So far, what I have observed up through Build 3. Flowers are fairly dense in the following areas basically describing a North-south axis in North Ithilien:

Bâr Aroth
Aelin Veren
Cirith Negen

And all of those areas Flora are with about 10 meters of the Road.

So far I have found no flowers anywhere in the Field of Cormallen - even while completing the quest leading through that area (One Massive Problem).
or around

Henneth Annûn
Cair Andros
Bâr Tarnaher
Torngroth
Tuntham

All while wondering around on foot with my Hunter and "Floaty Names" turned on. Flora are invisible otherwise, unless you specifically mouse over them.
Many different flora (especially Larkspir and Elfspear) spawn INSIDE clusters of other vegitation, making them only visible and accessible by "Del-U" unless you are lucky!
A couple of varieties, notably, Drakewort, seem to spawn in very isolated locations making them easy to spot.
Horsetail and Oxlip tend to be visible because their lighter color stands out against the darker bacground.

So far, I have found only one instance of Bell-o-Dale.