I’m not sure what a “plum thicket” is…?
I live in KY, & I primarily model Appalachian-type scenery…We don’t have mesquite trees, but we have plenty of thicket!
I don’t know if this will be of any help to you or not, but for the last few years I’ve been using these plastic scouring pads to represent woodland scenery…I can get these at the grocery for a buck & some change…
I cut & pull these things apart into random clumps roughly the size of the palm of my hand…some are a bit larger, some a bit smaller…random is the key word here…
I spray paint them either flat black or dark brown, & then glue on some ground foam…
I also get twigs & weeds from outside, & glue ground foam to them…
I mix these up together to make something that hopefully looks like eastern woodland scenery…
That photo is from my HOn30 layout, but I’ve used this same method in both HO & O scales…
I can get about 30 or so clumps of foliage from each 6-pack of scouring pads…