Black Locust: A Tree Crop for Organic Farms

Written by Joseph R. Heckman, Ph.D. - Professor of Soil Science, Rutgers University, NJ

Black Locust, Robinia pseudoacacia, is fast growing tree with many attributes that make it a valuable tree to grow as a crop. This species is well adapted to the Mid-Atlantic region. It is an especially valued for organic agriculture because it can serve so many useful ecological functions. In a silvopasture system it provides filtered shade and shelter. As a legume it adds nitrogen to soil. The flowers in spring provide nectar for bees and Locust Blossom Honey. When harvested as a mature tree the decay resistant wood is a natural alternative to pesticide treated wood for fence posts and other uses as durable lumber.

Black locust should be more widely grown as a crop because the USDA-NOP standards require that: “The producer must not use lumber treated with arsenate or other prohibited materials” (NOP 7 CFR 205.206 (2015). Furthermore, because organic livestock production emphasizes pasture feeding, there is increasing need for perimeter fencing which can use locust fence posts. Thus, natural untreated locust wood products are highly valued for organic agriculture.

Currently some newly established organic pasture-based farms are reporting a short supply of available black locust wood post for fencing. Black locust wood is very decay resistant with an effective useful life as a fence post of 50 years or more.

Black locust is a very fast-growing tree that casts only a light shade such that it functions well in silvopasture systems. The filtered sunlight along with biological N fixation supports the growth of pasture grasses. The tree is easy to establish. After only about ten years of growth on good sites, the trunks can be harvested as round fence posts. Within about twenty years they may be processed with a sawmill into lumber or squared fence posts. After harvest, the trees regenerate quickly from stump sprouts. Thus, once established, there is no need to replant this regenerative tree.

Locust wood makes an excellent fence post because it is very strong, durable, and decay resistant. Also, because there is very little swelling or shrinkage with changes in moisture, black locust wood is a good alternative to building with treated lumber.

The chemicals found in treated lumber may include mineral elements such as chromium, copper, and arsenic or toxic organic compounds associated with creosote. The chemicals found in treated lumber can leach out of the wood and pollute the soil. Also, during fence installation shavings from treated wood often litter the ground and contaminate the soil with toxic elements chromium and arsenic. Avoiding soil contamination is a big advantage of using natural wood products such as black locust.

Besides black locusts, Osage orange is another species worth considering for fence posts. An advantage of black locust is that this species grows relatively fast and straight to produce a usable log within a reasonable time.

Other agricultural uses spurring demand for locust posts are trellises for wine grapes, garden stakes, and tall structures for growing hops. Besides agriculture, black locust lumber is becoming an increasingly popular wood for domestic outdoor uses such as decking, patios, and playground furniture.

As already mentioned, black locust wood is very hard and decay resistant. Because the wood is so hard, pre drilling holes for nails, screws, or staples is often necessary. Usually, the sapwood on black locust is relatively thin but less decay resistant. The heartwood is the most decay resistant.

When sawmilled or processed into 6x6 inch, a post 8 ft. long may sell for $25. Round unprocessed posts may sell for $10 to $15, depending on size and quality. Longer poles for trellises may be worth several times that amount. Lumber from black locust may retail for $5 per board foot. Logs unsuitable for posts or lumber make excellent firewood which when seasoned may retail at $250 per cord.

Black locust, because it is a nitrogen fixing legume, can grow quickly on disturbed sites or other low organic matter content soils without added N fertilizer. Although locust is capable of re-vegetating poor field sites the soil must have good drainage.

Strongly acid soils, very sandy soils, and poorly drained soils are not suitable sites for growing black locust. It can be planted on poor sites to help to remediate gullies and soils with low organic matter content. However, it will grow much faster on good soils. Well drained sandy loams, loams, and silt loams with soil pH levels between 6 and 7 are good sites. On good soils, fast-growing black locust will produce usable wood within ten to twenty years.

New plantings can be established from seedlings purchased from a nursery or they may be propagated from collected seed. When collecting seed look for specimens that grow tall with a straight trunk.

When black locust trees are harvested, they do not need to be replanted. They will very quickly regrow sprouts from the stump. The second harvest will grow back even faster from stump sprouts to reach commercial size logs. Stumps should be cut low to the ground. The multiple sprouts that are likely to emerge from each stump should be pruned to allow only a single stem to regrow into the new tree. Stump resprouts may regrow 5 to 10 ft in the first year after harvest. Logs should be harvested in the fall or winter months. Spring or summer harvests are not optimum for tree regrowth.

In a silvopasture system, when trees are young, they must be protected from livestock with some type of fencing. Although goats can safely consume locust foliage, the tree is regarded as toxic to equine. If the trees are pruned to remove side branches this produces a clear trunk with fewer knots in the wood. Pruned branches with leaves intact may be fed to goats.

Horses should not be allowed to consume black locust foliage or the bark from black locust. When the locust posts are used for horse pasture, the bark must be removed.

Locust trees tend to grow straight trunks. However, sometimes black locust is attacked by an insect known as the locust borer which damages the stem. Damaged trees typically recover but they are more likely to grow crooked. These trees might not be suitable for lumber, but regardless of tree shape locust makes excellent firewood.

The locust borer insect prefers to lay its eggs on tree trunks with exposure to sunlight. Cultural practices to shade the trunks is believed to reduce the amount of injury from locust borer. Close space plantings or integration with other forest tree species are strategies for producing more shade on the tree trunks.

In late summer, leaf miners may attack the foliage. In severe infestations, the foliage may turn brown but there is no easy way to control this pest.

One of the most serious pests of black locust, or any young tree, is attack from deer. Young trees have small thorns, but this is not enough protection from grazing. During the spring, deer especially like to graze on the young fresh shoots of black locust. This greatly sets back upright growth and repeated grazing can kill trees. In the fall, male deer rub on young trees and injure the bark. Some type of deer fencing, or deer repellent are necessary to prevent deer damage at sites with high deer populations.

Black locust will tolerate acidic low fertility soils but will grow faster when soil fertility is improved. Begin with a soil test. If the soil pH is less than 6.0 apply enough limestone to raise soil pH to 6.5. Growth may benefit from a broadcast application of compost before planting

Spring plantings of one or two-year dormant seedling trees that are about 1 - 2 ft typically transplant easily with a high survival rate. Planting in rows spaced 8 ft apart and 8 ft spacing within rows, creates a square grid pattern for a population of 680 trees per acre. This spacing allows for easy passage of a mower for weed management. A relatively close spacing is considered desirable for purposes of promoting upright straight growth and shading of tree trunks to minimize borer injury.

Young trees that fail to thrive or are not growing straight should be thinned from the grove in the second or third year after planting to reduce competition among trees. Some trees will naturally die off if the plantation is allowed to remain overcrowded. Black locust without too much competition among trees on a good site are capable producing a 6-to-8-inch logs in about ten to fifteen years.

A 1930, publication by the USDA Forest Service provides some older cultural advice on growing black locust (Web search for USDA Farmers Bulletin No. 1628).

Joseph Heckman, PhD is Rutgers University Professor of Soil Science and teaches courses in Soil Fertility and Organic Agriculture. For nine years he served on the board of Northeast Organic Farming Association-NJ. He currently serves on the board of the Raw Milk Institute. On the family farm in Ringoes, NJ he raises grass fed Jersey beef, black locust trees and USDA Organic Hay Certified by PCO. Email: heckman@njaes.rutgers.edu