Shrubs or trees, to 20 m, dioecious, occasionally monoecious. Bark grey-brown often with an orange tint, shallowly furrowed, ridges flattened, wide. Branchlets red-brown to light greenish brown, pubescent initially, later glabrous; lenticels pale, elliptic, prominent. Buds ovoid to conical, slightly compressed, 3–7 mm, apex acute; outer scales dark brown, often pubescent, minutely ciliate. Stipules linear, 1–1.3 cm, thin, pubescent. Leaves broadly ovate, 7–25 × 6–20 cm; upper surface somewhat scabrous with a sparse covering of short hairs; lower surface sparsely to densely pubescent; base rounded to subcordate, sometimes oblique; margins serrate or crenate; apex abruptly acuminate; petiole 2–2.5 cm, pubescent or glabrous. Male inflorescences 1–5 per node, to 2.5 cm; male flowers with ciliate, ovoid, purplish-green calyx lobes, pubescent abaxially. Female inflorescences 1 per node, 1–1.5 cm, peduncle to 2 cm; female flowers with compact calyx tigtly surrounding the ovary, with much reduced styles (<0.5 mm), stigma branched, whitish. Syncarps maturing red, dark purple or blackish purple, 1–2.5 cm. (Wunderlin 1997; Razdan & Dennis Thomas 2021).
Distribution Canada Ontario United States Alabama, Arkansas, Connecticut, District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Vermont, Virginia, West Virginia, Wisconsin
Habitat On a variety of soil types up to 600 m asl in wooden valleys and floodplains. Typical associates in the northern part of its range include Acer rubrum, A. negundo, and Fraxinus americana. Further south it associates with Platanus occidentalis, Acer saccharinum, and Liquidambar styraciflua. A secondary succession species.
USDA Hardiness Zone 7
RHS Hardiness Rating H6
Conservation status Least concern (LC)
The tallest of the mulberries, Morus rubra is native to central and eastern North America, from northern Ontario (Canada) to southern Florida and west to southeast South Dakota and central Texas (USA). A secondary succession species, it prefers the moist, deep soils found in forested floodplains and valleys. The eighteenth-century explorer William Bartram, on his travels on foot through Florida, South Carolina and Georgia, often found Red Mulberries in deciduous forests, growing alongside magnolia, elm, oak, and acer, especially along riverbanks and ravines (Bartram 1998). Dirr (2009) writes of its handsome dark green foliage making it conspicuous in forests in summer, with golden autumn foliage in some years also making a noteworthy show, though it is often just a ‘subdued yellow.’
Red Mulberry was once common in Canada but is now listed as Endangered under the Ontario Endangered Species Act, 2007 (Parks Canada Agency 2011); this is in stark contrast to a global IUCN classification of Least Concern. Within Canada it is now mostly confined to the Ball’s Fall conservation area on the Niagara peninsula of southern Ontario. In 2014 only an estimated 217 trees remained in Canada, of which just 105 were considered to be mature (COSEWIC 2014). The main threat to the species here is hybridization with the introduced White Mulberry, M. alba, and this is a significant problem in the US, too (Burgess et al. 2005). Further south, various pests and pathogens are associated with Red Mulberry but none appear to be considered significant or urgent problems despite how scarce a tree it is becoming in the US. Despite living well within its natural range in Pennsylvania, Paul Meyer, writing in 2022, ‘can’t remember when I last saw M. rubra’ (P. Meyer pers. comm. 2022).
Red Mulberry has never really acclimatised outside its native range, lowering the prospects for meaningful ex-situ conservation. For Bean, ‘this mulberry thrives the worst [in Britain] of those here mentioned. At Kew it always has an unhappy appearance, and I do not know of good trees elsewhere. Probably our climate is as unsuited for it as for several other trees from the same region’ (Bean 1981). Nevertheless, today both Kew and the Oxford Botanic Garden do have healthy, albeit still relatively young specimens, and there is a fine (unconfirmed) tree at Pampisford, Cambridgeshire. References to the ‘red mulberry’ in some early plant lists probably refer to red-fruited Black Mulberry, as M. rubra was not introduced in Britain until 1629; Bean noted ‘It is distinguished from M. alba and M. nigra by the leaves being much more downy beneath’ (Bean 1981).
Native Americans, notably the Cherokee and the Creek (Muscogee), but also several other eastern and southern tribes, cultivated groves of Red Mulberries for their fruit and timber (Coles 2019). The Seminole tribe of Native Americans, who broke away from the Creeks and settled in Florida, used Red Mulberry branches to make bows; meanwhile, the Comanche and Cherokee used its bark to treat intestinal worms and the sap to treat ringworm (Coles 2019).
Early specialists who travelled to North America to introduce sericulture during the reign of King James VI & I in the early 17th century were jubilant to find an abundance of native American mulberry trees – only to discover that the rough and rather hairy leaves were not appreciated by Bombyx mori silkworms, brought to the new colonies at the same time. This led to a massive importation and subsequent propagation of M. alba varieties (notably M. alba ‘Multicaulis’ in the 19th century), which went on to hybridize with the native M. rubra and push the latter to the verge of extinction (Burgess et al. 2005). Nevertheless, several selections of this hybrid have become important garden plants; these are discussed under M. alba × rubra.