Betula nigra River Birch Size: 40 to 70' and may reach 90' in height; spread 40 to 60'. Hardiness: Zone 3b to 9. Habit: Pyramidal to oval-headed in youth, often rounded in outline at maturity; the trunk is usually divided into several large arching branches close to the ground; the tree is more handsome when grown as a multi-stemmed specimen. Rate: Medium to fast. Culture: Sun, partial shade.