August 2017 Plant Profile: Oxydendrum arboreum

Oxydendrum arboreum is one of the larger plants in the Ericaceae, or heath family.  In the wild it is an understory tree of acidic soils and rocky slopes.  Sourwood favors conditions that rhododendrons and other heath family members do.  While the dark green leaves are glossy, it is deciduous—with some of the most spectacular and long lasting fall color or any tree.  Fall color is typically orange-red to crimson.  Leaves start to color in late summer and deepen as fall progresses.  They are often held into November.  Leaves are alternate, have fine teeth at the edges and are 5-8” long with a tapered tip.  Leaves also have a sour taste, which accounts for the common and scientific names.  Oxydendrum from the Greek oxys, acid or sour, and dendron, a tree.

This is one of the best late flowering trees for the garden.  In late June the flowers begin to form at the end of upper branches (or those in the most sun) and this is another showy feature of the tree.   Flowers resemble Pieris, or lily-of-the-valley shrub, and here its affinity to other Ericaceous plants is revealed.  Flowers are at their best in late July and August and they are followed by dry capsules which contain the seeds.  These cream-colored capsules hang in long racemes give the look of a somewhat skeletal hand in autumn.  The capsules hang on over many months.   It’s a good tree for Halloween.

Sourwood prefers some summer water unless planted in areas where things aren’t as droughty.  It can be somewhat aridity tolerant in woodland conditions.   Sourwood handles full sun in the Pacific Northwest, and generally colors better with more sun.  It prefers well-drained sites but is tolerant of clay soils as well.  It is generally propagated by seed and requires little pruning. Oxydendrum arboreum can be is found in most local nurseries that specialize in trees.  Sourwood was named a Great Plant Pick for the maritime Pacific Northwest.   It should be more frequently planted.  I’m always delighted when I see one in home gardens around town.

Common name:  Sourwood or sorrel tree

Family:  Hamamelidaceae

Location:  Sourwood can be viewed at both UW Botanic Gardens locations:  the Center for Urban Horticulture (CUH) and the Arboretum.  At CUH there are two young plants from 2005, growing adjacent to the Miller Library’s east wall.  There are many in the Arboretum.  Most are along Arboretum Drive E and date from the earlier days of the collection.  There are several on the west side of the Drive between the Graham Visitors Center and the Native Knoll and many at the east end of the Woodland Garden.  There are five near the west end of Loderi Valley and four on the east side of the Drive at the head of Rhododendron Glen.  And there are four in grid 35-3W on the east side of the Pinetum, above Arboretum Creek.  These can be seen coloring up nicely along Lake Washington Blvd E in the fall.

Origin:  Oxydendrum arboreum is native to the eastern and southeastern United States.  It ranges north into Pennsylvania and south into Louisiana and the Florida panhandle. Isolated patches are known from suburban Chicago counties, parts of Long Island and into Rhode Island.  It is the sole member of the genus Oxydendrum.  Sourwood was introduced to western horticulture in 1752.

Height and spread:  In the wild, sourwood can reach up to 50-70 feet though there are a few that have reached over 100 feet.  In cultivation it is slow growing, and most specimens will reach 25-30 feet after many decades.  It is generally a slender grower, usually half as wide as tall.

Hardiness:  Cold hardy to USDA Zone 5