Home gardeners take much into consideration when planning a garden -- sun, shade, soil conditions, water. But the first, most important consideration is the hardiness of a plant -- whether it will thrive in your particular climate. In the United States, temperatures and conditions vary widely -- so much so that the U.S. Department of Agriculture divided the country into zones based on average low temperatures. In Maryland, the planting zones range from 5b to 8a.
USDA Zone 5b
Maryland can have cold winters. Temperatures can reach average lows of 15 degrees Fahrenheit below zero during the winter in the Allegheny mountains, although most of the state remains warmer. In fact, USDA zone 5b only covers the far western tip of the state. Plants that tolerate the cold winters of USDA plant hardiness zone 5 include astilbe (Astilbe spp.), which is hardy in zones 3 through 9, and phlox (Phlox spp.), which is hardy in USDA zones 2 through 9.
USDA Zones 6a and 6b
USDA zones 6a and 6b cover much of the northern part of Maryland, including Hagerstown and parts of Baltimore. Zone 6a is defined as having average low temperatures between negative 10 and negative 5 degrees Fahrenheit, and zone 6b has temperatures between 0 and negative 5 degrees. Plants that thrive in such conditions include asters (Aster spp.), which are hardy in USDA zones 3 through 8, and the butterfly bush (Buddleia davidii), which is hardy in USDA zones 5 through 10.
USDA Zones 7a and 7b
Unlike USDA zones 5b and 6, USDA zones 7a and 7b cover nearly the entire state of Maryland. These zones stretch from Frederick to the Potomac and beyond. USDA zone 7a has average low temperatures between 0 and 5 degrees Fahrenheit, and 7b features temperatures between 5 and 10 degrees. Plants that thrive in USDA zones 7a and 7b include rosemary (Rosmarinus officinalis), which grows best in USDA zones 6 through 9, and hyacinths (Hyacinthus spp.), which are hardy in USDA zones 4 through 9.
USDA Zone 8a
USDA planting zone 8a covers the very southernmost tip of the state -- namely Crisfield and the islands to the west of it. The average low winter temperatures in USDA zone 8a range between 10 and 15 degrees Fahrenheit. Plants that thrive in these temperatures include grapes (Vitis spp.), which are hardy in USDA zones 2 through 10, and gladiolus (Gladiolus spp.), which are hardy in USDA planting zones 6 through 10.