Nettles and dandelions are perennial weeds, which last from year to year. Other common perennial weeds are ground elder, bindweed, thistle and willowherb.  All perennial weeds must be dug out before mulch is applied or they will continue to flourish. The way that mulches suppress weeds is by stopping light reaching weed seeds, thus preventing germination. This is most effective with annual weeds such as chickweed and groundsel. It is a good idea to hoe off these weeds before applying Bark Chips or any other mulch. Any annual weeds that do germinate through the mulch will be easy to spot and remove.