P.S. here is a comment I found elsewhere on line and it has been our experience:

"Both Timbertech and Trex are made from plastic and wood sawdust. Both turn gray with the weather, not to a "silver" as claimed by their manufacturers, but to an ugly musty gray. Both products are susceptible to speckles of black mildew because the resins leach out at the surface and this exodus forms surface micro-pockets. Now the wood chips are exposed and the chips, being organic, bring on the mildew which then thrives in these pockets!"