Dense St. Johnswort is a Danish cultivar whose showy golden blooms flower from July through September—much longer than other Hypericum. One of the parents of this cultivar, Hypericum kalmianum, is endemic to the Great Lakes region of the United States. This low-maintenance perennial forms a tightly branched, mounded shrub with dark green, willow-like leaves and grows to 30 inches by 30 inches. It requires moderately fertile, evenly moist soil, and once established, it is relatively drought tolerant.
This is not the species valued for its medicinal properties (H. perforatum). St. Johnswort is poisonous to livestock when ingested, and in high doses, it can cause many of the same symptoms in humans.
St. Johnswort is one of the few plants tolerant of being planted near black walnut trees. Soil under these trees tends to be alkaline, influencing the growth of many plants. These trees produce a nontoxic, colorless chemical called hydrojuglone, which kills many plants as they take up the chemical from the soil.