First cultivated for silkworms in North America since the 1620s or before (Hatch 1957), this northeast Asian tree has became widely naturalized across warm- and mid-temperate regions. However, in Ky. the species remains uncommon in some rural areas, apparently due to intense herbivory by rabbits, deer or other herbivores (pers. obs. and J. Shaffer, pers. comm.). With complex chemistry (including tropane alkaloids and unusual amino acids in latex and unripe fruit), its potential use for mammlian food and medicine continues to gain interest (e.g. Kusano et al. 2002, Butt et al. 2008, Bhatt et al. 2010, Chan et al. 2016). In the Bluegrass, Short (1828-9) noted: "The White mulberry, lately introduced by seeds from France, seems to thrive well in this climate, so far at least as four or five year's experience can show." Rafinesque (1836a, 3:46) noted: "a well known tree, now widely grown with us, almost wild in some localities." It has become locally common on base-rich soils of Ky. in mature riparian woods (reaching 8 dm or more in dbh), as well as being a common weed in most urban areas. M. alba is often confused with rubra (FNA 3; Nepal et al. 2012). Its lower leaf surfaces have hairs largely restricted to the midvein, major laterals and axils between them, mostly appressed to spreading and 0.2-0.5 mm long (versus more uniformly dense, mostly spreading to erect and 0.4-0.7 mm); upper surfaces are glossier (without impressed veins as in rubra) and largely glabrous (versus appressed-hairy or scabrid). Blades tend to be smaller (averaging 6-15 x 3-8 cm versus 12-30 x 8-22), with narrower shape, less cordate bases and less long-acuminate apices. Sprout leaves in sun are often more lobed and paler (yellowish- versus bluish-green); these sometimes have 3-5 unusually deep linear lobes, and have been mistaken elsewhere for M. australis (e.g. J.D. Hall at MARY). Winter buds are generally smaller, with scales mostly brown (versus blackish at apical margins). Bark is somewhat orange-brown to reddish with solid ridges (versus more grayish with peeling plates); root bark is generally orange but can also have purplish rings. Branches tend to be more erect, leading to a more rounded bushy crown. Fresh female catkins are shorter (ca. 5-8 mm versus 8-12 mm). Flowering is usually 1-3 weeks earlier in Ky. (peaking in early versus late Apr), and fruits ripen mostly during mid-May to early Jun (versus late May to earlyJul); ripe fruits are usually 15-20 mm long, deep purple or pale reddish or whitish, and somewhat insipid (versus 25-40 mm, deep purple-black, and more deeply flavored). In the upper midwest, there has been some hybridization with rubra, leading to locally invasive introgression (Burgess & Husband 2006; Nepal et al. 2012). M. alba and hybrids with alba mothers were consistently more vigorous in experimental shade and sun, compared to M. rubra and hybrids with rubra mothers. Hybrids have rarely been identified in Ky., but they may be often overlooked; a few colls. from FAYE, JEFF, MADI and perhaps elswhere do appear intermediate based on leaf pubescence and texture.