I am a carpenter and a friend of mine and I worked on the same job last year and we did MMA type and mostly self defence training on every thing we could think of. Grass/dirt/gravel piles/plywood floors/in the back of trucks[not moving tho]/ and no floor joist before plywood was put on[about all you can do there is punch each other there]/and on concrete.
I recomend training on all kinds of surfaces in all kinds of weather and clothing, it really opens up your mind to the real possibilities of a real fight and is interesting and alot of laughs.
Some of our findings; compare to your findings
1 grass itches
2 with work boots on its hard to escape foot locks
3 work jackets are a choke looking for a place to happen
4 throws on a hill are often spectacular
5 you must be mat tough first, throws hurt on the hard surfaces
6 freaks out your co-workers, but there really nice to you from then on.
In all seriousness, we train at a well known academy with 10 pro fighters and have competed ourselves and have trained 10 years pluss. If you have trained with punches you know how that will effect how you play in your guard, this will open your eyes to a whole new world and how you look at a fight. Really adds a sence of urgency to consider your stradegy!
In keeping with the thread, sure is nice to train outside in the fresh air, even if the grass itches!