Something like this is what I meant for wetting the ground
http://www.homedepot.com/p/RL-Flo-Ma...401P/100188485.
As long as it is a quality product you can throw it in your trunk with some water in it without worry and spray down wherever you are going to do your photos. It'll make using less water cover more area, just spray down before you park the car so you don't get a dry spot under the car.
What are you using for light now? Or are you still using the iphone light?