How to Grow Penstemon
From the Greek pente, five, and stemon, stamen, in reference to the five stamens (Scrophulariaceae). This genus of over 250 species of hardy and half-hardy herbaceous annuals, perennials and sub-shrubs is almost exclusively North American. The name is sometimes erroneously spelt Pentstemon. Though many are grown in British gardens, some of the species do not thrive and it seems likely that the continental climate, with its colder winters and hotter summers, is needed to make these plants really happy. The very popular late summer bedding penstemons were derived from an initial crossing of P. cobaea and P. hartwegii, and they have a fairly wide
colour range through pinks and reds to deep maroons and purples. P. heterophyllus is a fine sub-shrub with blue flowers which usually attracts interest when well grown. Another striking plant is the herbaceous P. barbatus (syn. Chelone barbata), a tall grower, to 3 feet, with bright vermilion-scarlet flowers.
Species cultivated P. angustifolius, 1 foot, soft blue, July, western United States. P. antirrhinoides, 3 feet, lemon-yellow, July, California. P. azureus, 1 foot, blue, August, North America. P. barbatus, 3 feet, scarlet, summer, Colorado. P. barrettiae, 1 foot, bright violet, May—June, western United States. P. bridgesii, 2 feet, scarlet, July to September, North America. P. campanulatus, 2 feet, rosy-purple, violet or white, June, Mexico and Guatemala. P. centranthifolius, 3 feet, scarlet, summer, California and western Arizona. P. cobaea, 2 feet, purple or white, August, United States. P. confertus, 1 foot, purple and blue, summer, Rocky Mountains. P. cordifolius,
4 feet, scarlet, summer, partial climber, southern California. P. davidsonii, 1-2 inches, ruby-red, summer, spreads by underground stems, rock garden. California. P. diffusus, 2 feet, blue or purple, September, western North America. P. fruticosus, 9-12 inches, purple, summer, north-western United States; var. crassifolius, with minor leaf differences. P. glaber, 2 feet, purple, July, United States. P. glaucus, 15 inches, purple, July, Rocky Mountains. P. hartwegii, 2 feet, scarlet, summer, Mexico. P. heterophyllus, 1-3 feet, sky blue, July, California. P. hirsutus (syn. P. pubescens latifolius), purple or violet, 1-3 feet, July, United States. P. isophyllus, sub-shrubby, 4-5 feet, crimson-scarlet, white within, late summer, Mexico. P. laevigatus, 3 feet, white or pink, summer, United States. P. menziesii, 6 inches, purple, June, northwestern America. P. ovatus, 2 feet, blue to purple, August to October, United States. P. richardsonii, 2 feet, violet, summer, United States. P. rupicola, 4 inches, ruby, northwestern America. P. scouleri, 1i feet, purple, May to June, United States. P. spectabilis, 4 feet, rosy-purple, summer, Mexico and southern California.
The following are some good modern varieties: 'Blue Gem', azure-blue, summer; 'Chester Scarlet', summer; 'Evelyn', pink, May—October; 'Garnet', wine-red; 'George Home', summer; 'Newberry Gem', pillar-box red; 'Six Hills Hybrid', rosy-lilac, May—June.
Cultivation A rich, slightly acid soil is most suitable or a compost mixture of 1 part of leafmould or peat and 2 parts of good loam. A sunny aspect is required. A weekly watering with a soluble fertilizer or liquid manure is needed by the summer bedding penstemons to keep them growing and flowering well. Seed is available for many species. This should be sown under glass in February or March in a temperature of 55-65°F (10-18°C) and the young plants are set out in May after they have been hardened off. But to get exactly similar plants of hybrids it is necessary to take cuttings and raise them under glass in August. They should not be disturbed till the following April. Plants may also be divided in April.