Good idea, Dom, I hadn't thought about that.
@Dan, what you are seeing is about 1/4" of perlite I put on top of my mix. I find that the water seems to soak into the mix better when a layer of perlite is on top. I'm sure there's some kind of scientific explanation for this, but ... I'll leave that to you ;)
@Cathy, the tarp is retractible. I usually check the weather in the morning so I know what to wear/expect/etc., I intend to slide it over and tie it back on rainy days to prevent collapse.
Thanks for the compliments and ideas!