Giant hogweed (Heracleum mantegazzianum) is a large perennial weed that can reach heights of 15-20 feet. It tends to form dense canopies and has dark reddish-purple spotted stems covered with fine bristles, and leaf stalks with compound leaves which can expand outward 5 feet from the stem. The flowers are broad, flat-topped umbels of numerous small white flowers which extend 2.5 feet in diameter. Giant hogweed takes at least 3 years to produce its first flowering stalk.Giant hogweed causes photosensitivity in horses upon contact and subsequent exposure to sunlight. The plant exudes a clear watery sap which contains several photosensitizing furanocoumarins; several of which are known to be carcinogenic and teratogenic. The leaves and roots have the highest concentrations of toxins, during the early part of the growing season.