Nasturtiums (Tropaeolum majus) are popular annual garden plants valued for their vibrant, peppery flowers and rounded foliage. Native to South America, they are frequently planted for their aesthetic appeal and culinary uses. However, their vigorous growth often raises concerns about whether they pose an environmental threat. Understanding the difference between a plant that spreads freely in a garden and one that causes widespread ecological damage is key to managing them.
The Difference Between Invasive and Aggressive Self-Seeder
A truly invasive species is defined by two main criteria: it must be non-native to the local ecosystem and its introduction must cause or be likely to cause economic or environmental harm. This harm typically involves outcompeting native plants, significantly altering habitats, or disrupting the natural food web. The plant’s spread is difficult to control and often requires regulatory intervention.
Nasturtium is not classified as an invasive species in most temperate regions where it is grown as a frost-sensitive annual. Instead, it is commonly labeled an aggressive self-seeder or a volunteer. This distinction is important because while an aggressive plant can be a nuisance, it generally lacks the ability to escape into natural ecosystems and inflict widespread ecological damage.
However, in specific, mild-winter regions where Tropaeolum majus can survive year-round, its status changes. Nasturtium is formally listed as invasive in areas like coastal California, Hawaii, and Malta. Here, it forms large populations that smother and displace native flora, causing genuine ecological harm by shading and competing with local species. For most gardeners, the concern remains one of garden maintenance, not ecological disaster.
Mechanisms of Nasturtium Spread and Vigorous Growth
The reputation nasturtiums have for being overly enthusiastic growers stems from several specific biological traits. One of the primary mechanisms of spread is their large, robust seeds, which are produced in three-segmented fruits after the flowers fade. These seeds are heavy and drop directly to the soil beneath the parent plant, where they often remain viable through the winter.
The plant’s preference for poor soil conditions also contributes to its vigor in a garden setting. Nasturtiums flourish in low-fertility soil, which naturally encourages flower production over excessive leafy growth. Conversely, planting them in overly rich or heavily fertilized soil causes them to produce abundant foliage and stems, further contributing to their sprawling habit.
Nasturtium varieties, especially the trailing types, exhibit a rapid vining growth habit, with stems capable of reaching up to ten feet in length. These stems use their leaf stalks to twist around other plants, allowing them to climb and quickly cover significant ground. This combination of prolific self-seeding and rapid stem elongation allows the plant to quickly dominate a garden patch if left unchecked.
Strategies for Managing Nasturtium in the Garden
The most effective strategy for managing nasturtium’s aggressive self-seeding is deadheading. Removing the spent flowers before they form and drop their hard seed pods significantly reduces the number of volunteer seedlings the following year. Consistent removal of these developing seed pods is the most important action a gardener can take to control future spread.
Gardeners can also limit the plant’s overall vigor by being mindful of planting location and soil fertility. Planting nasturtiums in poor, well-draining soil prevents the lush, overly leafy growth that can overwhelm neighboring plants. Providing full sun exposure also directs the plant’s energy toward producing flowers rather than focusing solely on foliage expansion.
To contain the plant entirely, nasturtiums perform well when planted in containers or raised beds, which prevents the sprawling vines from taking root elsewhere. Lightly applying mulch around the plants can also help suppress any unwanted volunteer seedlings that manage to germinate in the surrounding soil. Finally, any volunteer seedlings that appear in the spring are generally shallow-rooted and can be easily removed by hand weeding.