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Plant of the MonthJanuary 2005
Petrophytum caespitosumby Iza Goroff Petrophytum caespitosum is a member of the Rosaceae, the rose family, a large family of about 100 genera and 2,000 species, some of which are woody, others are not. Although it is not obvious when one looks at them, the three species of Petrophytum are woody. The closest relatives to Petrophytum are in the genus Spirea; when Petrophytum caespitosum was botanically first described, it was placed in that genus. Petrophytum caespitosum has the easternmost range of the genus, from South Dakota and Montana extending southwest to New Mexico and California. The other two species are found in Washington state, one in the Olympic Mountains, the other in the Cascades.
Petrophytum caespitosum is a plant of limestone areas, primarily growing on rock faces. Once a seed germinates in a rock crevice the plant is able to start a colony, extending over the rock face beyond the crevice, catching humus for its roots as it goes. It is found in subalpine areas, so it is not as hardy as its alpine relative, Kelseya uniflora, next month's Plant of the Month. Presumably seed originating from its northernmost sites can provide sufficiently hardy plants for most of our gardens. Seed, cuttings, and rooted divisions are the means of propagations.
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January 2005
Petrophytum caespitosum is a small plant, forming tight, wide mats in time. It is usually no higher than 1" (2 cm) in leaf. Its rosettes are about an inch across, silky pubescent, quite bluish in color. A rosette can produce a vertical flower spike on a stem about 1- 4" high in late summer. The flower spike is about an inch long and less than 1/2" across, packed with small dull white flowers, each about 3/16" (4mm) across. Although the floral display is cute, Petrophytum caespitosum is grown more for its character.