I have the solution:
First, obtain advice, from a horticulturalist, as to the likely growth rate of the shrub. Then plot the increasing approach gradients that correspond with the likely height increases of the shrub over time (with an appropriate safety margin). Then put that plot in the RDS.
So the approach gradient could be 3.8 percent for e.g. the next month, then 3.9 percent for the month after that. The increase won't be linear, because of different growth rates in different seasons, but pilots are capable of deriving the applicable point on a curved plot e.g. daylight and darkness graphs.
Job done!