That depends on many things starting with the condition of your soil. While one reason to mulch is to conserve soil moisture you would not want to mulch when your soil is too wet. I mulch in the fall, but then my soil drains quite well so I need not concern myself about holding too much moisture, rather I need to think about having enough. Generally any time the plants are growing quite well is a good time to mulch.

g'day littlejohn,

i mulch from when i first build the garden and keep it mulched continually from then on. i do use the green type mulches ie.,. spoilt hay or sugar cane mulch.

len

Here is a link that might be useful: len's garden page

Check out the discussion a few lines down entitled "when is best time to mulch".