Seaweed contains plant nutrients, including potassium, (up to 12%) , but it is  low in nitrogen and phosphate. It can   contain in the region of  60 trace elements, growth hormones and other nutrients  particularly rich in iodine and calcium.   Spraying seaweed tea  is said to increase resistance to insect infestation. Seaweed has been used as a soil improver for centuries and is still used in the garden as a mulch, liquid feed and in making  compost.   It is also available commercially both dried and as a liquid.  

If considering collecting seaweed it must be noted that many countries have laws or regulations protecting the marine environment and it is likely that these will cover the harvesting of seaweed.

In the UK the Crown Estate licenses sustainable, commercial harvesting of seaweed from areas of foreshore and seabed not privately owned. However, while commercial harvesting of seaweed from areas of Crown Estate foreshore or seabed requires a licence, collection for personal use, in small qualities does not require a licence but the permission of the landowner will be required.  

The following guides are recommended for anyone thinking of collecting seaweed https://cdn.naturalresources.wales/media/686552/gn011-detailed-guidance-for-seaweed-harvesting-hand-gatheringenglish.pdf

https://www.cornwallgoodseafoodguide.org.uk/how-to-guides/seaweed-harvesting-code-of-conduct.php

Composting seaweed

Seaweed is a commonly used as a  constituent in home composting  in coastal areas. It acts as an activator speeding up the compost process. If using a cold composting technique  small quantities of seaweed  can just be added to the bin  in the same way as any other nitrogen rich  “green”. There are mixed views as to whether the seaweed should be washed to remove traces of  saltwater or sand with the consensus being that it is not necessary. However, not all plants tolerate salt so if in doubt the seaweed should be rinsed  in fresh water.

If using a New Zealand or pallet bin, in which layers of greens and browns are alternated, the seaweed can be added as a separate green layer or mixed with  other green material. Shredded or chopped seaweed cut in   1- or 2-inch will decompose in  a few weeks compared with  six months or more for uncut fronds so, as with other materials, it is better to cut the seaweed into small pieces. If the contents of the bin are to be turned to aerate the organic material occasionally in cold composting  or regularly as in hot composting cutting the seaweed into short lengths will also make it easier to turn during aeration and speed up the composting process.  

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The simplest method of making seaweed tea is to soak a large handful of seaweed in a lidded bucket of rainwater water for at least three weeks) and leave to soak for a minimum 3 weeks. Normally it is soaked for about two months although some suggested leaving it in soak for up to a year.  The liquid will need careful decanting or filtering if it is to be used in a sprayer or watering can with a rose to avoid blockages.

As with making other liquid feeds putting the seaweed in an old pair of tights or a porous sack to hold avoids the need for filtering. Ideally, the sack should be weighted to keep the seaweed under the water.  If making a larger quantity using a container such as a dustbin or water butt it may be better to hang the weighted bag by a string making it easier to remove.  The same  seaweed can be reused to make a second or third batch of tea before being added to  the compost bin or used as a mulch if it has not decomposed to a sludge.  

Follow the links to Plant Liquid Feeds   Plant Feed Makers  Compost Teas

Fresh or dry  seaweed can be used as a mulch on the soil surface  or  dug into the soil like manure. The seaweed is best applied to the soil within a day or at most 48hrs of collection as it starts decomposing if left in a bag, 

 It does not need washing before application and is not normally shredded before use. When collecting seaweed choose that with smaller leaves or fronds   as it is  easier to apply as mulch.  Large fronds, if not cut into manageable lengths,  can be difficult  to fit round are difficult to form around plants.  In unplanted areas this does not matter, and larger fronts are less likely to be blow away under windy conditions  than some other mulch materials if used, as recommended,  without shredding.  It is normally spread 4 – 6” deep over  exposed soil with a second application after about a week. 
Seaweed can also be used with a cardboard weed suppressant layer where  two layers of cardboard in 2 layers are spread on the ground and covered by a six-inch layer of seaweed.   

Dry seaweed can be crushed and added directly to the soil.

For further information on the gardening aspects of Seaweed Mulch follow the link to 

http://www.geoliv.dk/wp-content/uploads/2015/02/Seaweed-mulch-CC.pdf

November 2014 Diploma in Applied Permaculture Design by Cathrine Dolleris is licensed under a Creative Commons AttributionNonCommercial-NoDerivatives 4.0 International License.