It looks much like the watercress that grows beside it: a sprawling, hollow-stemmed herb that floats and roots in running water, with round, dark green leaflets in pinnate sprays. The difference is in the leaves' undersides, which are flushed a faint violet, and in the flowers, tiny four-petalled clusters of pale lilac where watercress flowers white, open from late spring into summer. It tastes peppery at first and then oddly sweet, and a crushed handful smells of wet stone and something like clove.

It grows only where spring water runs cold and clear the year round, in the spring heads and shaded brooks of the Elderweald and the forest-edge valleys of the Confluence Basins, and dies out where a brook warms or muddies.

Taken before sleep, it slows waking and lengthens the deepest sleep, and dreams come clearer, more coloured, and easier to remember. A dream-worker who has eaten it reaches the dreaming more easily and stays longer. More than a little brings dreams that will not let the sleeper go: heavy, looping, and frightening, and a waking that leaves the head thick for a day.

It grows through the year where the water never freezes, thickest in spring, and spreads by broken stems that root wherever they lodge. A bed can be lost in one summer if the brook above it is fouled.

Dream-workers keep it dried or fresh as a regular part of their craft, and buy it from herb-gatherers who know the spring heads. Healers give a sprig to the sleepless and the grieving. Used night after night it builds a craving for its dreams, and those who eat it to escape waking life slide towards Dream-Sickness. Gatherers mistake it for watercress and serve it at supper, and the household sleeps late and badly.

Basin folk say a sprig under the pillow lets the sleeper dream of the dead, and widows sometimes gather it on purpose. Wood Elves say the cress dreams the brook it grows in, and will not take it from a grove’s spring. The dream-workers hold that it opens no door by itself and only lets the sleeper walk further, which is why they fear it in untrained hands.

Hedge-witches sell it to the grieving and to the greedy, who hope the dead will tell where they hid their silver; many villages have buried a son who dreamed his father’s hoard, dug it up, and was found dead beside it of the nightmares that followed.