It stands to reason that the person who will know which gardening questions are asked with the greatest frequency is someone who works in a garden center or store. In fact it is the Nursery Specialist at a Big Box store who has supplied the list of questions and, of course, the answers too. I found the information in an article by Cathy Olson over on The Master Gardeners website.

Good answers to gardening questions come from a variety of different sources. A neighbor of mine in Carroll Valley, Pennsylvania has been the Nursery Specialist at a Big Box store in Westminster, Maryland for 13 years. Following are the questions that he most often encounters from his customers.
Early Spring/Summer Questions
Which plants, annuals or perennials, come back each year? We’ll start with an easy one: annuals have to be replanted each year, and perennials come back by themselves.
When is the best time to plant grass seed? In our climate it?s September or October. Second best time is April or May.
When is the best time to put down crab grass preventer? As last year’s crabgrass seeds germinate in February or March, this is the best time to apply a herbicide that will kill them.
When is the best time to use “weed and feed”? When the weeds are actively growing, at bud stage.
What can I plant that deer won’t eat? Ornamental grasses, herbs, coral bells, wild native plants (wild ginger, wild geraniums, Lenten roses, false forget-me-nots) are some plants deer do not like.
Should I prune my lilacs before or after they bloom? Immediately after blooming, to maintain the size of the shrub. Rhododendrons too. Take off no more than 1/3 of the growth. When watering, do so at ground level. To help prevent diseases, do not get water on the leaves.

See more at The Master Gardeners