Sea rocket

How to recognise it

Habit

Annual, hairless, much-branched plant, sprawling to ascending, 15–40 cm tall and often wider than it is tall.

Leaf

Fleshy, glossy and slightly bluish green; very variable in shape, from almost untoothed to deeply pinnately lobed with narrow segments. The leaf tastes sharp.

Flower

Clusters of scented flowers about 1 cm across, with four lilac, pink or white petals, from June to September.

Fruit/Seed

A corky pod of 1.5–2.5 cm in two segments: the lower segment stays on the plant, the upper, mitre-shaped segment breaks off and can float away. Each segment usually holds a single seed.

Roots

A long taproot that reaches deep into the damp sand, which is why it does not transplant well.

Look-alikes

Sea kale is much larger and perennial, with broad, blue-grey cabbage leaves and white flowers. Frosted orache and prickly saltwort grow on the same strandline but have inconspicuous green flowers; saltwort has spine-tipped leaves.

Flowering: June–September

Why grow it?

• One of the few plants that flower on the bare beach: nectar for bees, hoverflies and butterflies on the coast.
• Like many members of the cabbage family, it can feed the caterpillars of cabbage whites.
• It traps windblown sand and forms small hummocks: the start of new dunes.
• It belongs to strandline vegetation, a habitat that has become rare and where shorebirds also look for food.
• The young leaves have a sharp, mustard-like taste; on the coast they were sometimes eaten in the past.
• An intriguing annual for a sand garden or a garden close to the sea, flowering from June to September.

Sea rocket: where does it grow best?

• In the wild on the strandline of sandy and shingle beaches, where washed-up debris provides nutrients.
• Also at the foot of the foredune and on dredged sand, always on open, bare sand with few other plants.
• Full sun and wind: it tolerates salt, salt spray and blowing sand.
• In the garden only on loose, free-draining sandy soil in full sun, such as in a sand garden or among gravel. Salt is not needed.
• On rich, ordinary garden soil it is quickly overgrown by other plants.

Goes well with

• Sea kale: A perennial from the same coast, with large blue-grey leaves and white flowers in May–June.
• Sea holly: Silvery blue, spiny plant for dry sand; flowers in the same months and attracts many insects.
• Sea bindweed: Creeps over the sand with pink trumpet flowers, just as on the foredune.
• Sand couch: Low, blue-green grass of the first little dunes, which traps sand together with sea rocket.

Sea rocket: when and how to plant it?

1. Never dig up plants in the wild and do not collect seed in beach reserves: buy seed from a nursery or seed supplier specialising in wild plants.
2. Sow in October–November or in April directly where it is to grow, as the taproot does not transplant well.
3. Press the large seeds 1–2 cm deep into loose, bare sand; they germinate better when they are slightly covered.
4. Thin the seedlings to 30–40 cm apart: each plant spreads widely.
5. Keep the sand moist until the seedlings have a few true leaves; after that, water only during long dry spells.
6. Let a few plants set seed every year: that way it will self-seed on open sand.

Care

Water

Hardly needed once it has germinated: the taproot draws moisture from deep in the sand. Only in long dry spells during the first weeks.

Feeding

Not needed. On the beach it gets its nutrients from washed-up debris; in the garden poor sandy soil is enough.

Open sand

Keep the spot free of grass and other plants, because sea rocket cannot cope with competition.

Let it seed

Let the fruits ripen and drop. Only weed in spring, once you can recognise the young plants.

On the beach

Leave the strandline in place and do not walk through it: machine cleaning of beaches and trampling destroy the plants and their seed.

Tip: Give no compost and no extra water to a sand bed with sea rocket: the poorer and more open the sand, the better it self-seeds year after year.

Pruning and mowing

When: Not needed: sea rocket is an annual and dies off by itself once it has set seed.

How:

• Do not cut anything back during flowering: it keeps flowering until September.
• Leave the dead plants in place until the last fruits have dropped.
• Only clear the old stems if they really get in the way, and leave the little fruits on the sand.

Pruning calendar and basic rules

Propagation

Seed

The only way: sow in autumn or in April where it is to grow, in loose sand. Collect seed from your own plants in September–October, when the fruits are brown and corky.

Self-seeding

On open sand it self-seeds every year. Keep a few bare patches each spring for this.

Sea rocket: how does it get through the winter?

Hardy to −20 °C.

• Sea rocket is an annual: the plant dies in late autumn, after the first frost or because of storms.
• It overwinters as seed in the sand or in the stranded fruits, which can float in seawater for some time without losing their ability to germinate.
• The plant itself only tolerates light frost; the seed survives the harshest winter.
• Winter storms shift the strandline: in spring the plants come up wherever the seed has been washed ashore.

Diseases and pests

• Caterpillars of whites: They chew holes in the leaves. Leave them be: they belong there and the plant recovers.
• Flea beetles: Tiny jumping beetles make small holes in young leaves, especially in dry weather. Sow a little more thickly and keep the seedlings moist.
• Slugs and snails: They eat the seedlings in a damp garden. Sow on dry, bare sand, where few slugs come.
• Being overgrown: On richer soil it disappears under grass and weeds after a year or two. Keep the spot open or add a layer of clean sand.

Through the year

• March: Clear the sand bed of grass and weeds.
• April: Sow in place in loose sand. The first seedlings appear.
• May: Thin the seedlings to 30–40 cm.
• June: First flowers. Watch the bees and hoverflies on the flowers.
• July: Full flower. On the beach: look for it on the strandline at low tide.
• August: Flowering and the first fruits.
• September: Last flowers. Collect ripe, brown fruits for seed.
• October: Let the seed drop or sow for next year.
• November: The plants die off. Leave the dead stems in place.

Did you know?

• The fruit hedges its bets: the lower segment stays close to the parent plant, while the upper one breaks off and can drift with the sea to another beach.
• Rocket is an old name for the salad plant also called arugula: sea rocket belongs to the same family and has the same sharp, mustard-like taste.
• Carried by the sea and by people, sea rocket has spread far and wide: it has become naturalised in places such as Australia and the west coast of North America.