This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The salt marsh is as a rule protected from the sea by a shingle beach, or dune, or sandy bank, or line of cliffs, but usually one or other of the former. It consists of low-lying meadows by the sea, which have at some period become inundated by the sea, have become salt or brackish, and have continued to be so. In addition to periodical reflooding, there is a certain amount of creep of salt water through the sand or shingle. This makes the marshes salt.
Amongst the true salt-marsh plants may be found a large number of ordinary meadow types. But sometimes these are driven out by the salt-marsh plants entirely. Here one may find that one salt-marsh plant occurs to the exclusion of all others, e.g. Buckshorn Plantain, there Sea Aster, or again Juncus Gerardi, etc. Frequent plants in the salt marsh, which usually grow in extensive societies or associations, are Sandwort (several kinds), Sea Lavender, Sea Milkwort, Sea Plantain, Shrubby Sea Blite, forming small plantations 2-3 ft. high and several acres in area, Samphire, Sea Rush, Sea Club Rush, usually in water, in drains, etc.
Others not described in detail here are Marsh Mallow, Dittander, Sea Heath, Seaside Clover, Slender Hare's Ear, Hog's Fennel, Sea Aster, Golden Samphire, Sea Wormwood, Marsh Samphire, Sea Blite, Sea Arrow Grass, forming large tussocks in wet submerged places, Long-bracted Sedge, Perennial Beard Grass, Nit Grass, etc.
The four zones of sea-coast vegetation above referred to furnish a variety of habitats, as each is distinct in itself. On the sandy coast there are long stretches of sand where plants grow in extensive patches, or discontinuously. Such a coast may be diversified with rocks jutting out here and there, and there may be creeks and pools caused by storms which quickly become colonized by Samphire, Ruppia, etc. The sand may be grass-grown, and stretch inland.
The muddy coast also furnishes a diversity of habitats. As a rule, it is in the mouth of an estuary. On some coasts, however, the shore is eaten out of a clayey formation.
The rocky coast is usually made up of hard granitic, siliceous, sandstone, limestone, or other old rocks, or of modern chalk or sandstone, or Crag, covered by Boulder Clay. The influence of the soil will here determine the flora which comes down to the sea-coast, and is mingled with the true coastal vegetation, such as Scurvy Grass, Thrift, Sea Lavender. Here and there on such rocky coasts trees and shrubs may shelter the vegetation, and give it an extraordinary luxuriance where the climate is warm and moist, as in the south-west.
The shingle beach affords a uniform type of habitat suited only to a few types. The dunes also exhibit a uniform vegetation made up of a few special types, the sandy and saline soil suiting a minority. But in both of the last cases there are frequently many weeds of inland origin which are widespread and able to grow almost anywhere.
The salt marsh varies considerably in regard to the degree of moisture. In some cases there are few or no pools, in others these are general. In some cases there is a struggle between inland types and salt-marsh plants, in others the latter are entirely dominant.