Purpletop vervain, scientifically known as Verbena bonariensis, is a captivating plant prized for its unique aesthetic in garden settings. This herbaceous perennial, often called tall verbena or Brazilian verbena, features slender, upright stems crowned with clusters of small, vibrant purple flowers. Its airy, open structure allows it to provide vertical interest without obscuring neighboring plants, creating a “see-through” effect. Beyond its visual appeal, purpletop vervain is highly valued for attracting a wide array of pollinators, including butterflies, bees, and hummingbirds, making it a dynamic addition to any garden space.
Ideal Growing Conditions
Purpletop vervain thrives when its foundational requirements for light, soil, and planting time are met. Establishing the right environment from the start ensures robust growth and abundant flowering, helping the plant develop its characteristic tall stems and profuse blooms.
Sunlight
Purpletop vervain performs best in full sunlight, ideally receiving at least six to eight hours of direct sun daily. Inadequate light can result in leggy growth and a reduction in blooms. While it tolerates some partial shade, especially in regions with intense summer heat, full sun exposure yields the most impressive display.
Soil
Well-drained soil is important for purpletop vervain, as it is susceptible to root rot in waterlogged conditions. While it adapts to various soil types, including sandy, loamy, and clay soils, good drainage remains the most important factor. The plant prefers a slightly acidic to neutral pH, typically ranging between 6.0 and 7.5. Incorporating moderate organic matter can improve soil structure and fertility, supporting healthier growth.
Planting Time
The optimal time to plant purpletop vervain is in the spring, after the last risk of frost has passed in your area. If starting from seed indoors, sow them eight to twelve weeks before the average last frost date. Seeds may benefit from a period of cold stratification if planting in spring, or they can be fall-sown directly outdoors. Transplanting young plants should occur once outdoor temperatures are consistently warm enough to prevent frost damage.
Routine Care and Maintenance
Consistent routine care helps purpletop vervain maintain its vibrant appearance and encourages a prolonged blooming season. While low-maintenance, specific practices related to watering, fertilization, and pruning enhance its performance.
Watering
Established purpletop vervain plants are drought-tolerant, capable of withstanding dry spells once their root systems are developed. Consistent moisture during the establishment phase is beneficial for young plants, requiring regular watering until they are well-rooted. During extended drought, occasional supplemental watering ensures the plant remains healthy and continues to bloom. Allowing the top inch of soil to dry out between waterings helps prevent overwatering and root issues.
Fertilization
Purpletop vervain is not a heavy feeder and thrives without extensive fertilization, especially in well-drained soil with adequate organic matter. Excessive fertilization can promote lush foliage at the expense of flower production and may lead to weaker stems. If the soil is poor or sandy, a light application of a balanced, slow-release granular fertilizer or compost can be applied every four to six weeks during the active growing season. For container-grown plants, a balanced liquid fertilizer may be applied periodically, as nutrients in pots can deplete faster.
Pruning and Deadheading
Pruning purpletop vervain helps maintain its shape and encourages bushier growth, while deadheading spent blooms promotes continuous flowering. Removing faded flower clusters directs the plant’s energy into producing new blossoms rather than setting seed, extending the bloom period from early summer until the first frost. In early spring, cutting back old, dead stems to a few inches above the ground can invigorate new growth. Pinching back young plants can also encourage more branching and a fuller habit.
Managing Unique Characteristics
Purpletop vervain possesses distinct characteristics that, when understood, contribute to its successful integration into a garden. Its prolific self-seeding habit and winter hardiness vary by climate, requiring tailored approaches to ensure desired growth and persistence.
Self-Seeding Management
Purpletop vervain has a strong tendency to self-seed, which can be advantageous for naturalizing areas or ensuring its return. If unchecked, this can lead to a dense carpet of volunteer seedlings. To manage its spread, deadhead flowers before they produce and disperse seeds. Alternatively, volunteer seedlings can be thinned in spring, or unwanted plants can be easily removed by hand. Allowing some seeds to drop can ensure a continuous presence without replanting annually.
Winter Preparation
Purpletop vervain is a tender perennial, hardy in USDA Zones 7 through 11. In milder climates, it can overwinter successfully, particularly if mulched. In colder regions (Zone 6 and below), it is commonly grown as an annual, completing its life cycle in one growing season and not surviving winter temperatures. For perennial zones, cutting back old foliage in late winter or early spring, as new growth emerges, prepares the plant for the upcoming season. Leaving dead stalks through winter can also provide interest and food for birds.