The Mid-Atlantic region typically includes the states of Delaware, District of Columbia, Maryland, New Jersey, New York and Pennsylvania. From a climate standpoint, in relation to gardening, the Mid-Atlantic region has been extended by the National Gardening Association to include southern Ontario of Canada.
According to this map of the Mid-Atlantic states, the region lies in USDA Plant Hardiness Zones 5 through 7 and in AHS Heat Zones 3 through 6. Stretching over a long expanse of territory, the Mid-Atlantic states are home to a wide range of weather and climate. The Atlantic Ocean along the coast or the Great Lakes in northern New York and Pennsylvania moderate temperatures. The diverse topography of the area, from the mountains of Maryland to the beaches of Delaware, also contributes to diverse weather patterns, including sea breezes, mountain-induced downslope warming patterns, mountain snows, and valley and river fogs. However, generally speaking, this region enjoys 4 distinct seasons characterized by mild springs, hot and humid summers along the coast, but cooler in the mountains, mild falls, and chilly winters with some snow.
The frost free growing season ranges from 144 to 231 days with the last frost dates ranging from late March (Interior Plains; Chesapeake Bay, Southeastern Pennsylvania, Southern New Jersey) to late May (Interior Pennsylvania and New York; St. Lawrence Valley) and the first frost dates starting as early as mid September (Inland Plains of Lake Erie and Lake Ontario) to early November (Interior Plains; Chesapeake Bay, Southeastern Pennsylvania, Southern New Jersey).
While the climate of this region provides enough natural moisture, sunlight and summer warmth to successfully grow a wide variety of plants, there are still a few challenges for gardeners such as harsh winter temperatures, late spring frosts or unexpected early fall cold snaps, long dry spells or torrential rains. Selecting plants suited to the diverse Mid-Atlantic climate will be a key step to success.