The orientation of your flowers might have an effect. In previous Animal Crossing games, having the flowers touch diagonally was better, but in other games, having them in straight rows was better.
-Water flowers even if they don't need it
-Plant large patches of the same type of flower to have multiple flowers next to each other
-Bury fertilizer next to the flowers. Fertilizer is available after you upgrade Timmy and Tommy's shop to the Department Store.
Taken from A Guide on Thonky.com
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Hope that helps