Sand Dunes and Sea Rocket: discover a coastal cabbage and its importance in dune creation and conservation
On beaches all around the British Isles from the Isles of Scilly to the Moray Firth and beyond, and even across Europe, from the shores of the Mediterranean up to the Baltic, if you take a closer look the dunes, just above the high tide line, you’ll see more than just marram grass; you’ve a good chance of finding a beautiful plant called sea rocket (Cakile maritima).
Sea rocket is a member of the Brassica family, and therefore closely related to the cabbage crops that we’re used to finding on our dinner tables. One thing sea rocket has in common with its cabbage cousins are their simple flowers made up of four petals. Sea rocket can be found flowering throughout the summer and you might see plants with petals ranging from pure white through lilac to vibrant pink. And keep your eyes peeled for pollinators too - the flowers attract a wide range of insects from bumblebees and butterflies to burnet moths.
In the spring the first few seedlings start to appear through the sand, and as the plants grow characteristic fleshy leaves develop. In some places Sea rocket has leaves with lots of notches or a pinnate appearance, but in others the leaves are almost completely oval or cloud shaped. Variation in leaf shape can be due to the harsh conditions plants experience when they grow, for example as a result of strong winds, but patterns of variation in plant appearance between populations has fascinated scientists for years as it can hint at the existence of local adaptation, where populations evolve to be particular well-suited to local environments.
Sea rocket plants are generally annuals, so only live for one growing season before setting seed. Come the end of summer leaves begin to drop from the plant and odd shaped fleshy green fruits start to form. The fruits that develop are curious things, sometimes made up of two compartments that can each contain one seed. The top segment is longer and pointy, detaching from the plant in the autumn. This segment floats in sea water and high winter storm tides wash them away to other parts of the beach or other beaches to be buried, ready so they can germinate in the future. The bottom fruit segments generally stay attached to the plant, which dries out, taking on a windswept look in the autumn and can be found being blown around beaches like giant tumble weeds depositing seeds or being buried by shifting sands driven by winter storms.
These curious plants brighten up beaches but they’re also an important plant of the dune landscape. Sea rocket is a pioneer species - in germinating and establishing above the high tide lines and putting down deep tap roots, they provide a structure around which sand particles gather, creating embryonic dunes. By colonising this strand line, these mini dunes provide a habitat for other species, such as sea holly (Eryngium maritimum) and sea sandwort (Honckenya peploides) to create a special foredune community alongside sea rocket, and allow plants such as marram grass to follow along behind.
Pick up pretty much any wild flower book and you’ll read that rea rocket is widespread on sandy and some shingle beaches throughout the UK. Having visited many miles of beaches around the fantastic coastline of Great Britain and the Baltic during the course of my PhD, unfortunately it often appears that sea rocket isn’t flourishing as it once was. In some places, such as Lindisfarne, there were incredible displays of over 600 plants in a sheltered cove, however increased dune erosion from stronger storm surges, and excessive trampling of embryonic foredunes by thousands of pairs of feet appears to be leading to very few or no Sea Rocket plants at all.
Foredunes are an integral part of dunescapes, either as mobile dunes or the first stage of dune succession. Sea rocket has a key role in the dune landscape and whilst populations do naturally fluctuate from year to year, the expected rise in sea levels and increasingly intense winter storms as a result of climate change, mean we need to do our bit too to make sure these plants thrive on beaches and play their role in creating dynamic dunes.
Unfortunately due to the current crisis we can’t get out to beaches to see these fabulous plants as they emerge and start to flower. However the decreased footfall on our beaches might mean fewer sea rocket plants are walked on this year - who knows, maybe they’ll have a particularly good year as a result! But we have something to look forward to when lockdown is lifted - when you can next get out and about at the beach, maybe visiting a Dynamic Dunescapes site, take some time to have a look for this fascinating dune plant.
About the author
Kate studied Biology at the University of Bath and is just finishing her doctoral studies on local adaptation in Sea Rocket. As part of her PhD she conducted fieldwork on beaches all around Great Britain, Northern Germany, Denmark and Sweden. She currently works at Plantlife as Road Verge Campaign Manager.