Arnica montana, Arnica chamissonis, Arnica cordifolia

Arnica is a member of the Aster family, the Compositae. We harvest the flowers for medicinal use, usually in the month of June – first few weeks in July. It likes moisture to protect its roots, which run just a few inches beneath the ground, from the heat of the summer sun. To this end, it also appreciates fertility, and a good side-dressing of mature compost once a season is helpful to ensure good blooms next year.

Elemental associations: Fire
Phytochemistry: Essential oils, sesquiterpine lactones, bitter glycosides, alkaloid, flavonoids, tannin (D. Hoffmann)
Actions: Rubifacient, Anti-inflammatory, Anti-bacterial, Vulnerary
Specific systems: Skin. External use.

Arnica is the premiere remedy for bruises, strains, sprains and swelling, due to its anti-inflammatory action. It stimulates good circulation at the site of application, virtually eliminating bruising if immediately applied. Some theorize that this action is obtained in part by irritating the local capillaries; regardless, it works incredibly well.
It is an old folk remedy in Europe, where it (A. montana) grows at high elevations, above the treeline in Alpine meadows. It is a protected species, to be harvested with great respect in the wild. Here in the United States, it does not grow under 4000 feet ~ we rely on alternate species, which have proven equally effective (A. chamissonis, for instance).

Indications: Bruises, muscle strains (acute), sprains, tendon inflammation (acute), osteoarthritis
Contraindications: The material preparations are not for internal use
Preparation/Dosage: The fresh flowers can be made into a tincture, 50% alcohol, 1:3 to 1:10. This can be applied externally to great effect, and is the traditional preparation. 
A homeopathic remedy can be made by dilution/succussion for internal use; dilute at least to a 6X, and take 5 drops at a time. Alternatively, 1 drop of the tincture can be taken in 1oz of water.
An infused oil can be made with the freshly dried (at least 3-4 days) flowers. Apply externally as needed, or add to anti-inflammatory salves.