Hallo Lisa,

eigene Erfahrung habe ich nicht damit, aber hier steht, dass sie am besten ohne Stratifikation keimen - aber mit Kunstlicht, also sollten sie wohl seeehr hell stehen!:

Seedling Development: Germination of river birch seeds is best (about 35 percent) with unstratified seeds under artificial light (1). Germination is epigeal. During early stages, seedlings are fast-growing and have a high soil-moisture requirement. Because of abundant seed production, rapid germination, and vigorous early growth, river birch is one of the pioneer species of new forests growing on stream bank alluvium (17). Germination and development, as well as growth at all stages, is inhibited by even moderate shade (3). Despite the high moisture requirement of seedlings, river birch can tolerate flooding no more than 3 months during the growing season (14). Stumps of young trees sprout vigorously (5).

von hier: http://forestry.about.com/library/silvics/blsilbetnig.htm

Vielleicht kriegst Du ja noch andere Infos;

lg und gutes Gelingen
Moni

Birken'????'?
das is doch wie unkraut!
einfach in balkonkasten streuen, nächstes frühjahr haste da nen kleines birjkenwäldchen!