Q.Walnut Tree Causing Damp Patches
I have a large walnut tree and now have damp patches like water damp on my ground is it the tree causing this.
Certified GKH Gardening Expert
Unfortunately, your photo did not come through. It is likely not the cause. It probably doesn't help, though.
Likely, under the tree, rain will not evaporate as fast as in the open. That may be what is causing the spots in your lawn.
Walnuts are also notorious for killing off certain plants and grasses, though.
Here is an article that will help:
https://www.gardeningknowhow.com/garden-how-to/info/allelopathic-plants.htm
https://www.gardeningknowhow.com/edible/nut-trees/black-walnut/black-walnut-compatible-plants.htm
https://www.gardeningknowhow.com/edible/nut-trees/pecan/pecan-tree-toxicity.htm