Yarrow isn't toxic.
Benefits of Yarrow
Ancient peoples used different parts of the herb to treat various medical conditions, including heavy menstrual bleeding, poor circulation,
inflammation, fever, and gastrointestinal problems. The leaves appear to have been the part of the herb most used, as they were applied to wounds to stop bleeding, chewed to relieve tooth pain, and steeped in water to prepare infusions (teas) to treat a variety of ailments.
Today, yarrow is often valued as a diuretic to help with weight loss, fluid retention, menstrual cramps, heavy menstrual bleeding, indigestion, and bloating, as an
antispasmodic to relieve muscle spasms, and as an astringent, which makes it helpful for tightening and firming tissues. Yarrow can also be used in a sitz bath to relieve painful menstrual cramps.
Yarrow has many constituents that are responsible for the herb’s healing properties. Some of those substances include sesquiterpene lactones (stimulants), an alkaloid called achilleine (anti-hemorrhage), flavonoids (anti-spasmodic), tannins (astringent), camphor (pain reliever), coumarins (anti-inflammatory), and terpinen-4-ol (diuretic).
and in companion gardening Yarrow increases the production of essiential oils in herbs, so does, German Chamomile.