Answer: There are several possible answers to your question. As flowers age they normally fade and turn brown and eventually dry up. While mums are very long lasting flowers, they will eventually fade. Many gardeners remove the old flowers; this is called "deadheading".
Mums generally prefer full sun all day or at least for half a day. Impatiens, on the other hand, prefer shade to partial shade. It sounds as though the mums may not be receiving enough light, especially since your impatiens are doing so well in that location. Daily watering may be too much depending on your soil. Impatiens appreciate ample water but mums do not do well in a soggy soil.
There are different kinds of mums, but most are usually grown for their long lasting late summer and fall flower display. The technique used to achieve this result is called pinching. The mums are usually trimmed back repeatedly until about the Fourth of July. This causes the plants to branch and fill out and delays the formation of flower buds until later in the season. Once the trimming stops, the plants direct their energy to flower bud formation and bloom prolifically about eight to ten weeks later. You might want to consider this technique in the future.
Finally, you may have a disease problem on your hands, especially if newly-opened buds are turning brown.
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