Common St. John's wort is native to a broad region across Europe, parts of North Africa, and western and central Asia. It has been introduced to both North and South America, along with areas in Eastern Asia and the sub-Antarctic region. In several major regions, particularly the Americas and Oceania, common St. John's wort is sometimes thought to be invasive and may have an impact on local ecosystems.

Common St. John's wort can cause allergic reactions and severe or life-threatening side effects in humans taking certain prescription drugs. The plant is dangerous to cats and mildly toxic to dogs, with symptoms like dermatitis, gastrointestinal distress, and photosensitivity. Prompt veterinary treatment is recommended for pets exposed to common St. John's wort, as contact or ingestion can lead to serious health issues.

The native range of common St. John's wort includes Eurasia and northern Africa. It has spread to parts of the Americas, Oceania, and southern Africa and is listed as noxious in areas of the United States and Canada. It is cultivated in farms and gardens for several uses. However, common St. John's wort is toxic to some livestock, depletes soil moisture, competes with native species, and may increase the risk of fire. It spreads voraciously as individual plants can produce up to 100,000 seeds annually, seeds can remain viable for up to 10 years, and plants can also spread laterally through rhizomes. Hand pulling, tilling, mowing, and burning are typically not effective means of control and may even stimulate reproduction. It is resistant to some herbicides but is susceptible to several species of leaf-feeding beetles.

The best season to remove common St. John's wort is during spring or fall. Ideally, the plant should be removed before it matures and starts to flower or fruit to avoid propagation. Here are a few effective ways to control the spread of common St. John's wort: