Covering up for winter

Should I cover over rhubarb and asparagus crowns with weed membrane or cut around them?  My asparagus didn't come up this spring and wondered if that was the cause.

Thanks for that.

I agree with Aidy (who's obviously had a busy day - is it raining in tropical Blackpool?) Its been sunny in Hull for a few days now - the sun shines on the righteous!! I put compost or manure round the rhubarb. Don't grow asparagus - its bad for gout apparently. My artichokes seem to survive winter without the need for straw etc, but a dressing of compost helps. However, I'm the only one who eats them.