Heavy rain damage, frost, insufficient water, and the natural flowering cycle can all turn mum blooms brown. Deadheading brown blossoms and cutting back damaged plants will help keep mums looking their best and blooming profusely.
Mums are a vigorous late-blooming garden plant with showy flowers that turn brown if weather conditions become adverse. An early frost, very heavy rain, or a hail storm can turn mum flowers brown and mushy overnight. When mum blooms are finished flowering, the individual blossoms naturally turn brown and fall off.
Mums are perennial plants that flower reliably in the fall; they are inexpensive and available in a wide variety of colors. Damaged mum plants can be cut back almost to the ground and they will usually come back healthier than ever.
Mum blooms can be ruined if nipped by early frost or damaged by heavy rain, but browning can sometimes be prevented by covering the plants with a plastic tarp when frost or rain is expected. If covering is not possible or is not effective, pinch off all brown blossoms and any damaged leaves to encourage new growth.
Some mum cultivars are more resistant to frost than others; pinching mums back hard by the Fourth of July helps to ensure vigorous bloom before first frost. Hot summers can cause mums to flower later than usual. Some gardeners mow their mums to the ground in early July if the temperature is especially high in order to encourage better flowering in the fall.
Mums grow well in full sun and tolerate neglect and severe treatment. Keeping spent blossoms pinched off prolongs their blooming period. Choosing frost-resistant varieties and mulching heavily to keep the soil moist help to prevent brown blossoms and promotes healthier flowering.