Black locust

Botanical name

Robinia pseudoacacia

Type

Deciduous tree, thorny, nitrogen-fixing legume

Height

Typically 40 to 60 feet, trunks 12 to 30 inches across

Lifespan

Usually about 90 years

Suckers

Sprouts from roots and stumps

Hardiness

Zones 6b to 7b across the service area

A grove of black locust springing up around a lone tree is the signature of this species. It is easy to plant and hard to get rid of.

How do you identify black locust?

Leaves are compound with oval leaflets that fold at night. Young trees and shoots carry pairs of short thorns at the base of the leaf. Bark on older trees is thick, deeply furrowed and dark brown, with ridges that cross. In late spring it is covered in drooping white flower clusters, followed by flat brown pods.

• Compound leaves with small oval leaflets
• Paired thorns on young growth
• Deeply furrowed dark bark
• White flower clusters in late spring

Where does it grow here?

Black locust is a planted and naturalised tree on valley and foothill lots, including Reno (4,500 feet), Carson City (4,700 feet), the Carson Valley (4,700 to 4,900 feet) and Dayton (4,400 feet), in zones 6b to 7b. It takes poor, dry soil but grows far better with irrigation; the nearest NOAA stations record about 7 to 9 inches of precipitation a year. It is uncommon in the Tahoe basin.

What goes wrong with black locust?

The Forest Service says the tree grows fast but is short lived, typically about 90 years. It reports locust borer attack combined with drought killed 55.5 percent of black locust in the Oklahoma panhandle in the 1930s, and that in an ice storm study 36 percent of planted trees were badly broken. Dead or ragged crowns, small holes with sawdust and snapped limbs are the signs to watch. An arborist identifies the cause; we do not spray.

Sprouting is the second problem. The Forest Service says root and stump sprouting is more common than seedlings and that fire and cutting stimulate it. Removal without a plan for the roots leaves a thicket. Thorns on suckers are a hazard for tires and bare skin.

What does an arborist check on a black locust?

We look at how many stems the tree has and where they join, since multi-stem locusts split at the union under snow. We check the crown for dead twigs and borer holes, and the base for decay. We also look at where the suckers are coming up, because a thicket near a fence, septic field or neighbor's lawn changes the plan. Some lots are better served by removing the whole grove in one visit; some need only the dead trees cleared.

Pruning, removal and firewood

Dead wood and hazardous limbs come off at any time. Otherwise, late winter is the usual window for deciduous trees. Prune to remove deadwood and shorten long limbs, because broken limbs are the failure mode.

Removal is usual for a tree that is dying, split, leaning toward a house, or growing in a place where its suckers are a problem. Stump grinding takes out the stump; the sprouts can be cut back. See stump grinding and tree removal. A certified arborist assesses the job on a free estimate. For firewood, call the office; we do not publish wood types or terms on the site.