I just started putting in my garden this year. I've made all of my beds, planned the areas, and planted rhubarb and blueberries, but not any veggies. (I'll do some overwintering carrots and garlic, put plan to garden in earnest next spring).
I planted four blueberries along my south-facing fence, and a rhubarb as well. Everything I have read says it is important to mulch blueberries so that the ground stays moist and cool, so I put down 5 inches of pine shavings over the entire blueberry bed.
Today it is HOT for our area: 95 degrees F. The last three days have been hot and dry. I'd watered my blueberries three days ago, and went today to make sure they were not drying out too bad. when I stuck my hand under the mulch and into the dirt I was startled at how cool the soil was!
I went and got my trusty thermometer to compare the soil temps. First I measured the un-mulched rhubarb (same bed, same watering schedule, no mulch).
1.5 inches into the dirt it was 125 degrees F!!! The sun really soaks into that soil and holds in the heat. The soil was bone dry.
1.5 inches into the blueberry plant's dirt was 75 degrees. The soil was pleasantly moist. Five inches of shavings made a 50 degree difference and held in the water for the roots perfectly!
4 inches into the dirt around the rhubarb it was 95 degrees with very little moisture. 4 inches into the dirt around the blueberries it was 70 degrees with the perfect amount of moisture.
I am officially sold now on the concept of mulching. I plan to use straw over the drip lines of my garden next summer after the soil warms up. i can't believe how well this worked.