Water the yellow autumn mum garden deeply and regularly throughout fall. Depending upon the amount of rainfall, two weekly soakings to 6 inches in depth are sufficient. Keep the soil moist into winter, until the ground freezes.
Fertilize the yellow autumn mum in its second season with a 5-10-5 fertilizer. Use 1 lb. per 100 square feet of mum bed and water it in to a depth of 6 inches. Reapply once a month throughout the growing season.
Pinch 1 inch from the tips of the stems when the mum reaches 6 inches tall. When the resulting new shoots reach 3 inches, snip them again. Continue to do this until July.
Pour a 4-inch layer of mulch around the yellow autumn mums after the soil freezes. Remove the mulch in spring and cut the plant all the way to the soil.