If you can find a woodworker in your area I am sure they will give you sawdust. I am into woodworking and use my shavings all over the yard. Just stay away from walnut. Sawdust keeps weeks down and will hold moisture in the ground.

I put lava rock. It adds a bit of color. Of course you need to be careful about turning your ankle and you can't very well kneel in it, but it's pretty.

My concern with wood mulch is insects. I've already caught a couple of carpenter ants wearing tiny little bibs and licking the fresh wooden planters. I'm afraid that a bed of wood chips would look like an all-you-can-eat-for-$5 buffet to them.

My city gives away free wood mulch, but the concern there is what are you getting along with the wood? These are chipped-up trees that have been cut down around the city, but why were they cut down? What are you going to introduce into your yard along with the mystery mulch? Termites? Asian long-horn beetles? Poison ivy?

I've all but decided to go with rubber mulch. The main reason, aside from those above, is maintenance. I can't expect my aging mother to haul sacks of wood mulch every year. Supposedly the rubber bits stay in place and can be "cleaned" with a leafblower.

I wouldn't use it IN the planters, just around them to cover the landscape cloth.