Verbena bonariensis is a tall, airy perennial grown as an annual in Zone 6b. It produces clusters of small purple flowers on long, branching stems from midsummer until frost, and attracts butterflies and bees. Its open structure lets it stand among other plants without blocking them.
In upstate New York it rarely survives winter, so grow it as an annual or start fresh from seed each year. It self-seeds readily once established and can reappear the following season. Start seed indoors 8–10 weeks before last frost; germination is slow and benefits from light. Plants tolerate heat and dry spells once rooted.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.