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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Petrophytum caespitosum' from the website Trees and Shrubs Online (treesandshrubsonline.
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It is, perhaps, stretching the term ‘shrub’ somewhat to include mention here of this species, but its base is purely woody and as it occurs in nature the main stem may be half an inch or more in diameter. The plant is a low, prostrate evergreen forming a close, compact tuft or mat an inch or two high. One writer describes it in the state of Idaho as making dense and perfectly flat mats of tough woody branches growing over rocks, in the cracks of which the seed had originally germinated. Leaves oblanceolate, not toothed, 1⁄4 to 1⁄2 in. long, 1⁄8 in. or less wide, tapering gradually to the base, bluntish at the apex except for a minute tip; grey-green covered with silky hairs. Flowers white, very small, produced during July and August densely packed in cylindrical racemes 1 to 21⁄2 in. long, the racemes being borne at the top of an erect stalk 1 to 4 in. long. The stamens (about twenty to each flower) are conspicuously exposed.
Native of the S.W. states of N. America, where it often occurs on limestone formations. It is hardy even in the eastern United States and is grown successfully in Scotland. It is adapted only for the rock garden, or moraine, where there is perfect drainage and unobstructed sunshine. Very distinct in its dwarfness, its narrow entire leaves, and the close packing of the small flowers near the top of a quite erect spike.
Eriogynia hendersonii Canby
Spiraea hendersonii (Canby) Piper