(if zoning OK..)Would the tenants pay more, get better tenant quality, or adequately save on turnover if you were to turn it into a garden for the tenants..?

Just spray is on with a pump sprayer, wait 10 minutes, and rinse with a garden hose.

Apartment is a garden level basement unit.  

I said, if my house is on fire and I have a garden hose around I think I knwo what I have to do..

My friend thinks this made the issue worse. 3 slab readings in 5 years plus all the normal signs in doors, windows, walls, floors indicate that it is getting worse.

It has a nice sized garden with two garden sheds.

The backyard was completely redone, new retaining walls, side garden with automatic watering, etc.

Unless the area is really really cold it’s not that hard to work indoors with a bullet heater running even part of the time.

You can’t expect tenants to maintain elaborate gardens anywhere, though.

The wealthier neighborhoods have more and better stuff to steal.One technique I've heard about for 40 years is the brazen daytime robbers dressed as movers who show up at a house with a large moving van and empty it out while the neighbors watch (it happened on the street where I lived in the Rose Garden neighborhood of San Jose, California).