Nice image...
Just a though on the population... The distribution looks a little too uniform, both in terms of size and spacing. You could try adding some extra variation by duplicating the population and feeding the fractal for the density shader into two colour adjust shaders.
Drop the density on both populations, change the seed in the population (not the density fractal) and tweak the two colour adjust shaders.
Population 1:
smaller objects, colour adjust: decrease black level a bit (uncheck "clamp below black")
Population 2:
bigger objects, colour adjust: increase black level a little, decrease white level a little
Attached a sample clip of what I mean using shaders. Just an idea... haven't actually tried it yet.
What this should do is give you a higher density of taller flowers near the centre of the "clumps" and providing some falloff of height towards the open spaces. Dropping the black level in pop'n 1 should also put a thin scattering of small plants in the open areas.