According to the Hyannis Country Garden, the most common reason for hydrangea flowers to turn brown is that they are drying out. Additionally, being planted in full or afternoon sun can cause the hydrangea flowers to dry out and become brown more quickly than they otherwise would.
Fix this problem by watering the hydrangeas deeply and applying a thick layer of mulch around the base of the plant to help lock in moisture, according to the Hyannis Country Garden. In some cases, a soaker hose installed under the mulch may be necessary to keep the hydrangeas hydrated during the hottest days of the summer.
According to the Hyannis Country Garden, if the plant is well-hydrated, but its flowers continue to turn brown, moving the hydrangea may be necessary to create a longer flowering period. Transplant the hydrangea to a location that has strong morning sun and afternoon shade for the best bloom. Hydrangeas need three hours of strong sun to produce flowers, so keep this in mind when selecting a location. Hydrangeas require a lot more water than many other types of plants. For this reason, it is recommended to group these plants together in a garden to minimize watering needs.