This distinct species of ash (Wallender 2008) occurs on calcareous soils across east-central states, with some more or less disjunct sections across its range (Little 1971; K). It is most abundant in subxeric woods on rocky slopes, and it is also locally common on mesic to submesic sites with moster or deeper soils. Although many large trees of quadrangulata remain in woodland-pastures of the Bluegrass region, seedlings almost never invade old fields, unlike americana and pennsylvanica (e.g. Campbell 2016). The national champion is at the western edge of the Bluegrass in JEFF (38.2745N -85.6903W): 181 cm dbh, 24 m tall, 26 m wide in 2009 (KDF 2020). In contrast to other eastern species, flowers of quadrangulata are perfect (versus largely dioecious). Samaras wings are decurrent with no narrowing to the base; the apex is obtuse and length/width is usually 4-4.5 (versus 5-7). Samaras mature relatively early in Aug-Sep (versus Sep-Oct), but, unless there are strong winds, they usually do not fall until spring (versus during the autumn). Leaflets are usually 7-11 (versus usually 5-9 in native trees, 7-13 in nigra and some cultivated trees), declining in size from medial to terminal (versus the terminal one sometimes as large or larger than adjacent pair), drying to bluish- or blackish-green on both sides (versus drying bluish-green, olive or brownish above, often paler to whitish below). Young petiole bases, and often also rachises and young twigs, are partly covered with long repeatedly curling hairs (versus glabrous or with short hairs or long curling hairs). Leaf scars are on distinct raised pedestals, confluent with stem angles, most evident on year old twigs (versus not so except in alien excelsior). Buds are blackish but largely covered with light brownish to grayish granular glands and hairs, without broad glands (versus blackish or partly covered with broad reddish-brown glands, without general covering of hairs). The four-angled fresh twigs are distinctive (versus terete or slightly angled in nigra and excelsior), but the angling is often obscure to absent on less vigorous shoots. A blue dye was made from its bark in olden times, but no analysis of the chemical basis has been published. Horses often nibble the outer bark in woodland pastures of the Bluegrass region, but generally refrain from complete consumption of the cambium. Initial experiments with the Emerald Ash Borer in Ohio have shown that quadrangulata may be the only eastern ash with some significant resistance to this pest; although larvae can feed in the bark, adults tend to avoid this species (Tanis & McCullough 2012, Villari et al. 2016, Qazi et al. 2018). Curiously, it is much harder to inject insecticide into quadrangulata, compared to other ash species (D. Leonard, pers. comm.).