Will a shortwave longwire antenna work if it is laid on the ground or does
it have to be hung in the air from a tree or something similar?

Up to 3 Mhz it probably works better than a higher antenna.
There is a beverage antenna which is highly directional and normally suspended
18 inches to 2 feet above the ground and aimed at the target station. It is
also a quarter mile or more long.
It was used in WW2 to listen to Jap AM stations from Hawaii.
I built (?) an underground antenna boput 12" down many years ago, and as I
recall it was very quiet. But interesetingly the drop in noise was greater than
the drop in gain, so overall it was favotrable. Most of my DXing ws in the AM
band.

Your posting has made me think about a FRACTAL underground beverage.