Who said anything about rot? Except you of course.
Please do clarify your "root issues" comment, then. You love to add content, and troll responses, but seldom do you engage an actual discussion. As someone who has been top dressing containers for as long as I've used containers, I'm intrigued to tap your vast knowledge, so that I can know what (and why) I've been doing wrong for so long now...
I'm also curious to know how you qualify/quantify grass clippings being less "porous" than hay, in particular. Not that I'm not fond of hay - but it's not free, in most cases. Does it not break down in almost the exact same way as any other stem/blade plant? (i.e, the fluff soon gives way to a compacted mass) And with all of these "root issues", caused by "non-porous" top dressings, what advice would you give someone who does the unthinkable, and uses something like... leaves?
(OP: excuse the sarcasm - I'm rather fond of oak leaves, as top dress, when I can find them)
Maybe I should also ask why you feel the need to second guess the OP, on the issue of "locking in" moisture? The location is listed as Australia, and there is a lot of arid space in Oz. Do you think it's wrong to try to slow the escape of moisture in an arid climate? Probably would have been good to ask specific location, if not answering the question directly. I'm disappointed that you're off your game. Usually, I'd expect to see you pulling a weather report for every area of the continent before replying.