Elegant Martha Washington geraniums —also as known as Regal, Royal, Lady Washington or French geraniums — are cool- and shade-loving hybrid relatives of the heat- and sun-loving common garden geranium (Pelargonium ×domesticum). The common name is derived from a popular variety introduced by that name in the United States in the early 1900's.
Martha Washington geraniums are descended from Pelargonium cucullatum, a redflowered, frost-tender evergreen perennial geranium native to South Africa, and P. grandiflorum, a large-flowered, tender evergreen South African shrub. Modern Martha Washingtons possess strong, upright, woody stems to about 2 feet tall, clothed with stiff, green, pleated, pointy-lobed leaves; their blossoms, which are held in clusters in spring, the cooler parts of the summer, and fall, are richly colored in single or bicolor shades of burgundy, wine, ruby, reddish, or white. Outdoors they are winter-hardy only in USDA Zones 9b-11.
In Northern New Mexico, Martha Washingtons thrive best with afternoon shade, particularly in the summer. They do well under portales, on bright windowsills indoors, and as shade garden edgers. They tend to bloom best when slightly stressed, so keep them just moist but never soggy, and water them only when the surface of their soil feels dry to the touch. To encourage a longer bloom time, feed monthly with a medium-strength water soluble bloom fertilizer. Another option is to apply a time-release bloom fertilizer, such as Osmocote™ 14-14-14 (in the green bottle): one application will feed the plants for 3-4 months. Do not let the plants become root-bound, or they will dry out too quickly. If their roots have filled their pot, transplant them to a pot that is larger by a half-inch to an inch larger all around.
After the plants finish blooming, cut them back by about a third to keep them compact. To overwinter them, move them into the house or a cool greenhouse (50-60ºF) just before outside frost threatens. Give them full winter sun and reduce fertilization from monthly to once every 2 months.