Thanks for submitting your follow-up question re Black Locust mulch to the UVM Helpline.   Due to the holidays, it is taking a bit longer than normal to respond to your question.   

One of the experts at UVM is currently doing research into toxalbumins that are in black locust.

In the meantime, I found a reference regarding the toxicity of black locust which you might want to consider if you have young children playing in the area you plan to mulch.   

From the Texas AG&M "Common Poisonous Plants and Plant Parts" document: https://aggie-horticulture.tamu.edu/earthkind/landscape/poisonous-plants-resources/common-poisonous-plants-and-plant-parts/

Thanks for your patience as we continue our research into your question.

Hello again Bob and Happy New Year!

We have researched your question about Black locust (Robinia pseudoacacia L.) and the potential for composting ornamentals and vegetable crops.

As you know, the bark and leaves are toxic to humans and animals if consumed; it is especially an issue for horses as it can cause acute toxicity. Whenever we hear of plant toxicity, we need to take a moment of pause and recommend caution.

Our research was inconclusive about the persistence of toxalbumins (robinine and the other toxins in black locust) when decomposed via composting. The literature does suggest that heat will kill the toxins but in a cool compost situation or natural decomposition, you would not have reached hot enough temperatures that are reached in a hot composting situation.

Therefore, out of an abundance of caution, we would not recommend applying the material to edible crops.

For ornamentals, here’s what I would suggest – conduct a germination test in the compost to see if the material will affect plant growth. Basically, you pot up the material with some seeds and see if they germinate. More information on conducting this type of test can be found
at: https://content.ces.ncsu.edu/conducting-a-bioassay-for-herbicide-residues
(note, this article is testing for herbicides but the methods used are the same). If you get poor germination, then don’t use in your gardens.

On the topic of black locust, as you may know, it is considered an aggressive, and is on the Vermont invasive watch list, see https://vtinvasives.org/invasive/black-locust. Therefore, it is important to avoid spreading seeds, pods and flowers – keep this in mind as you are contemplating using this material.

Best of luck with your gardens this year!