Growing Purpletop Vervain in a pot

Once established, purpletop vervain is remarkably low-maintenance and drought-tolerant. Water regularly during the first growing season to help roots establish, but thereafter only water during prolonged dry spells in summer. Overwatering or planting in soil that stays damp encourages weak, floppy growth and increases the risk of winter losses. In most temperate European gardens, rainfall alone is sufficient. Feeding is not necessary and can actually be counterproductive. Verbena bonariensis grows naturally in poor soils and too much fertility produces lush foliage at the expense of flowers, as well as reducing winter hardiness. Avoid adding fertiliser or rich compost around established plants. If your soil is very poor or sandy, a light sprinkling of general-purpose granular fertiliser in April is the absolute maximum required. This plant is reliably hardy in zones 8–10 but can be borderline in zone 7a, especially in wet winters. Improve survival by ensuring excellent drainage and avoiding autumn feeding or mulching with organic matter, both of which promote soft growth vulnerable to frost. A gravel mulch is beneficial. In very cold gardens, treat it as a short-lived perennial or allow it to self-seed as insurance. Pests and diseases rarely trouble verbena bonariensis. Powdery mildew can occasionally appear on foliage in late summer during dry spells, but it seldom affects flowering and doesn't warrant treatment. Aphids may cluster on young shoot tips in spring; a strong jet of water or a spray of insecticidal soap will control them if necessary, though natural predators usually keep numbers in check. Slugs generally ignore the foliage.