Control flower, grass,.. spawning on one block

When you place a flower it is placed on a block randomly(little bit on a right, to a south,etc.).Is there a way to make a flower be placed at a exact location on block that you want to (let's say on a middle of the block)?

The offset of the flower is calculated from a hash strictly dependent on X, Y and Z position of the block and nothing else. Without modding the game code the only thing you can do is find a specific block where the flower lands exactly where you want it. If you just need it to exist visually (without being able to interact with it), you can cheat with "falling block" entities that don't fall, like this:

/summon falling_block ~ ~ ~ {Time:-2147483648,BlockState:{Name:"dandelion"},NoGravity:1}

You cannot break this entity by punching it and you also can't punch through it's full-block-sized hitbox, but at least you can position it however you want. It will also break and turn into an item after a while, but with this value of Time, that will take more than 3 years.