I had to use a few hacks to import the model because unfortunately importing it as a tree automatically made the game apply the standard tree blowing in the wind animation which looked terrible on this model (the whole thing rocks back and forth and the flowers stretch and contract like the stems are made of elastic). So what I did is import them using the water tower asset, which can be placed anywhere. Then I edited the .crp file so they appear in the trees menu. They don't actually need water, but since I used the water tower as a template they still bring up the water overview when you place them. Unfortunately I never figured out how to change this.

If you want to make a nice looking garden, I would suggest using the prop version of the flowers which can be found in the collection linked above. You can place the flowers as props in the park editor then save it and place the garden as a normal park in your city.

I like your flowers and the way they have in game icons... but the fact that they won't be plopped as regular trees it is impossible to layout a natural looking garden. Also they destroy each other when placed too close... Also why do they need water?