Black Locust Genetics & Management: What You Need to know
I’m on my way back from Hungary, having had the time to write this on the plane. Hungary is known for black locust, Robinia pseudoacacia, and I’d like to share a bit of what I’ve learned while working with this species over the past 10 years. I’ll run though how Hungary grows black locust currently, what improved Hungarian black locust genetics look like, and keys to success for growing black locust in the United States.
A Brief History
Hungary’s borders contracted after World War I, and they lost much of their natural forest. As a result, they now grow much of their wood in plantations: hybrid poplar, pine, and black locust. Black locust is a fast-growing hardwood that does very well in sandy soil. Land that was once dry grassland, the Hungarians have turned into locust plantations and locust-dominant forests.
Today, 20-30% of Hungary’s forest area is Robinia-dominant, creating tens of thousands of jobs in Hungary, stemming from timber production and locust honey. This is possible in part because locusts pests are not present in Europe. The locust borer, Megacyllene robiniae, does not maim and deform drought-stressed low-vigor trees, as it does in North America. The Hungarians can thus grow high-quality wood on low-quality soils. We can also grow high-quality wood in North America, in the right soils and if we manage our trees well.
There is a certain mystique surrounding “Hungarian” black locust – and at Propagate we aim to demystify agroforestry. 99% of Hungarian locust is genetically the same as any black locust we’ll find in the North America, but much of it is managed as a plantation and not harvested from wild stands.
How to Grow Black Locust
Growing locust from seed is economically viable, but presents an entirely different approach compared to starting with improved clones. Seedling black locust trees present high genetic diversity: some trees are curved and scraggly, while others will grow straight (pinnate growth form). Growth rate also differs greatly. Hence: Hungarian foresters plant at high density, and cull heavily.
Initial between-row spacing is around 8 feet, and in-row spacing is around 3.25 feet, for 1,673 stems per acre. Planting density is constant across soil types, and the following yields (shown above) are from a slightly-below-moderate site. By Year 2, 10% of the trees will have died, or have been out-competed and are ignored. In Year 7, a “cleaning” is done to remove 28% of the lowest-quality trees, sometimes for firewood, at an average of 2.36” DBH. The trees are often cut and left on the ground. As the years progress, foresters continue to low-grade the stand, always removing the thinnest trees. Year 12 presents the first fencepost harvest, removing another 33% of the remaining trees, at at an average of 4” DBH. Two more fencepost harvests occur. In year 17, 39% of the remaining trees are cut at 5.5” DBH. In Year 22, 36% of the trees still remaining are cut at 6.7” DBH. The final harvest occurs in Year 30-34, when any remaining trees are cut for sawlogs at an average of 8.7” DBH.
This management scheme presents between a 4 and 8% IRR, depending on attainable prices of firewood, fence posts, and locust lumber. The upside of planting seedlings is a lower cost per stem than planting clones, and higher in-species biodiversity. The downside is lower yields. Overyield, our agroforestry planning software, makes this math easy. We have several templates available which a user can select to generate farm-specific financials for seedling black locust.
Thoughts on Genetics
Improved black locust genetics consist of straighter growth form and higher growth rates. Straight trees, often known as Shipmast or Pinnate, yield more milled lumber per tree, and higher quality lumber than wood from a curved or forked tree. Propagate has selected a clone, which we’ve called “Daybreak,” that exhibits a straight growth form and a moderately high growth rate. It is currently outperforming the Steiner Group black locusts in our trial in New York State, by an estimated 50-60%. There are still better clones, which exhibit flagpole-shipmast-telephone pole straightness and 190% the growth rate of seedling locust – namely: Silvanus Forestry’s Turbo Obelisk.
Silvanus Forestry was started in the 1980s and founder Jenő Németh improved upon Hungary’s prior decades’ breeding work. From 50,000 seedlings, down to 125 clones, down to 70 clones, and finally down to a handful of multifaceted elite clones, we’re looking at a straight sawlog in 14 years, 30 feet of post wood from 1 tree after 10 years, and an IRR in the mid to high teens. Silvanus’ Turbo Obelisk tracts are planted on a grid, with 4 meters (13 feet) between trees, and thinning is not necessary. Agroforestry spacing of 6 x 2 meters (20 x 6 feet) and wider is also appropriate, with form and lift pruning being 100% imperative.
Different genetics necessitate different management, but proper site selection is always important. Moderately-drained to well-drained soil, with at minimum 30 inches to either the water table or a restrictive layer is ideal. Planting into beach sand or construction sand is not viable in North America, and 16 inches of rain during the growing season is necessary. The droughty summers of the Pacific Northwest are usually a no-go.
Management Tips
“You get out of it what you put into it” also rings true, across both seedlings and clones. Good management is essential, so let us walk through the three highest leverage points in terms of success with Robinia. First, when planting in well-drained soil, Silvanus highly recommends using an auger and not a tree planter, with the root collar of the tree 4-8 inches below the soil line. Second, controlling grass competition is important. Robinia is sensitive to herbicide, and thus cultivating next to the tree row in the early years is the best option. Mowing is an inferior management strategy. Third, pruning is the highest value added activity in the game. We’d go as far as to say: don’t bother planting black locust if you’re not going to prune it. Pruning your timber trees presents the same necessity to continuously overcome present bias, the idea that we think we’ll “have more time” in the future, when in reality the opposite is true. Pruning timber trees is easy to learn and rewarding to do.
Conclusions
Hungary has produced excellent results with black locust, both with seedlings and clones. Clones yield double the return on investment (IRR), compared to seedlings, with best case scenarios of 16% and 8% respectively. Growing black locust is more nuanced than growing softwoods destined for framing lumber: black locust wood prices are up to six times greater than pine, and thus management interventions can carry six times the weight.
At Propagate, it used to take 40 hours to plan a farm with GIS programs, spreadsheets, and custom write-ups. We’ve since built software, Overyield, that produces that same farm plan in less than 4 hours. We have a number of black locust templates to choose from, along with dozens of other tree species with which to analyze how forestry and agroforestry can benefit a piece of land.
We’ll be sharing more on the economics of Black Locust next week. If you are interested in learning more about this species, or if it’s a potential fit for your land, contact us and we’ll be happy to assist.