Hi Scud!
It's quite complicate to do with a generative.
You need to considere that a plant is only a distorted spline (
knot clamped for example) . The flowers, leaves and other branch are points renderer of this spline, randomly paced at the top of it.
-Define the max size a plant thanks the "Slice" option of a knot object.
-Add the 
path animation renderer. It will allow you to use a sphere mask for the following modifiers:
Point scale over the point renderer and the thickline renderer
Maybe duplicating or transform you basic spline as 
shared element will allow you to make the leaves following the growth of the plant more easier.
Hope it's help
