The genus Lupinus is recognized for its striking, tall flower spikes, which bring vibrant color to both cultivated gardens and wild landscapes. This popularity, combined with their vigorous nature, often leads people to question whether lupines should be considered invasive plants. The status of any lupine species depends on the specific plant and its geographic location. The designation of a plant as invasive is a classification based on ecological behavior outside of its natural range.
What Determines If a Plant Is Invasive
A plant is labeled an invasive species only when it meets three specific criteria in a new environment. First, the plant must be non-native, introduced to the ecosystem by human activity, either intentionally or accidentally. Second, it must have the ability to spread rapidly and become established in its new habitat. Finally, its introduction must cause or be likely to cause environmental or economic harm.
A species that spreads quickly in its native region is merely aggressive, but it is not invasive because the local ecosystem has evolved mechanisms to keep it in check. Many Lupinus species possess traits that predispose them to aggressive behavior, such as producing a large number of seeds and thriving in disturbed soils. When certain species are transported outside their native habitat, they can easily overcome local competition and establish self-sustaining populations.
Specific Non-Native Lupines That Pose a Threat
The most problematic lupine species globally is the Large-leaved Lupine, Lupinus polyphyllus, and its widely cultivated hybrids, often sold commercially as Russell Lupines. This non-native species, originally from western North America, has become a serious ecological threat across parts of Europe, including Scandinavia, Iceland, and New Zealand. Its aggression stems from a unique biological advantage shared by many legumes: the ability to fix atmospheric nitrogen.
The plant harbors symbiotic bacteria in its root nodules that convert atmospheric nitrogen into a form usable by the plant, essentially fertilizing the soil around it. This mechanism gives L. polyphyllus a competitive advantage, particularly in nutrient-poor environments like heathlands and low-fertility grasslands where native plants are adapted to low-nitrogen conditions. The resulting nitrogen enrichment changes the soil chemistry, allowing the non-native lupine to form dense, single-species stands. This monoculture formation displaces native flora, drastically reducing local plant diversity and altering the invaded ecosystem.
The Ecological Importance of Native Lupines
Native lupine species are valuable components of their local environments, unlike non-native invaders. For example, the Wild Blue Lupine, Lupinus perennis, is a naturally occurring species in eastern North America and is a foundational species in its ecosystem. This native plant is the sole host for the federally endangered Karner Blue butterfly (Lycaeides melissa samuelis), which lays its eggs and on which its caterpillars feed.
The survival of this specialized insect is linked to healthy, native lupine populations. Where non-native species like L. polyphyllus have been introduced, they can outcompete the native L. perennis for resources and even hybridize with it. The resulting hybrids often do not provide the specific nutritional requirements the Karner Blue caterpillar needs, effectively turning the area into an ecological sink that cannot support the butterfly’s life cycle. Native lupines contribute to soil health and support specialized insect communities.
Responsible Planting and Control Methods
Gardeners and landowners have a clear role in managing the spread of problematic lupines by making informed planting decisions. Before introducing any lupine, check its native status for your specific region, and opt for native species like Lupinus perennis over commercial Russell hybrids. Sourcing seeds from reputable native plant nurseries helps ensure genetic integrity.
For existing, established non-native plants, control efforts should focus on preventing seed production. Control methods include:
- Deadheading, which involves removing the spent flower stalks before the seed pods mature.
- Physically removing the entire plant, including the root system, for small infestations.
- Using repeated mowing, timed to the plant’s growth cycle, to deplete energy reserves in larger infestations.
- Implementing long-term monitoring and follow-up removal of new seedlings, as lupine seeds remain viable in the soil for many years.