First impressions are important. It's mid-summer and some people are a little less energetic about pulling weeds, mowing the grass, tending to the garden, etc ...
It's simple: deferred maintenance with the landscaping of a home translates to potential buyers second guessing the amount of care given to important components such as the furnace, the roof, and more.
Hire a professional if you aren't up to the task. The market is different throughout the country, but one thing that is most likely universal: The homes that are maintained well outsell similar homes that are not maintained well.
One of your neighbor's lawns outshining yours? Compliment them and ask for the name of their landscaper. They do it themselves? See if they'll help. You'll be surprised at how involved a neighbor will get into helping you sell your home. After all, they have a vested interest in that too.