Nasturtiums are popular, edible flowering plants known for their vibrant colors and round, shield-like leaves. The most common types are technically annuals, but they frequently reappear in the same spot, behaving much like perennials. This apparent return is not due to the original plant surviving the winter, but rather a natural process of self-seeding. Gardeners in most climates must rely on this mechanism for a fresh batch of plants each spring.
Nasturtiums: The Botanical Classification
The widely cultivated nasturtium, Tropaeolum majus, is classified as a tender annual in most temperate regions. The plant is native to high-altitude areas of South and Central America, such as the Andes mountains. This origin means it is accustomed to warm seasons and is highly susceptible to frost.
The plant’s natural life cycle is completed within one growing season, typically succumbing to the first hard frost. Outside of USDA hardiness zones 9 through 11, Tropaeolum majus is grown as an annual. In these milder, subtropical zones, the plant can survive the winter and be considered a short-lived perennial, regrowing from its roots. Other species within the Tropaeolum genus, such as T. polyphyllum, are genuinely perennial and possess tuberous roots that survive colder temperatures underground.
The Phenomenon of Self-Seeding
Nasturtiums seem to return due to their efficient method of self-seeding, which mimics the persistence of a perennial. As the plant finishes its flowering cycle, it produces distinctive three-segmented fruits, each containing a single large seed. These seeds are relatively large and heavy, causing them to drop directly onto the soil below the parent plant.
The seeds possess a hard outer coat that protects the embryo and allows them to survive winter dormancy. This process, known as scarification, is naturally aided by the elements and soil microbes over the colder months. When the soil warms sufficiently in the spring, typically between 55° and 65°F, the seeds naturally germinate and a new generation of plants emerges.
Practical Steps to Encourage Their Return
To reliably encourage self-seeding, a gardener must take specific steps in the late season. The most direct action is to stop deadheading the flowers toward the end of summer. Allowing the spent blooms to remain on the plant ensures the formation and maturation of the seed pods.
Once the plant has been killed by frost, leaving the garden bed undisturbed over winter is beneficial. Avoid aggressive raking or turning the soil, as this can bury the dropped seeds too deeply for successful spring germination. The seeds need to remain near the soil surface to receive the warmth and light needed to sprout.