Great St. John’s wort is a large, robust midsummer to late summer perennial that produces showy yellow flowers. It is a good option for areas with rocky soils and can be grown on slopes. This species is native to the Chicago region according to Swink and Wilhelm’s Plants of the Chicago Region and current research.
Size and method of spreading:
Great St. John’s wort grows 2 to 5 feet tall and spreads 1 to 3 feet wide. Plants spread through offsets produced by underground stem structures (rhizomes).
Native geographic location and habitat: (include C-value if appropriate)
The native range for great St. John’s wort includes the northernmost Eastern and the Midwestern United States. C-value: 10.
Attracts birds or pollinators:
The primary pollinators of great St. John’s wort are bumblebees. Moth and butterfly caterpillars use great St. John’s wort as a food source.
Great St. John’s wort has leaves that are typically much longer than wide (lanceolate), but they can be more oval in shape (lanceolate-ovate to ovate). They have smooth edges (entire margins), medium green upper surfaces, pale green undersides, and are hairless (glabrous). The leaves grow in pairs (opposite arrangement) that are either directly attached to the stem (sessile) or slightly clasping it.
The flowers of great St. John’s wort are about 2 inches in diameter. The five petals are slightly twisted or curled, bright yellow, and surround a slightly bulbous light green center structure (pistil with five styles) and numerous yellow, stringlike protrusions (stamens). The stamens occur in bundles or groups of five (fascicles). Great St. John’s wort flowers occur in groups of one to five (cymes) on the terminal ends of upper stems.
Great St. John’s wort produces a hairless, five-chambered, pyramid-shaped fruit that is approximately 1 inch tall and one-half inch wide. The fruit starts yellowish green, but dries to a brownish black. When the seed is ripe, the fruit opens at the top (capsule).
Great St. John’s wort is easy to care for and a vigorous grower that may require occasional division.
List of pests, diseases, and tolerances:
No serious pests or diseases affect great St. John’s wort. Aphids may feed on the foliage and become minor pests.