Ageratina altissima var. altissima

Native

Moderate moisture. Open woods. Shade.

Late July to September

White

1′ to 5′

This is a 1′ to 5′ tall perennial forb that rises on 1 to 3 stems from a rough, knotty, fibrous-rooted rhizome. It grows only in shade. It is a late bloomer, one of the last flowers to be seen in the woods in the fall.

The stems are erect or ascending, branched near the top, leafy, and hairless or covered with fine, short hairs.

The leaves are opposite, thin, and egg-shaped to broadly lance-shaped. They gradually taper from above the base to a sharp, drawn-out point forming concave sides along the tip. The bases are often rounded or heart-shaped. They are on ½″ to 2½″ long leaf stalks. The blades are 1½ to 5 times longer than the leaf stalk. The lower leaves are 2⅓″ to 6″ long and 1″ to 5″ wide, becoming gradually smaller as they ascend the stem. They have 9 to 25 sharp teeth on each margin. The upper surface is mostly hairless, the lower surface hairy along the veins.

The inflorescence is a flat-topped to dome-shaped branched cluster. On smaller plants they are compact and appear at the end of each upper stem. On larger plants they are open and also appear on long stalks from the upper leaf axils. The clusters are up to 3″ wide and have many flower heads.

Each flower head is about ½″ wide and ¼″ tall. It contains 12 to 25 disk florets and no ray florets. Each disk floret is about ¼″ wide, consisting of a bright white flower tube with 5 long spreading lobes. A white, forked style protrudes from the floral tube well beyond the lobes.

The fruit is a tiny dark achene with a small tuft of white hairs about ⅛″ long.

Common boneset (Eupatorium perfoliatum var. perfoliatum) leaves are longer, narrower, stalkless, fused around the stem at their bases. The flower heads are much smaller. It is usually found in full sun, in prairies and meadows, often in wet areas.

Tall boneset (Eupatorium altissimum) leaves are mostly stalkless or are attached to the stem on short leaf stalks. The flower heads are much smaller. Each flower contains just 5 disk florets. The fruit has a tuft of light brown hairs. It is usually found in full sun, in prairies and meadows.

Taxonomy
This plant has recently been reassigned to the genus Ageratina. It is still often referred to Eupatorium rugosum.

Toxicity
Poisonous to livestock. If eaten by cows the milk may be poisonous to humans. Unusually abundant in southern Minnesota in 2004. A number of horses in the New Ulm area died in the summer of 2004, and it is thought that white snakeroot is the cause.