invasive pests are kept in at least one botanic garden worldwide. In contrast to this, only a quarter of threatened plant species are cultivated in gardens. Nevertheless, there are 21 plant species that are extinct in the wild but cultivated in at least an average of ten botanic gardens. Over half of the 610 cultivated ornamental plant species, rated as invasive, were cultivated in the assessed institutions. Why is this the case? Philip assumes that by the time of establishment of the gardens it must have been the craving for sensation, for something extraordinary and exotic, being bored by the local flora.

Already barred? – A danger foreseen is half avoided

Philip also found that many floral pests were introduced over a hundred years ago. The purpose and philosophy of botanic gardens has changed over this period of time from presenting decorative rarities to conserving the native vegetation. Following this clue he investigated whether the proportion of native plants being established in botanic gardens has increased.

Are botanic garden managers aware of the problem and addressing it with non-escaping strategies?  A working example is the management plan of the Christchurch Botanic Gardens. However, less than 10 % of all assessed botanic gardens don’t have regulations and control mechanisms to keep their “inhabitants” inside the fence.

Philip also detected another threat. Although the number of invasive plants being cultivated in a single botanic garden decreased, the total number of botanic gardens has strongly increased from 1950, especially in South America and Asia. So the overall risk might stay the same but it is distributed from a low number of traditional institutions to a large number of new institutions.

Only one in a hundred going astray

In this context, the tenth rule comes into play: an average sized botanic garden grows 4000 living plant species, of which 40% or 1600 are non-native. Of those, typically 10% or 160 species would ecologically be able to become naturalized and of those 10% or 16 might become invasive. Peanuts? If you transfer this number to all evaluated 3000 institutions there are up to 48,000 potentially invasive plants. And that is a lot, isn’t it? And as it takes in average between 30 and 100 years for a plant to climb up the invasiveness ladder Philip expects some of the numerous ornamental plants present in botanic gardens to become invasive as well in the future.

For Philip the evidence is clear: botanic gardens definitely bear a part of the blame. Legal regulations or at least voluntary codes that prevent plant invasion should be established. Plants rated as invasive should only be cultivated commercially in regions where they naturally occur or where climatic patterns make it impossible for them to establish a viable population in the wild. Botanic gardens should shift interests and programmes towards conservation instead of sensation, and visitors should be educated about the traits of invasive plants and the importance of conserving intact local ecosystems. Perhaps participation in a programme such as this would also bring Granny to rethink what she grows and what not to grow in her front-yard.

The author Susanne Kandert is a postgraduate student in the Master of International Nature Conservation taught jointly at Lincoln University and University of Göttingen. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.