Eelgrass (Zostera marina) looks like long, flat ribbons of bright green grass growing underwater. The leaves are typically 20 to 50 cm long and 4 to 10 mm wide, though in ideal conditions they can stretch up to 3 meters. Unlike seaweed, which is floppy and rubbery, eelgrass has structured, blade-like leaves that sway with the current, creating dense underwater meadows that resemble a mowed lawn seen in slow motion.
Leaf Shape, Color, and Texture
Each eelgrass leaf is flat, smooth, and ribbon-shaped with a narrow, rounded tip that sometimes ends in a tiny sharp point. The leaves emerge in bundles from a single base, fanning upward through the water column. Fresh, healthy eelgrass is bright green, though the exact shade varies with depth and light availability. Plants growing in deeper water tend to be darker green, while shallow specimens can look almost lime-colored in strong sunlight.
The leaves have faint parallel veins running their length, giving them a subtle texture you can feel if you run a finger along the blade. In the wild, eelgrass leaves are often coated with a thin biofilm of algae, tiny diatoms, and even microscopic animals like hydrozoans and mussels. This layer can make older leaves look brownish, fuzzy, or slightly slimy compared to the clean bright green of newer growth. When this coating gets heavy, it visibly dulls the plant and can shade the leaves enough to cause decline.
Roots and Rhizomes
Below the sediment, eelgrass spreads through a creeping underground stem called a rhizome. This rhizome is thick and whitish, about 2 to 5 cm long between branching points, with nodes spaced 1 to 3.5 cm apart. From each node, a cluster of thin, hair-like roots anchors the plant into sandy or muddy bottoms. If you pull up a clump of eelgrass, you’ll see these pale roots tangled through the sediment, looking similar to the root system of a terrestrial grass but finer and more delicate.
Flowers and Seeds
Eelgrass flowers are easy to miss because they have no petals. The plant produces a small spike, a thin unbranched stem tucked within the leaf sheath, carrying 1 to 20 tiny flowers that stay below the water’s surface. The most visible part is the anthers, which have a noticeable pinkish or reddish-purple tint and are only about 1 mm long. Unless you’re examining the plant up close, you’ll likely overlook them entirely.
The seeds develop into small fleshy fruits, roughly 3 to 4 mm long with a short beak at the tip. Each fruit contains a single seed. These fruits are inconspicuous and green to brownish, blending in with the leaf sheaths where they form.
How It Changes Through the Seasons
Eelgrass looks dramatically different depending on the time of year. In summer, meadows are at their thickest and tallest, with the longest leaves and highest shoot density. This is when the beds look most lush, forming dense canopies that can carpet large areas of shallow coastline. Leaf length, width, and overall shoot weight all peak between May and July.
By late fall and into winter, the plants thin out considerably. Leaves become shorter, narrower, and sparser. From November through April, the above-ground portion of the plant weighs significantly less than during the warm months, and meadows can look patchy or nearly bare in colder regions. The rhizomes survive underground, and new growth returns as water temperatures rise in spring. If you visit a known eelgrass bed in February and see very little, that’s normal.
Dwarf Eelgrass vs. Common Eelgrass
A closely related species, dwarf eelgrass (Zostera noltii), looks like a miniature version of common eelgrass. Its leaves max out at about 22 cm and are much narrower, only 0.5 to 1.5 mm wide compared to the 4 to 10 mm width of common eelgrass. The easiest way to tell them apart is leaf tips: common eelgrass has rounded tips that may end in a tiny point, while dwarf eelgrass leaves develop a distinctive notch at the tip as the plant matures. Younger dwarf eelgrass leaves start rounded, so the notch only helps with identification on established plants.
How to Tell It Apart From Look-Alikes
The species most commonly confused with eelgrass is widgeongrass (Ruppia maritima), which also grows in coastal and brackish waters. Widgeongrass has shorter, thinner blades than eelgrass, and its leaves tend to branch from a more visible stem rather than emerging in flat bundles from a basal sheath. In places like Chesapeake Bay, widgeongrass has largely replaced eelgrass as the dominant underwater grass, so what you spot in shallow estuaries may not be eelgrass at all.
Surf grass (Phyllospadix species) is another look-alike, but it grows attached to rocks in wave-exposed areas rather than rooted in soft sediment. If you see grass-like plants on a rocky shoreline, that’s almost certainly surf grass, not eelgrass. True eelgrass is found in calm, sandy, or muddy bottoms in bays, estuaries, and sheltered coastlines, typically in water less than a few meters deep where sunlight can reach the leaves.