If you're just getting around to buying some plants--and you wouldn't be the only one judging from the shoppers at Home Depot yesterday--you might want to look out for plants that are rootbound. It happens when plants have been on the shelves for a while; they start outgrowing their little nursery pots and the roots start twisting all around and getting matted up. One way to avoid this is to buy your plants earlier in the season, but sometimes that's not possible. Another thing you can do is while you're at the nursery or local big box store, just dump the plant into your hand and examine the roots. If it's rootbound, go on to the next one. However, if you really want that one plant and all the ones like it are in the same condition, don't worry. It's been my experience that it will end up doing just fine.
Take this rosemary for example. I actually bought it about six weeks ago so there's no excuse for it having been so rootbound. And, as sometimes happens, I didn't take my own advice--I didn't dump it out to check the roots. So I got all the way home and was ready to plant only to discover this tangled mess. Most gardening books will tell you to "shake out the excess soil and tease the roots apart." These roots were in knots so "teasing" was not an option. But I didn't just want to stick it in the pot as it was because there's no chance for the roots to spread into the surrounding soil. So, I just cut into it vertically from the base with gardening scissors and ripped it apart so I could spread out the root base and plant it in the soil. It was an aggressive approach, but take a look. My rosemary has not only recovered, it's thriving. Bookshelf: Container Gardening Books