Nasturtiums are known for their vibrant, trumpet-shaped flowers and circular leaves, both edible and adding a peppery zest to culinary creations. While many gardeners are familiar with annual varieties, certain perennial species return year after year. These perennial forms provide sustained beauty and flavor, representing a less common aspect of the nasturtium family.
Understanding Perennial Nasturtiums
Unlike common annual nasturtiums (Tropaeolum majus), perennial types survive winter and regrow from their rootstock. This perennial nature stems from specialized underground structures, such as tubers or rhizomes, which store energy and allow the plant to overwinter. These structures are a key adaptation for enduring colder temperatures or dry periods.
Specific perennial species include Flame Nasturtium (Tropaeolum speciosum), known for its bright red-orange flowers and vining habit, and Mashua (Tropaeolum tuberosum), valued for its edible, colorful tubers. Flame Nasturtium thrives in cooler, temperate climates, often scrambling through shrubs or hedges. Mashua, originating from the Andes, produces tubers that can be harvested and cooked, offering a peppery, slightly sweet flavor.
Ideal Growing Environment
Perennial nasturtiums generally flourish in locations with a balance of sunlight and protection from intense heat. They perform best in full sun to partial shade, with afternoon shade particularly beneficial in warmer climates to prevent scorching. Adequate light exposure is important for robust flowering and healthy foliage.
Well-draining soil is crucial to prevent root rot, especially for tuberous varieties. A slightly acidic to neutral soil pH (6.0-7.0) supports optimal nutrient uptake and plant vigor. Incorporating organic matter, such as compost, improves soil structure and drainage while providing slow-release nutrients.
Consistent moisture is important throughout the growing season, particularly during dry spells, but waterlogging should be avoided. Perennial nasturtiums do not tolerate overly wet feet, which can lead to tuber decay or fungal issues. Hardiness zones vary by species; Tropaeolum speciosum is often hardy in USDA Zones 7-9, while some Tropaeolum tuberosum cultivars tolerate colder temperatures, sometimes surviving in Zone 6 with winter protection.
Cultivation and Year-Round Care
Plant perennial nasturtiums by setting out tubers or established plants in spring after the last frost. Plant tubers shallowly (2-4 inches deep) with growth points facing upwards. Providing a support structure like a trellis or fence is beneficial for vining types, encouraging them to climb and display their flowers effectively.
Perennial nasturtiums generally do not require heavy feeding; excessive nitrogen can promote leafy growth at the expense of flowers or tubers. A balanced, slow-release fertilizer or compost in early spring usually suffices. Regular pruning helps manage vigorous growth, promotes bushier habits, and encourages continuous flowering. Deadheading spent flowers redirects the plant’s energy towards producing more blooms rather than seeds.
Overwintering strategies are important for ensuring the return of perennial nasturtiums in colder regions. In zones where the ground freezes, tubers of species like Mashua can be lifted after the first light frost, dried slightly, and stored in a cool, dark, frost-free location in breathable material like peat moss or sawdust. For Flame Nasturtium in borderline zones, a thick layer of mulch applied in late fall provides insulation and protects the underground rootstock from freezing, allowing it to survive and emerge in spring.
Propagation Methods
Perennial nasturtiums can be propagated through several effective methods, allowing gardeners to expand their collection or share plants. Dividing tubers, a straightforward approach for species like Mashua, is best done in early spring before new growth begins, ensuring each section has at least one eye or growth bud. These divisions can then be replanted directly into the garden.
Stem cuttings offer another viable propagation technique, particularly for vining perennial types. Cuttings of 4-6 inches, taken from healthy, non-flowering stems, can be rooted in a moist, well-draining potting mix. Removing lower leaves and dipping the cut end in rooting hormone can increase success rates, with roots typically forming within a few weeks.
While less common for perennial types, some species can also be propagated from seed, though germination can be erratic or require specific dormancy-breaking treatments. Seeds are generally collected from mature, dried pods in late summer or early fall. Sowing them directly outdoors in fall or starting them indoors in early spring can lead to new plants, although resulting plants may take longer to establish and flower compared to those grown from tubers or cuttings.