The USDA hardiness zone map for Maryland shows three hardiness zones within the state's border. Zone 5 is a tiny wedge in Maryland's mountains. Parts of the coast along the Chesapeake Bay are zone 7b (the warm range of zone 7). Zone 6 and the rest of zone 7 are sandwiched in between. Common flowers of Maryland may be different from one region to the next, due to climate and temperature range, yet all regions share certain plants that are hardy and grow well.
The black-eyed Susan (Rudbeckia hirta), Maryland's state flower, grows well in sun. It naturalizes well and the flowers hold their color for a long time. Vaughn Deckret of The Maryland Natural Resource says it should be on the top of the list for anyone who wants to start gardening with native plants. Butterflies are drawn to black-eyed Susans, and goldfinches like the seed, he says. It blooms from mid-summer into fall, and is hardy in all areas.
Plant breeder Robert Darby hybridized the first hardy hibiscus in Maryland in the 1950s, breeding plants hardy to zone 4, according to Jennifer Schultz Nelson in an article for the University of Illinois Extension. He gave his hybrids Maryland names: Lord Baltimore (Hibiscus "Lord Baltimore") and Lady Baltimore (Hibiscus "Lady Baltimore"). Lord Baltimore has large, dark-red flowers and grows 4 to 5 feet tall; Lady Baltimore is light pink with a dark eye-zone and grows to 4 feet. Although it's hardy to zone 4, provide winter protection in zone 5.
Butterfly weed (Asclepias) is also native to Maryland. Vaughn Deckret calls the plant "a butterfly magnet," noting that it is not unusual to see several butterflies at once on the orange flowers. Butterfly weed flowers are lightly fragrant and bloom in May and June. A mature plant grows to 3 feet high. Plant in full sun and a well-drained soil. Butterfly weed is hardy in all areas of Maryland.
Russian sage (Perovskia) is often used in Maryland landscapes, even planted in masses in highway median strips. The plant seems to thrive in the hot summers; the silver-green foliage and bluish-purple flower spikes give gardens a cool feel. Russian sage blooms in summer and into fall. It prefers full sun and well-drained soil, and is hardy to zone 4.
Vaughn Deckret recommends cardinal flower (Lobelia cardinalis) as a good native for Maryland gardens. It's brilliant red flower spikes attract hummingbirds. Cardinal flower likes moist, evenly-wet soil, and can tolerate some shade. It is hardy in all areas of Maryland.