Experts and BHG readers answer.
Leaves on Mandevilla Turning Brown
Thanks for writing. Without being able to see your mandevillas in person, I'm afraid I can't say what's wrong. Brown leaves that drop off could point to a number of problems --- including too much or too little water; too much fertilizer; damage to the roots at planting time; exposure to chemicals; and more.
For more help, you might bring a sample of an affected leaf to the staff at your county extension office or a reputable local nursery. After seeing the leaf in person, they should be able to help determine the cause of the browning.
---Justin, Senior Garden Editor, BHG.com