Your snake is not going to get mites just being in the grass. Mites must be specific to reptiles to affect your snakes and cannot live off a host for very long, so you aren't just going to find them laying about in the grass. Ticks are also very unlikely, as you'd be able to find one long before it ever attached itself to the snake. I have had snakes for 8 years and I take them all out and let them crawl in the grass (excluding the garter because he's too flighty) and I have yet to see a single mite or tick on any of them.
I also do not take my snakes out below 75F, and below 80F there must be a decent amount of sun (no overcast days) and little to no wind.