When you plant your seedlings, move aside the hay to leave a couple of centimetres free of hay around each plant. Mulch inhibits the growth of unwanted plants which find it very difficult to make their way up to the sunlight through 12 centimetres of hay. You will find that those which do achieve this are spindly and shallow-rooted and are easily pulled out. Similarly, on a cold morning the temperature of the soil under the hay is well above the air temperature. This ensures that your plants are subject to a more constant root temperature which, in turn, aids their growth.
Worms love mulch because it creates the conditions under which they thrive.
This makes digging your garden superfluous. As mentioned above, your hay will decompose and provide valuable nutrients to your soil. In addition however you may decide to use the mulch as a convenient cover for composting some of your kitchen waste. This method of composting breaks down waste very quickly, provides food for the worms, and gives a continuing and varied source of enrichment to your soil. There's something very exciting about trying new techniques.