Beginning gardeners often worry about when to plant the plants they buy, yet the answer is simple: pretty much any time you want to.
You can plant almost any time the ground isn’t frozen. That said, the two most popular seasons for planting are spring and fall (in the latter case, until about 2 weeks before the ground freezes for the winter), while the plants are more or less dormant… and when cooler temperatures mean digging holes is not as arduous for you.
Of course, some plants do have special needs.
Annuals, tender bulbs and most vegetables, for example, must be planted in the spring, of course, as they don’t survive the winter in most climates. As for hardy bulbs, including tulips, daffodils, and hyacinths, but also garlic, fall is the best time.
For most other plants, take your pick: either season is appropriate. In cold-winter regions, however, it is better plant slow-rooting plants such as rhododendrons and magnolias in the spring so they’ll have time to settle in and produce new roots before winter comes. You can however plant them in the fall as well as long as you mulch them thoroughly after planting.
I haven’t mentioned summer planting yet, although it is perfectly feasible, especially for plants grown in pots (as opposed, for example, to bare-root plants) since their rootball is left intact when you plant. Plants put in the ground during hot weather may need extra watering to settle in, but otherwise do fine.
Bare-root plants will find it a bit more of struggle if you plant them in the summer, as they have fewer or damaged roots and tend to wilt on hot days, but should do fine as long as you keep their soil moist until they show by new growth that they have settled in.
The main reason not to plant in the summer, though, is for your sake: planting during hot, humid weather is simply uncomfortable for the gardener!