During the winter, we don’t want to feed our mandevilla since the days are so short.  Once summer arrives and they are growing quickly, they’ll need to be fed every 2-3 weeks with a high phosphorus, soluble plant food like Jack’s Blossom Booster (10-30-20).  We also don’t want to keep the plant soggy wet all the time, just lightly moist.  Always check to see if the soil is moist before watering.  Poke your finger a couple of inches into the soil before watering.  if the soil feels damp, wait a couple of days and check again.  It is better to underwater a bit that overwater…plants drown very quickly.