September 2023 - Sourwood

Common Name: Sourwood; also called sorrel tree or lily-of-the-valley tree.

Botanical Name: Oxydendrum arboreum. The genus name comes from the Greek words “oxys” meaning acid and “dendron” meaning a tree and refers to the bitter taste of sourwood leaves.

Native Range: Its native range extends from southwestern Pennsylvania to Florida and Louisiana. Never very common in the wild, it may be found on well-drained gravelly soil on ridges above streams. In the Appalachian Mountains, it is found on rocky wooded slopes, often growing in combination with other heath family members (e.g., azaleas and rhododendrons) that share the same acidic soil preferences. Sourwood grows in USDA Hardiness Zones 5 to 9 and is suitable for planting in New Jersey, which lies just a bit north and east of its native range.

Height: In cultivation, sourwood typically grows 20’-25’ tall with a straight, slender trunk. In the wild, it may reach 50’-60’ tall.

Spread: Its crown often is narrow and oblong, typically 6’-12’ wide, but can extend up to 20+ feet.

Form: A deciduous understory tree. Sourwoods have conical or rounded crown with spreading drooping branches, lovely in outline. Open-grown trees are pyramidal with a conical or rounded crown and are branched to the ground. If grown in shadier sites, the trees develop a taller, more columnar crown on limbless trunks.

Growth Rate: Slow. Grows 14’-15’ over a 12-to-15-year period.

Sun: Best in full sun; can tolerate part shade but with somewhat diminished flowering and fall color.

Soil: Medium moisture. Best grown in acidic, moist, organically rich, well-drained soils. Intolerant of drought.

Leaf Description: Alternate, simple, elliptic to lanceolate, finely toothed or entire, lustrous, drooping green leaves (3”-8” long) that taper to a point at the tip.

Flower Description: Sourwood blooms from the ends of its twigs after they have finished growing for the year and after the leaves are fully formed. This is long after most trees have finished blooming. Perfect, waxy, urn-shaped, lily-of-the-valley-like, white flowers emerge, June to July, in slender, drooping, one-sided terminal panicles (4”-10” long), giving the tree an overall impression of weeping softness. Flowers have a slight fragrance.

Fruit: Pale yellow fruit capsules are almost as showy as the flowers. These darken and remain well after leaf drop in fall.

Bark Description: The thick, grayish-brown to dark brown bark on mature trees is divided by longitudinal furrows into ridges covered with blocky scales. These irregular ridges have the shape of shattered auto glass. The bark can be tinged with red.

Fall Color: Leaves produce consistently excellent fall color, typically turning crimson red and burgundy on the upper surface and more white-washed beneath. Can have variability, with touches of yellow mixed in. In addition, as the flowers transform into green peppercorn-sized capsules that ripen to brown and silver-gray in September and hang in pendulous chains, these panicles contrast well with the leaves' fall color and provide continuing ornamental interest even after leaf drop into winter.

Wildlife Benefit: The flowers of sourwood are a vital source of midseason nectar favored by many bees, wasps, beetles, and flies. It is particularly valuable since they bloom in the heat of summer, a flower-poor time of year.

Tolerates:
• Tolerates deer.
• Does not tolerate urban pollution.
• Does not tolerate full shade.

Possible Disease and Insect Problems: No notable insect or disease problems. Leaf spot and twig blight infrequently occur, neither of which is serious.

Uses: Sourwood is a small tree that is an excellent specimen plant with multi-season interest for lawns, patios, shade gardens or woodland edges. It is an attractive ornamental throughout the year.

Where to be found on municipal property:
• A 18” Sourwood can be found in front of the Graduate College on the Princeton University campus.
• A 10” Sourwood can be found along Boudinot Street.

Additional Facts:
• Sensitive to pollution, soil compaction, and root disturbance, sourwood is not for urban areas.
• Sourwood honey is a highly prized product.
• Sourwood roots are enmeshed in a mycorrhizal network. The fungi can be fed with a yearly helping of rotted bark or leaves.
• However, mulch at the base of a tree should never be placed up against the root flare. Allowing the mulch to rest in contact with the basal bark puts the tree at risk of injury from rot diseases. Sourwood is among the trees that are particularly vulnerable.