Grass-type Pokémon are quite powerful, but become more powerful when holding a Rose Incense in their hand. This item increases the power of all grass-type moves. This is required for anyone running a Grass-type Pokémon in their party. Unfortunately, if you are looking for this item, you will not be able to do so until you have defeated the Gym Leader in Canalave City. This is where you can find Rose Incense in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get your hands on this item, you'll first need to get the hidden Surf move from Cynthia's grandmother. You get this hidden move as part of the story, so there's no way to miss it. After you get your hands on the hidden move, you will need to go to Canalave City and beat the gym leader there in order to use it outside of battle.
After completing the above steps, go to Hearthome City or Pastoria City. Hearthome City is closer and it is easier to reach the object from there. Travel to Route 212 south of Hearthome City. Continue south until you come across a pool of water with some box-shaped hedges next to it. There is an opening on the left side of the hedges where there is water.
Use Surf to swim to the opening in the hedges. Collect the item in the middle of the hedges and you will receive the Rose Incense.