What is the particle size of the "pine bark mulch" you are using? Ideally, the particles would be smaller than 1/4". Bigger particles = more drainage = less water retention. Also, this mix needs to be properly moistened in order for it to work. If it is completely dry it will be hydrophobic - meaning it will not retain moisture.
If these conditions are met, it should not be drying out too quickly - certainly not at this time of the year in the upper midwest.
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