Hemlock is a tall, much branched and gracefully growing plant, with elegantly-cut foliage and white flowers.
It is a biennial plant, usually growing from 2 to 4 feet high, but in sheltered situations sometimes attaining nearly double that height. The root is long, forked, pale yellow and 1/2 to 3/4 inch in diameter. The erect, smooth stem, stout below, much branched above and hollow, is bright green, but is distinctively mottled with small irregular stains or spots of a port-wine colour and also covered with a white 'bloom' which is very easily rubbed off.
The leaves are numerous, those of the first year and the lower ones very large, even reaching 2 feet in length, alternate, longstalked, tripinnate. The upper leaves are much smaller, nearly stalkless, with the short footstalk dilated and stem-clasping, often opposite or three together, more oblong in outline, dipinnate or pinnate, quite smooth, uniform dull green, segments toothed, each tooth being tipped with a minute, sharp white point.
The umbels are rather small, 1 1/4 to 2 inches broad, numerous, terminal, on rather short flower stalks, with 12 to 16 rays to the umbel. At the base of the main umbel there are 4 to 8 lance-shaped, deflexed bracts; at the base of the small umbels there are three or four spreading bractlets.
The flowers are small, their petals white with an inflexed point, the stamens a little longer than the petals, with white anthers.
The fruit is small, about 1/8 inch long broad, ridged, compressed laterally and smooth. Both flowers and fruit bear a resemblance to caraway, but the prominent crenate (wavy) ridges and absence of vittae (oil cells between the ridges) are important characters for distinguishing this fruit from others of the same natural order of plants.