Do dead locust leaves mat down or do they decompose? Can I use them on my gardens over winter?
Thank you for contacting the Toronto Master Gardeners with your inquiry regarding your locust leaves.
As the temperature drops, trees, shrubs and perennials are preparing for the long winter ahead, this means falling leaves. During the summer, trees were busy absorbing minerals from deep in the soil and a large portion of these minerals end up in their leaves. Adding these mineral packed leaves to you soil will add some of these minerals and nutrients back into the soil. Aeration of heavy clay soils, prevention of sandy soils from drying out too fast, soaking up rain water and increase moisture retention are just some of the other benefits of adding leaves to your soil.
In general, for leaves to be effective, they must be chopped or shredded first. Large, broad leaves such as maple, oak and elm can become matted and may smother plants if they are not shredded, blocking air and water from reaching the roots. Shredding the leaves will break them up to prevent this problem, and will help them decay more quickly to improve drainage, feed earthworms and enrich the soil. Composting diseased leaves, such as maple leaves with tar spots, is a controversial question: here are two links which provide information on both sides of the argument:
Diseased Leaves in Mulch, Using Diseased Leaves In Compost
However that being said, very small leaves such as those from locust trees are already small enough to be used as mulch and do not need to be broken down,but shredding can still be used if desired. You never mention if you are composting honey locust leaves or leaves from black locust. Leaves from both tress are okay to mulch and compost, however it should be noted that the inner bark, roots, and twigs of black locust are poisonous to animals and humans when ingested: https://www.cbif.gc.ca/eng/species-bank/canadian-poisonous-plants-information-system/all-plants-common-name/black-locust/?id=1370403267062
Once you are ready, layer the leaf mulch 2-3 inches deep around perennials, shrubs and trees, spreading them as far out as the drip line. Avoid piling the mulch right up against stems or trunks, which could lead to fungal diseases and rot.
Here’s a link with some helpful tips about using leaves for mulch (compost): Composting Leaves- A Worthwhile Challenge