Variation and nomenclature need deeper investigation. This largely southeastern taxon is usually considered a variety typical of wet sites (F, W): A. rubrum L. var. trilobum Torr. & Gray ex K. Koch. It is possible that carolinianum is largely octoploid (Santamour 1965), but this is speculation; estimates of ploidy need to be linked with herbarium collections. These plants are generally distinct from typical rubrum, with leaves relatively small (on average), less cordate, and mostly 3-lobed (with at most a minor extra pair of basal lobes) versus more deeply 3- to 5-lobed, and often pubescent below versus generally glabrous (F, W). A possible natural hydrid with saccharinum has been observed in MONT (in yard south of Jeffersonville). Recent analysis of chloroplast DNA (Saeki et al. 2011) suggests that var. trilobum (as var. tridens Wood) should be considered a distinct species, partly because saccharinum (with sister drummondii) appears to be nested within this overall complex. It is possible that significant additional variation exists among the plants mapped here: Ladd & Thomas (2015) have applied the name var. trilobum mostly to "small multi-coppiced trees in upland gravel washes"; whereas plants of lowland swamps may grade into drummondii. Virtually all cultivars of "rubrum" appear to be derived from these plants rather than typical rubrum. Despite abundant planting in urban areas, these trees rarely appear to naturalize on uplands, in marked contrast to saccharinum. Germination can occur in gutters or wet ground around houses, but seedlings rarely survive.