To all Sawdust users:
Some folks think that because Cedar or other kinds of bark are used to keep weeds down, sawdust from those sources will be bad for plants. This is not the case. Bark and other types of mulch inhibit weed growth primarily by denying light to emerging weed seedlings.
On the negative side, mulches also encourage garden pests and diseases by giving them a cool damp place to live and multiply. We recommend you keep your garden clean, clear, and dry, except at the root zone of your plants. When you plant according to the Mittleider Method the close-planted vegetable plants will quickly shade the ground and minimize water evaporation, without any need for other ground cover.
Walnut sawdust is the only material – at least in North America – that we have found to be a problem for vegetable plants.