Nasturtiums are popular garden additions, celebrated for their vibrant, often peppery flowers and distinctive foliage. These easy-to-grow annuals quickly fill spaces or cascade from containers. Their vigorous growth and rapid spread often lead to questions about whether nasturtiums might be considered invasive.
Understanding What “Invasive” Means
The term “invasive” in plant species carries a specific ecological meaning, distinct from a plant that simply grows vigorously in a garden setting. An invasive plant is a non-native species introduced to an ecosystem where it causes significant environmental, economic, or human health harm. This includes outcompeting native plants, disrupting natural ecosystems, or altering habitats.
Invasive plants often exhibit rapid growth, prolific seed production, and the ability to thrive in diverse conditions. Many also lack natural predators or diseases in their new environment, which would otherwise control their populations. It is important to differentiate these ecologically harmful species from plants that are merely aggressive self-seeders within a cultivated garden.
Nasturtium Growth Characteristics
Nasturtiums (Tropaeolum majus) exhibit several growth habits that contribute to their reputation for spreading quickly within a garden. They are prolific self-seeders, producing large, easily dispersed seeds that readily germinate. Seeds can drop directly or be moved by water, leading to volunteer seedlings.
Plants have a rapid growth rate, with trailing varieties extending several feet, and bush types forming dense mounds. Some nasturtium varieties, particularly trailing types, can also root from their stems where they come into contact with the soil. These characteristics allow nasturtiums to quickly fill garden beds, often outcompeting less vigorous plants.
Are Nasturtiums Truly Invasive?
While nasturtiums are vigorous self-seeders and can be aggressive in a garden setting, they are generally not considered ecologically invasive in most regions where cultivated as annuals. In temperate climates, frost usually eliminates them, preventing widespread ecological disruption. They rarely escape cultivation to establish large, self-sustaining populations in natural ecosystems.
However, Tropaeolum majus is recognized as an invasive or problematic species in specific mild-winter regions where it can perennialize and escape gardens. For example, it is listed as invasive in coastal California, where it can spread rapidly and smother native flora, particularly in riparian areas.
In parts of Hawaii (e.g., Hawaii Volcanoes National Park, Kauai, Molokai, Maui), Tropaeolum majus has naturalized in disturbed mesic areas and is actively managed. It has also been identified as problematic in Australia and Malta, forming dense infestations that can displace other vegetation. It is worth distinguishing Tropaeolum majus from Tropaeolum ciliatum, a different nasturtium species known to be highly invasive in some regions.
Managing Nasturtium Growth
Gardeners who wish to enjoy nasturtiums without them becoming overwhelming have several effective management strategies. Deadheading, which involves removing spent flowers before they can form and drop seeds, is a straightforward method. This practice reduces the plant’s ability to self-seed.
Regularly removing volunteer seedlings is also an effective control measure. Since nasturtium seeds are large and germinate readily, these young plants are usually easy to identify and pull. To fully contain nasturtiums, planting them in containers, hanging baskets, or raised beds can restrict their spread and prevent them from rooting into the surrounding garden soil. Choosing less vigorous or dwarf varieties can also help manage their growth habit within defined spaces.
Benefits of Growing Nasturtiums
Despite their vigorous growth, nasturtiums offer numerous advantages to the home gardener. All parts of the plant are edible, including the leaves, flowers, and immature seed pods, providing a unique peppery flavor that can enhance salads and other dishes. The flowers add a vibrant, colorful garnish, while young leaves can be used in pestos or as a spicy addition to greens.
Nasturtiums are also valued as companion plants in vegetable gardens. They can act as a “trap crop,” luring pests like aphids away from vegetables such as tomatoes, beans, and brassicas. Beyond pest deterrence, nasturtiums attract beneficial insects and pollinators (e.g., bees, butterflies, and hoverflies), contributing to a healthier garden ecosystem. Their sprawling growth can also serve as a living mulch, helping to suppress weeds and retain soil moisture.