I have been thinking about what to use for mulch for the top of the container. I was thinking about that wood mulch that takes a long time to decompose. 

I have been fascinated with lava rock recently. I was sitting in the back room a few days ago and seen a bucket that contained some lava rock from the grill we used when it was gas. I picked one up and found it to be light weight and porous. Of course they are good sized. I did not know they sold this stuff at garden centers in different sizes from lava sand up to lava stone. Shows you how much I have been in garden centers over the past years.

I think I will go with this for potting mix cover around the plants. https://www.homedepot.com/p/Vigoro-0-5-cu-ft-Decorative-Stone-Red-Lava-Rock-440897/100427379