The rosebay willowherb is a handsome plant with pink flowers that blooms during the middle part of summer. It is a common perennial herb throughout Finland and reaches a height of 0.5–1.5 metres. Its stem is non-branching, hollow, and often reddish in colour. The narrow, lanceolate leaves are attached to the stem in a whorled pattern.
The rosebay willowherb is a common pioneer plant throughout Finland and quickly takes over areas cleared by fire or cutting, but recedes within a few years as other vegetation grows.
Young, reddish shoots of rosebay willowherb can be gathered soon after the snows melt and far into early summer. The leaves can be gathered for herbal beverages later in the summer, from June to July, just before flowering begins. They can be stripped off by hand from the top down and dried whole or after wilting. The leaves should be clean, young, healthy, and undamaged. The fresh leaves should be a nice green colour when dried. Bits of the stem should not be gathered along with the leaves. Only the pink petals of rosebay willowherb flowers should be gathered because the green parts can have a bitter flavour. The best time for picking the flowers is in June to July, as soon as they have opened. The best time for gathering rosebay willowherb roots is early spring or late autumn.
Rosebay willowherb leaves contain significant quantities (at least 15% of the recommended daily allowance for adults) of dietary fibre, phosphorus, riboflavin, and vitamins C and A.
Young shoots of rosebay willowherb can be served similarly to asparagus. The finely chopped young leaves add variety to salads and vegetable dishes. Rosebay willowherb is perhaps best known for its use in herbal beverages. It can also be mixed in with regular tea infusions. Like regular tea leaves, the leaves of rosebay willowherb are often wilted before drying. The wilting process enhances the aroma of the leaves. Rosebay willowherb flowers can be used to prepare a beverage that has an appealing colour. A flour for use in bread dough or porridge can be made from rosebay willowherb roots. The roots have also been used as a coffee substitute.