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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Penstemon' from the website Trees and Shrubs Online (treesandshrubsonline.
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This genus is almost purely N. American and consists of some 150 species, most of which are perennial herbaceous plants. The shrubby species are mostly native of California. The leaves are opposite; the corolla tubular, two-lipped; stamens five, as the generic name indicates, but one of them is always sterile. The shrubby species are easily increased by cuttings of moderately ripened wood; they like a sunny position and a lightish well-drained soil. A very beautiful genus.