The global horticultural trade in ornamental plants is well known to be a primary source of non-native invasive plant introductions worldwide. In the United States, non-native species make up as much as 80% of the ornamental nursery stock and account for most nursery revenue. Although only a small percentage of these species escape cultivation and become invasive, the large number and diversity cultivated, especially in California, results in ornamental plants contributing to more than half of the invasive weeds currently damaging California's wildlands and remnant natural areas in urban landscapes.
The majority of ornamental plants are purchased and grown by the public. Thus, educating consumers to make informed choices towards purchasing non-invasive ornamentals has been an effective mechanism championed by local groups and organizations, such as PlantRight (https://plantright.org/) and Cal-IPC (https://www.cal-ipc.org/), to reduce ornamental plant invasions across the state. Both organizations also work with California's ornamental nursery industry to remove potentially invasive plants from sale and promote non-invasive alternatives (https://plantright.org/about-invasive-plants/plant-list/).
Expansion of residential and urban areas into wildlands has substantially increased the risk for ornamental plant invasions of natural areas. However, perhaps even more concerning today are the impacts of urbanization and ornamental vegetation on wildfires in the Wildland-Urban Interface (WUI), i.e. the zone of transition between wildland and human development. Each year, wildfires consume hundreds of homes and cause too many wildfire-related deaths in the WUI in California. Fire is, and always has been, an integral component of California's natural ecosystems due to the Mediterranean climate. However, in recent years, the frequency and intensity of fire events have increased dramatically such that the 2018 wildfire season was the deadliest and most destructive wildfire season ever recorded in the state. Increases in human-caused fire ignitions, due to the rise in numbers of people living near wildlands, combined with climatic changes characterized by very high summer temperatures, increased drought periods, and strong winds, have markedly increased the risk of devastating wildfires in WUI areas.
Ornamental vegetation used in landscaping can increase the vulnerability of homes to wildfire as this vegetation can act as “ladder fuel” and ignite houses and other structures. Thus, in addition to providing consumers with lists of invasive ornamental plants and suggested non-invasive alternatives to protect California's environment (e.g., https://plantright.org/about-invasive-plants/plant-list/), plant lists of flammable ornamental plants and suggested low-flammability alternatives could reduce the risks of destructive wildfires in WUI areas. Obviously most, if not all, plants will burn in a high intensity fire but it is possible to suggest ornamentals that have lower flammability. For instance, FIRESafe Marin (https://www.firesafemarin.org/) has developed a list of common native and non-native fire-safe plant species suitable to the climate of Marin County (https://www.firesafemarin.org/plants/fire-resistant/table), based on reviews by fire, forestry, and botany experts. Scientific studies of plant flammability in terms of quantitative measurements of ignitability, heat release, and fire spread rate of ornamental, invasive, and native species are also being conducted in California and other regions of the world. Results of the plant flammability studies, paired with knowledge of the relative invasiveness of ornamental plant species, will provide useful guidelines to homeowners for reducing fire risk and preventing the spread of invasive plants in the wildland-urban interface.