In general we know how cells grow, and even plants. Actual plants are controlled by variables that vary species to species, can't be easily measured, and are too numerous to count.
There are a lot of unanswered questions related to how plants grow. If you look through the recent papers at Algorithmic Botany (linked at the start of the thread), you'll notice that they're partnering with other labs to do fundamental biological research (specifically, on the growth of veins in flowers and leaves). That's because they reached the limit of existing knowledge and needed to learn more to improve the computer models.
That's where I did my master's, though I didn't meet Holly and Steve. I'm told they left for Weta shortly before I joined.
