In which month should I map maritime vegetation?
One way to approach this problem would be to look at herbarium collections of the taxa that you're interested in. Usually, a flowering or fruiting specimen is easiest to identify; but no matter what character is best for your taxon, if you look at the herbarium collections and see that
-most of the collections identified to species, or 
-the especially good-looking collections, or 
-flowering/fruiting collections, 
were made in a certain month or season, that could help you to narrow down your time window.

As a bonus, in reviewing herbarium collections, you'll have given yourself a step towards being able to identify the species.

Some herbaria have some collections online; here, for instance, are records and images for online Puccinellia collections at a number of UK herbaria.