Nasturtiums are valued for their vibrant, edible flowers and distinctive foliage. These annual plants are often grown for their peppery flavor and ability to attract beneficial insects. Gardeners frequently wonder if they need to replant them every year or if they will return on their own. The short answer is yes, nasturtiums are prolific self-seeders and will reliably come back in subsequent seasons.
The Natural Process of Nasturtium Self-Seeding
Nasturtiums ensure their return through self-seeding. Once a flower is spent, it develops a three-segmented fruit, each containing a single, large seed. The seeds are large, resembling small, dried peas, and they ripen quickly after the bloom fades.
Mature seeds drop directly from the parent plant onto the soil surface. They are protected by a thick, durable outer coat, known as the testa, which is designed to withstand environmental stress. This hard casing prevents immediate germination, a mechanism called dormancy.
The seeds typically require a period of cold, damp weathering over winter, similar to natural stratification, to weaken the testa. This physical weathering allows water to penetrate the seed and break dormancy. This process ensures the seeds do not sprout prematurely in autumn, leading to “volunteer” plants the following spring once the soil warms sufficiently.
Conditions Necessary for Successful Germination
Successful natural germination depends heavily on the surrounding environment. Nasturtium seeds thrive on minimal disturbance, making self-seeding most successful in neglected or less-trafficked areas of the garden.
The seeds need good contact with the soil but should not be buried deeply; about half an inch is optimal for germination. Too much soil prevents the emerging seedling from reaching the light. Soil temperature is a major factor, with ideal germination occurring when temperatures consistently range between 55°F and 75°F.
Moisture levels must be balanced; the soil needs to be consistently moist to soften the seed coat, but not waterlogged. Excessive moisture can lead to rot, while overly dry soil prevents the seed from swelling and breaking dormancy. A lack of intense competition from established weeds also gives young, self-sown seedlings a better chance to develop.
Encouraging or Preventing Volunteer Plants
Gardeners can manage the return of nasturtiums based on their goals for the space. If the aim is to encourage a dense stand of returning plants, simply allow the flowers to fade and the seeds to mature and drop naturally. In the autumn, avoid heavy mulching over the area, as a thick layer can smother the seeds and prevent them from making soil contact or receiving the necessary winter weathering.
Conversely, preventing the plants from returning requires intervention before the seeds drop. Aggressively “deadheading,” or removing the spent flowers before the seeds have a chance to form and harden, is the most effective control method. The developing seed pods are noticeable and should be removed entirely from the area.
If seeds have already dropped, a heavy layer of mulch applied in the fall will cover the seeds, preventing light and proper soil contact needed for spring germination. Any “volunteer” seedlings that sprout in unwanted locations are easily managed by simply pulling them out while they are still small.