Our District

Seaweed: the ultimate plant food

Seaweed is an almost complete plant food - Irish farmers grew potatoes without soil purely by layering seaweed onto sand and rock.

What type of seaweed is best? All types contain fabulous nutrients and plant hormones, in forms readily available for plants to use. Fine, soft types like sea lettuce are easier to spread, and break down rapidly while at the other extreme, tough, leathery kelp might take months to break down (slow-release fertilizer).

To rinse or not to rinse? I never rinse seaweed, without any ill effects, but on the other hand it won’t do any harm to hose it off if you feel so inclined.

Where do I get it? Seaweed mostly grows on rocks, so Kāpiti’s sandy beaches aren’t the best source. But it does wash up after southerly storms. Further afield, Plimmerton and Lyall Bay are good collecting grounds - throw a sack in the car boot if you are heading that way.

Holding water: The gelatinous quality of seaweed makes it a great water-reatainer (eg for container grown plants- bury some towards the bottom of the pot.)

Jumping with life: A pile of seaweed left for a day or so will soon be teeming with worms and a huge variety of other microbes and macrobes. Perfect for your compost or soil as all those creatures help break down nutrients into forms plants can use.

Check local signs for restricted areas, and don’t collect seaweed from within marine reserves. It might look like it’s just lying around on the beach, but it may be playing an important role in the marine ecosystem.

What about that mulch stuff that washes up? The fine mixture of twigs, leaves etc washed downstream after heavy rains ending up on the beach can also be laid directly on the garden as mulch. Or mix it with grass clippings for great compost.

Use seaweed as mulch: Lay it on the ground around fruit trees, rhubarb, berries, or any plant that is looking a bit sick. To chop seaweed more finely for use as mulch around vegetables, you can lay out seaweed on a path or lawn and run over it with a lawnmower (but watch out for stones and shells.) or chop with shears or spade in the wheelbarrow.
Add it to the compost heap: Though it’s often brown, seaweed counts as a ‘green’ when layering a compost heap, in that it’s rich in nitrogen (similar to grass clippings or manure). It’s also disease- and weed-free.
Make a liquid seaweed tonic: Especially if you just have small amounts of seaweed, a little can go a long way. Put in a bucket with a lid and cover with water. You can start using the liquid after a few days but it gets progressively stronger – after a few weeks, the seaweed will totally dissolve into the water. Dip out what you need into a watering can or bucket and top up with fresh water. Dilute to the colour of weak tea and water onto roots or spray onto leaves as a foliar feed. It will get smelly after a while – to avoid this, introduce air by stirring regularly or using a small aquarium pump.