I wanted to post the file itself but in some way i must have screwed it up Hannes. That would be easier,sorry.
But the general principle is easier then it might look by reading it below:
*I made two very crude objects like in the image below.
*The scale of these objects are animated from small to big.
*I made another square object for anchoring the first two objects. This plane object stands vertical.
This objects scale is animated in the Y xis from small to big.
This makes the trunk part and leaves grow in the Y axis and is the most important part.
I used big numbers in the "a" spacing of the population to get few trunk parts (two plants in this example).
To get the trunk as a solid i just used a small value in the "b" object spacing (i made ropes, pipes with this approach earlier.)
*To get the displaced look of the trunk and leaves i displaced the square object with a powerfractal that is controlled with a surface layer by altitude (to get only displacement above a certain point of the ground.)
Hope this makes sense Hannes.