Slender vervain (Verbena bonariensis) is an herbaceous perennial known for its airy structure and vibrant blooms. This plant adds vertical interest to garden spaces, often appearing to float above other plantings. It has also become naturalized in various regions outside its native South America. Its growth habit allows it to blend into diverse garden designs.
Identifying Slender Vervain
Slender vervain is recognized by its tall, wiry stems and delicate flower clusters. The plant grows 3 to 6 feet tall, with a spread of 1 to 3 feet, creating an open, transparent appearance. Its stems are slender, erect, square-shaped, and often reddish. Stems are sparsely leaved, contributing to its airy quality.
Dark green, lance-shaped leaves with serrated margins form a rosette at the base, up to 5 inches long. As the stems ascend, the leaves become more sparse. Small, tubular flowers, typically rose-violet or lavender-purple, cluster into rounded heads 2 to 3 inches across. These clusters bloom from mid-summer until the first frost, attracting a variety of pollinators.
Where Slender Vervain Thrives
Native to South America, slender vervain prefers full sun exposure, requiring at least six hours of direct sunlight daily for optimal growth and flowering. It can tolerate some light shade, but its bloom capacity and overall vitality may be reduced.
It thrives in well-drained soils, including sandy or loamy types, and tolerates poor soil conditions if drainage is good. It prefers slightly acidic to neutral pH (6.0-7.5). Once established, it is moderately drought tolerant, but benefits from consistent moisture during its growing season.
Cultivating Slender Vervain
Slender vervain is an easy-to-grow plant, often treated as an annual in cooler climates (USDA Zones 7 and below) and a perennial in warmer regions (USDA Zones 7-11). Plant young plants after the last frost in spring. Ensure the plant’s crown is level with the soil surface.
It can be grown from seed (germinates readily without pretreatment) or propagated from stem cuttings. For earlier blooms, seeds can be started indoors 6 to 8 weeks before the last frost. While low-maintenance, consistent watering benefits establishment, though mature plants are quite drought-tolerant. Fertilizing is not essential, but a balanced, slow-release fertilizer can be applied monthly during the growing season to promote healthy growth and flowering.
Managing Slender Vervain in the Garden
Slender vervain’s vigorous growth and prolific self-seeding can cause it to spread widely in gardens. To encourage a bushier, more compact plant, pinch back young plants by removing the top inch of growth 2 to 3 weeks after planting. This encourages branching and more flower stems.
To control spread, deadhead spent flowers to prevent excessive self-seeding. Removing browning flowerheads before they produce seeds can significantly reduce the number of volunteer seedlings. If you appreciate its naturalizing tendency, allowing seeds to drop provides a continuous supply of new plants. If an area becomes too crowded or unwanted plants appear, seedlings are easy to pull or hoe out, especially in early spring. When removing established plants, pull them up completely, ensuring as many roots as possible are removed.