Coastal Processes FAQ - Why aren't there any plants growing on the beach?

The dune grass stops at the base of the dune; why doesn't it grow all the way to the water?

Many beach goers may have observed that the flat, dry beach area between the dune and the water's edge is comprised of sand, pebbles, and shell fragments. Typically there may be a visible wrack line, or collection of organic debris and detritus, marking the landward excursion of the highest tide. There are no plants growing in this active section of beach because the sand in this area of the beach is impacted by waves and tides, either on a daily, monthly, or seasonal basis. The energy imparted to the beach by the tides and waves is too great for vegetation to root and take hold.

In Delaware, like most mid-Atlantic beach areas, wave energy varies tremendously between a calm summer condition and a stormy winter condition. In the calm summer months, small waves build the high wide beach that is used for sunbathing and recreation. However, winter storms cut into this portion of the beach resulting in a landward displacement of the high tide line by as much as 100 to 125 feet. At the same time, the vertical beach elevation drops by as much as 10 feet.

Although there may be viable seeds hidden in the organic detritus of the wrack line, germination and seedling growth will typically only occur when a storm tide has left the debris at the highest and most landward edge of the beach, far out of reach of the daily high tides and waves. Occasionally, after an unusually calm winter with few or no coastal storms and associated storm tides, seedlings can be found on the more active section of beach.

Sand dunes develop beyond the active part of a beach which is influenced daily by waves, tides, and currents. In Delaware, there may be several parallel rows of dunes which range in size from two to twenty feet or more. The dominant process forming sand dunes is that of wind action blowing over the dry part of the beach and transporting sand inland where vegetation plays an important role in trapping the sand. When the wind encounters an obstacle such as a clump of vegetation, wind speed is reduced, causing wind-borne sand to settle.

Dune grass is specially adapted to grow taller as sand accumulates around it. As the plants grow up through the sand, more wind blown sand is trapped. As sand accumulation continues, a dune is formed.

Dune grass and other vegetation such as sea rocket, will often spread through the growth of new shoots and rhizomes. The grass will extend itself down the seaward toe of the dune and onto the upper beach area as long and as far as it can until it is eroded or adversely impacted by waves, tides, and currents. Cape American beach grass (Ammophila breviligulata) spreads naturally down the face of the dune towards the beach. It will survive and continue to grow until it is removed by a coastal storm and associated high waves and tides. The seaward edge of vegetation typically marks the edge of the vertically stable beach, or the area that is not impacted and eroded by storms.