Like the description says, you need to keep the first flower that appears in the level alive until the end of the level.
You need to manipulate the growth by shooting the flower. When you are hitting it, it will go towards the beginning and when you are not shooting it it will grow towards you.
If you shoot it too much or too little - the flower will end when it reaches the beginning or the end of the grid.
I would recommend getting this on level 9 as described and showed on this video I used as a guide. All credits goes to the uploader "JustAnotherMetalhead"