There may be several types of leaf feeding beetles, caterpillars, slugs, etc. that feed on the foliage. Since you are not noticing them by day, you may want to go out at night with a flashlight to try to identify the culprit. Handpick and drop into soapy water. The plants should be able to tolerate some leaf feeding damage. 

Water during dry periods and make sure mulch is not thicker than several inches and away from the base of the plants. You can send more photos if you notice additional symptoms. http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=286965

Marian