Crushed lava rock (ugh!)
Reading back over the postings re: landscape fabric I felt I should pass on an experience I have had with red lava rock.
I didn't buy it for mulch but for curiosity - 1 bag only. If I ever get to post pics of my gardens you will note mostly bare ground. I find time in the shade using a shuffle hoe peaceful. After buying the lava rock I found myself in a position where I was going to be potting up lots of divisions of hosta for a new garden the following year. I decided to add a layer of the lava rock at the bottom of 4 larger pots for drainage.
The results: disasterous! Three of the four did not survive the following spring until planting time. Sickly little eyes opened and then nothing. When I tried to repot them and saw the rotted roots I threw them out. The single survivor was Sagae which was half the size of the potted division. Of 30 other divisions potted using my regular method I lost none.
That seems more than a coincidence and I consider the lava rock the cause of failure of the hostas to survive. I find it hard to believe the processor of the lava rock would go through the expense of adding a herbicide to it to control weeds. Do they do this? I was so irritated my customary curiosity was bypassed and I never checked pH of the soil. More probably the rock raised the soil pH way above what the tender hosta liked.
When potting up a lot of divisions like that I use drip coffee filters in the bottom. They are biodegradable and keep the soil in, and being designed to let water pass through when brewing a cup I don't find my pots waterlogged.
One of my SOLs likes everything neat and tidy. My daughters the gardener but he edges all the gardens and used the rubberized mulch on top that looks like red lava rock. I give it to him, it does look neat but I like hosta in a natural setting. I like all my gardens to look natural, which is hard with hosta when you need to leave so much space to grow out.