formerly Arenaria congesta
Also known as dense-flowered sandwort, ballhead sandwort is a tufted perennial forming mats from 5-15 cm wide and with one to several erect flower stems from 15-40 cm high. The stems and leaves are typically glabrous, although they may occasionally bear some glandular hairs. The ascending leaves are linear, 1-nerved, and measure from 0.5-8 cm long. They are glabrous to glaucous but may sometimes have ciliate margins and tipped with a sharp, rigid point. The basal leaves may have secondary leaves in their axils, while the 1-4 pairs of stem leaves are opposite and much reduced in length.
The inflorescence is most often tightly congested in a terminal head, although some varieties will have somewhat open cymes. The sepals are 1-nerved and measure from 3-6 mm long. They range from lanceolate to ovate in shape with blunt to pointed tips and with wide, white membranous margins. The white petals are up to 9 mm long, or about 50% longer than the sepals. They are linear-oblong in shape.
Sharptip sandwort: var. cephaloidea - Inflorescence a dense, tight cyme. Bracts closely enveloping the sepals. Sepals acute. Found from central and eastern Washington into Idaho. Rare in Oregon.
Ballhead sandwort: var. congesta - Leaves 3-8 cm long, 0.5-1 mm wide, leafy and thread-like. Inflorescence a dense, tight cyme. Bracts closely enveloping sepals. Sepals obtuse, 3.5-4 mm long. Gravelly to sandy soil in the open. 1300-2000 m. Found from central Washington south to California and east to Montana, Utah and Colorado.
Ballhead sandwort: var. crassula - Leaves 2-3 cm long, 1-2 mm wide, fleshy and needle-like. Inflorescence a dense, tight cyme. Bracts closely enveloping sepals. Sepals obtuse, 3.5-4 mm long. Dry ridges and rock crevices from 2200-2500 m. Found from southwest OR to the Klamath Region south to the high North Coast Range and Modoc Plateau of northern California.
Gland sandwort: var. glandulifera - Plants glandular-hairy. Inflorescence open, umbel-like. Bracts confined to base of inflorescence. Sepals acute. Found in Douglas Co, WN and Valley & Washington counties in Idaho.
Loosehead sandwort: var. lithophila - Absent from Oregon and Washington. Found from Idaho to Montana and south to Utah and Colorado.
Ballhead sandwort: var. prolifera - Plants glabrous. Inflorescence open, umbel-like. Bracts confined to base of inflorescence. Found along east side of the Cascade Mts. from Okanogan CO, Wn to northern Oregon.
Subcongesta sandwort: var. subcongesta - Absent from Oregon and Washington. Found from Idaho south to the southwestern US.
Suffrutescent sandwort: var. suffrutescens - Leaves 1.5-8 cm long, 0.5-1.5 mm wide, leafy and needle-like. Inflorescence open, umbel-like. Bracts confined to base of inflorescence. Sepals obtuse, 3-4 mm long. Rocky slopes and outcrops, 1200-3300 m. Found from southwestern OR to the Klamath Region and south into the High Sierra Nevada Mts.
Ballhead sandwort may be found on dry slopes from the lowlands to alpine slopes in the mountains.
Ballhead sandwort may be found to the east of the Cascade Mts. from Okanogan County in north-central Washington south to south-central Oregon and westward to the Siskiyous of southwestern Oregon and the Sierra Nevada of California. It may be found eastward to the Rocky Mts. from the Yukon south to Wyoming and Colorado.