Seaweed: Everything You Need To Know
For those that live near the coast, gathering seaweed for use in the garden might be an option. In this article I discuss its merits as a mulch and soil amendment.
It is getting into late fall, and this is often the time of year that I go down to the beach to collect seaweed, so I thought I would write an article to address all the questions that often come up when people as me about using it as a mulch or soil amendment.
What is seaweed?
Generally speaking, “seaweed” is various types of red, green or brown algae that can be found in bodies of water. Despite the fact that we call it “seaweed”, it can also be found in ponds, lakes and oceans.
How to collect it?
The best time to collect seaweed is after a big storm. The wind and waves batter and bash the seaweed loose, then throw it up on the shore, above the high tide line. Never collect live seaweed. Aside from that being illegal in many places, its simply not good for the ecosystem, and there is no advantage to collecting it live. It is also the case that live seaweed will have more salt water attached to it, whereas the dead seaweed above the high tide line may have been rained on many times. To collect, I just bring large containers, fill them, and then bring them to my car. This is an ideal thing to do with a friend, because it can be heavy when it is wet. Sometimes (almost always) the seaweed has “sand hoppers” in it, which are like tiny shrimp. They are very mobile and will escape the containers and go everywhere - so for those with nice cars - keep the containers wrapped in a tarp or garbage bags in the trunk; or better yet, outside the car on a rack or trailer; or even better yet - in a friend’s truck!
Is it a good mulch?
The best way to asses the usefulness of any potential mulch is to think about it in terms of how it contributes to soil moisture retention, how well it suppresses weeds, its nutrient content, and how long it lasts. In light of all those considerations I can say that seaweed is a good mulch, but it is not an excellent mulch. It is not as good as other mulches for moisture retention, is fairly poor at weed suppression, and it breaks down quickly. On the good side it does have reasonably good nutrient qualities in terms of nitrogen, potassium, phosphate and magnesium.
Is it a good soil amendment?
Given its reasonably good nutrient properties, seaweed is a good soil amendment, and that is how people traditionally use it - by tilling it in like manure in fall. Even laid on top of the soil, it will break down and improve the soil. Another "old-timer" trick is to lay it in trenches when planting potatoes. I have done this many times and it works well.
What about salt content?
Most of the salt in seaweed is actually on the seaweed. Seaweed has an unique ability to filter salt out - so there really is not that much salt in the seaweed - and so long as it has been rained on a few times (like after a big storm - which is the best time to collect it anyway) there will not be that much salt on the seaweed. Many articles on seaweed recommend hosing it down before adding it to the soil. A biologist friend of mine once estimated that a wheelbarrow of seaweed might not have much more than a tablespoon of salt in it - so I never bother to hose it down - and my plants have never suffered. Of course - if it is added in the fall, then the risk of having too much salt is even lower, because the fall, winter and early spring rains will wash away any salt that might be present.
Does it deter slugs and snails?
I have read many articles about how seaweed deters slugs and snails because it is salty and has sharp edges when it is dry. Having used it for years, I can say with absolute confidence that anyone who writes that sort of rubbish has never used it as a mulch. Slugs and snails are not the slightest bit deterred by seaweed. Anything that is prone to slug and snail damage (like kale) will be just as vulnerable to attack as with any other mulch. It does not deter slugs and snails - or any other pests as far as I know!
Is it bad for worms
This is another thing I have read in a number of articles, and it is based on nothing. Lay down four inches of seaweed, and after a few days pull it back, and there will be worms underneath it having a great time. Worms are not bothered by seaweed - and as far as I can tell, they like it.
What about buying seaweed extract?
Seaweed extract has all the things that are in seaweed - such as nitrogen, potassium, phosphorus, calcium, magnesium, and sulfur, as well as traces of iron, sodium, zinc, copper, folic acid, protein, phenol, and manganese. In this sense it is a good soil amendment - but from a dollars and cents sense - I think it is a waste of money because it is expensive and unnecessary. For those that live inland and cannot get seaweed for free - I think there are other things that are just as good, like leaves, grass clippings and manure.
What’s the best way to use seaweed?
I think seaweed is best used in a number of ways. Because it breaks down fast, it is a great thing to add to the compost pile to get things cooking. As mentioned above, it can also be worked into the soil like manure, or added to trenches when planting potatoes or other similar plants. However, my favorite way to use it is as a mulch in combination with leaves. I add an inch of seaweed, and then put about three inches of leaves on top of the seaweed. In this way, I get the nutrient benefits of the seaweed, and the moisture retention, weed suppression, and staying power of the leaves.
Final thoughts
Seaweed is a wonderful thing to use in the garden, and anyone who can gather it legally should do so. At the same time, I think it is important to point out that there is really nothing special about seaweed - as a mulch or soil amendment - and the best thing about seaweed is that it is abundant and free. For those that can’t source it easily, it makes absolutely no sense to drive hours to collect it, or to spend ridiculous amounts of money on extracts or other forms of magic pixie dust that are made from it. For me - the best soil amendments are the ones that can be sourced for free, nearby. I use seaweed because I can drive to the ocean in 10 minutes. If the ocean was 60 minutes away, I would not use it - and I would still have an excellent garden.
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