Seeds from plants in the Pacific Northwest usually do not require an after-ripening period. Germination occurs fall through spring. Calcium ions in water appears to inhibit germination of some biotypes. Plants typically produce an average of 15,000-33,000 seeds per plant. Seedlings survive best on disturbed open sites.
Cultivation readily controls plants on agricultural lands.
Improving soil fertility and reseeding with desirable vegetation can help reduce or eliminate infestations in pastures. Mowing and overgrazing reduces seed production, but promotes vegetative spread from rhizomes. Burning stimulates seed germination and vegetative reproduction. It is introduced from eastern North America and expected to expand range. In addition, three native Hypericum species occur in roughly the same regions as common St. Johnswort .