From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156738-983444
Question: I have several rose bushes in containers; on all of them the
blooms turn brown before they fully open. Any ideas on what is causing
this problem?
Answer: Based on your description it is difficult to identify a
definite cause of the problem, but here are a few possibilities.
Drought stress can cause the symptoms you describe, and insects also
can cause buds to be deformed and fail to open. Roses need a rich soil
and regular watering to keep them evenly moist but not soggy, so it is
possible that there is a soil problem. Container plants are a
challenge when it comes to watering, sometimes needing it twice a day
on very hot, dry, and windy days or if the pots are too small for the
plants. You might see if the texture of your potting mix has
deteriorated over time; or if it has been allowed to dry out, it may
require some care in rehydrating it thoroughly. A top-dressing of
compost is helpful for all roses, and you might also consider
fertilizing (either a timed-release granular or water-soluble type
specifically for roses) according to package instructions. Finally,
inspect the blooms very carefully for signs of aphids or other
insects. Japanese beetles may be burrowing inside the blooms, as might
thrips, which are so small they are nearly invisible. A sharp spray of
water from the hose will knock away aphids; handpicking or using a
neem-based spray will take care of the beetles, and insecticidal soap
used according to the label instructions may work on the thrips. The
soap also is effective in case of a serious infestation of aphids.