Once your potted mums, or chrysanthemums, start to die off, you can still plant them in the garden, and they’ll continue growing. Perennial varieties will even sprout the next year.
When your plant is brown and no longer blooming, but the base of the stem is still green, it's time to plant it in the garden. Pull it out of the pot while holding it at the base.
Don't remove the dead, browned stem, as it will protect the living plant underneath. Plant your mums in full sun or partial shade if your area remains hot into winter.
If the soil isn't too clayey, you don't need fertilizer, but you can add a phosphorus-based one to support root growth in winter, and a nitrogen-based one for new growth in spring.
If your area gets snow, add an inch of mulch around the base of the plant, avoiding anything with chemicals in it. In spring, trim back the dead material to encourage new growth.