Answer: Mums are usually divided in the spring as they begin to grow. This allows you to discard the woody center of the plant and keep only vigorous divisions from the perimeter of the plant.
To control the overall height and keep the plants dense and well branched, pinch out the growing tips every few weeks from now until about the fourth of July. This delays flowering until fall and also increases the number of flowers per plant.
Most mums will increase somewhat in diameter from year to year but they do not run. However, there are a few varieties of mum that really do spread aggressively. If you planted one of these, you may want to dig out a good portion of the new growth.
Good luck with your mums!
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