Butternut grows naturally in the north-eastern United States and also – though rarely - in southern Canada. It is a large multi-stemmed tree with grey bark and a broad crown. The decorative oddpinnate leaves are 25-50 cms (10-20 inches) long, with 11-19 leaflets in opposite pairs. The flowers are unpretentious, but the fruit is a hard-shelled nut 6-8 cms long inside a sticky fleshy covering, ripening in September. The shell is sharp-ribbed into sections and needs to be handled carefully to avoid lacerations.
In America suitable plants have been selected to produce new forms and subspecies whose shells break more easily and contain bigger tasty nuts. The species is also used worldwide in landscaping.
At Mustila, the walnuts are among the most eye-catching of the broadleaf species because of their foliage. The butternut is among the first species to have been planted on Etelärinne (Southern slope). It takes on autumn colour, a beautiful yellow, later than the Manchurian walnut, which is the hardiest of the genus.