Clay pots may make attractive garden accessories, but clay soil can be a gardener's biggest problem. Clay soils consist of microscopically fine, highly water-absorbent particles. Wet conditions can compact these particles into an impenetrable, glue-like substance. Many evergreen trees aren't up to the task of breaking through heavy, wet clay. Their roots can't spread far enough to get sufficient oxygen and nutrients. A handful of evergreens, however, manage well in clay.
Norway Spruce 'Acrocona'
Norway spruce (Picea abies) is a native European evergreen. Happiest in cold climates, it's hardy to United States Department of Agriculture Hardiness Zone 3, where winter lows flirt with minus 40 degrees Fahrenheit. Trees in the wild can grow as high as 200 feet; cultivated trees are typically 40 to 60 feet high. Gardeners with small yards and clay soil have the option of planting the 'Acrocona' Norway spruce cultivar. This dark green spruce grows slowly, reaching a height of 5 to 10 feet after 10 years. Fully grown plants measure up to 20 feet tall. Acrocona's clusters of spring cones bring red accents to the landscape. The cones age to light brown in summer. This sun-loving tree tolerates clay. It thrives, however, in moist, well-drained soil, notes Ohio State University Extension horticultural agent Jane C. Martin.
Austrian pine (Pinus nigra) is a European native evergreen hardy to Zone 4. This 40- to 60-foot tree progresses from symmetrically pyramidal to flat-crowned and open shape as it grows. Older trees have distinctive plates of nearly black bark. Pairs of glossy, dark green, rigid 3- to 6-inch needles line their heavy, spreading branches. Susceptibility to Diplodia tip blight makes this evergreen unsuitable for parts of the United States where that fungus is established. States where the disease is present extend from Maine south to North Carolina, and west to the Dakotas and Oklahoma. California also harbours Diplodia, according to USDA Forest Service. Austrian pine tolerates clay and alkaline (pH above 7.0) soils. It grows best in full sun.
Soaring Ponderosa pine trees have topped 250 feet in the wild. Cultivated Ponderosa pines often exceed 100 feet. These evergreens are answers for problem clays soils, note University of Minnesota extension educator Jeffrey Gillman and his colleagues. A lack of branches along the lower halves of their trunks gives the pines a top-heavy appearance. It also exposes their colourful, yellow-orange-to-rust, exfoliating bark. The twigs' long, yellow to grey-green needles occur in clusters of two or three. Dense, heavy wood makes drought-tolerant Pondersa pines commercially valuable lumber trees. These evergreens need locations where their size won't threaten surrounding trees and structures. They grow in full sun to full shade and acidic (pH below 7.0), sand or gravel-based loams, as well as clay.