Having had a G scale outfit for some time before a house move, I can confirm that Mother Nature tends to get quite heavily involved even in that much larger scale. Unfortunately, while her efforts at assisting with scenery can be managed in a beneficial way in G scale, she doesn't understand OO! Moss on an OO scale layout will be overscale for one thing, but problematic if allowed anywhere near points! If it's allowed to grow on wooden boards it will encourage damp, but dry weather will kill it all off, cue an unsightly brown mess.
I knew someone who at one stage had a very large outdoor circuit for the same reason - full length trains (of 1930s era LMS carriages) - and he actually didn't bother with scenery at all on outdoor sections. He just used a plain circuit on narrow baseboards about waist level. It looked well overall and certainly gave his locomotives good exercise. He never mentioned how he managed track cleaning.