This is a widespread eastern species of subxeric woods on acid soils; it is usually diploid (2n = 34) but occasionally tetraploid (FNA 9). A. arborea has been confused with laevis in some regions of Ky., and hybrids may be expected; see notes under laevis. An unusual coll. from TAYL (T.J. Weckman #1353 at EKY) has proximal pedicels 3-5 cm long, which is longer even than laevis; but no other characters indicate a hybrid. The name A. canadensis (L.) Medik has been misapplied to arborea in some treatments before 1950, causing continued confusion in academia and horticulture. Typical canadensis (F, FNA 9, W) is a more shrubby species that differs as follows: mature at 0.5-8 m tall (versus 2-20 m), often forming "stoloniferous" colonies of stems (versus rarely so); leaf blades mostly 3-5 x 2-3 cm (versus 4-7 x 2.5-4 cm), the apices subacute or obtuse to rounded and mucronate (versus acute to acuminate), the margins with mostly 6-15 teeth per side on proximal half (versus 11-21), the teeth acute with little or no point (versus acuminate with ascending point); racemes usually erect, mostly 1.5-4 cm long (versus drooping, mostly 3-5.5 cm), with proximal pedicels mostly 0.5-1.5 cm (versus 1-2 cm); sepals erect to spreading, mostly 1.8-3.1 mm long (versus soon reflexed, mostly 2.3-3.7 mm); petals mostly 6-12 mm (versus 10-15 mm). A. canadensis occurs mostly in wetlands of Atlantic states, from Ga. to N.S. It has also been collected on the Cumberland Plateau of Tenn., especially along rocky banks of larger streams in Cumberland Co. (Rodgers #74, #461, ?#614, Estes #6236, #6228), Fentress Co. (Underwood #3894, Shaw+ #5-286) and Morgan Co. (Estes ?#9254, Medley #10518); some colls. from Tenn. suggest hybridization of arborea and canadensis. A. canadensis or its hybrids are sometimes planted in Ky., as well as laevis hybrids (X grandiflora). There is no evidence that canadensis or its hybrids are native or naturalized in Ky. See also note under intermedia.