Most people think that mums should be planted in the fall (as that’s when many nurseries sell them), but this is a misconception; the best time to plant mums is in early spring, after all the danger of frost has passed. Mums will grow if planted at other times, but even then they need to have time to develop a root system before hot weather sets in and they would prefer to be planted in spring in order to maximize root development.
Why plant mums in early spring? For one thing, that means they will have plenty of time to establish their root system, which leads to more intake of nutrients and more blooms. Although they do need plenty of drainage, mums love to drink water. They also want to be fed every two weeks. So developing a good root system is very important so that your mums can drink up these nutrients that they need.
Another reason to plant in spring is that mums don't particularly like growing in pots. They would prefer to be able to stretch their roots. If you pick up a mum at the nursery in August, it has been growing in a pot at the nursery all summer. It has probably been well-cared for and well-fed, but it hasn't had the room it desires. Planting your mum early allows it to spend the entire summer in the best possible growing conditions.
Finally, planting your mum in spring gives you time to give it special treatment that it might not otherwise get. At the very least, when the plants reach six inches, you should pinch off each bud along with about three quarters of an inch of stem. Repeat this process when the plants are a foot tall. This will create more new shoots, which means more blooms. If you are somewhat more adventurous, cut the plants back by half when they reach twelve to eighteen inches high. This will maximize the amount of new shoots, which leads to even more flowers come autumn.