The fertilized plots will probably have more lush (softer) foliage and have more volume. If you don' t mind the expense then fertilize away. If you' re going into a dry period you could burn the plants, so beware about overfertilizing. I do most of my fertilizing when the grass is just greening but people do fertilize lawns all summer (they water). A high K and P fall fertilization will help the plants overwinter.

Dan O.