Oh, the wonders of mulching plants. What wonders you might ask? Well, first, the layer of old leaves, straw, wood-chips, or other mulchy material will help to keep water in the soil for longer instead of evaporating in the heat. This is especially important since I don’t have a sprinkler system worked out, so it could take about an hour to fully water the entire garden. I want that moisture to last as long as possible. Secondly, the mulch will suppress weed growth. In my particular case, mulching has a third benefit of (over time) lowering the pH. Both of the composts that I am using happen to be quite alkaline, or having a high pH. This is a bit of a problem, since most garden plants grow best in a slightly acidic soil. So, as the mulch decomposes and the acidic tannins leach out of the leaves and pine needles, it will help to regulate the pH at an appropriate level. Finally, at the end of the year the mulch will be turned into the soil and decompose, adding organic matter, nitrogen, and other minerals.
Here is a picture of a happy little mulched lettuce plantlet!