Here’s a Horticulture Tip from University of Missouri Extension:
Mums are fun to grow for fall color in your landscape, but to get a lot of flowers, you’ll need to pinch off the growing tips in early summer. This will promote a lot of extra growth, and many more flower buds will be produced. But by mid-July, it’s time to stop removing those growing tips and let the plant mature. You’ll be rewarded with a great show this fall.
This has been Tim Baker, with a Horticulture Tip from University of Missouri Extension.