Mexican ageratum grows just 6 to 12 inches tall, forming a compact, spreading mound covered in fluffy, tassel-like flower clusters from spring until hard frost. The color range spans pink, white, and blue, with the soft blue cultivars offering something genuinely rare among summer annuals. Prefer rich, moist, well-drained soil in full sun, spacing plants 10 to 12 inches apart to allow good air circulation and reduce the powdery mildew that can trouble the genus. Moderately drought-tolerant once established, it rewards consistent moisture with the densest bloom.

The genus name means not old in Greek, a nod to the remarkable flower longevity that keeps the display fresh across months of hot weather. Taller varieties cut well for arrangements. Use the compact forms as edging or low massing plants in beds and containers, where their fine texture contrasts well with bolder foliage. Deer and rabbits leave it alone. From May through October, few annuals match its steady, dependable output.