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Seaweed will benefit your garden any time of year, but it is especially useful as a mulch to protect plants during hot, dry weather. In our garden, we’ve come to rely on seaweed as a valuable, yet free, source of fertilizer, mulch and organic pest control all in one natural material.

Benefits of seaweed for gardening

Gathering seaweed for the garden has always been a favorite outing for our family. We usually take a small skiff to a nearby beach and load up with as many sacks as we can safely transport home. It’s fun for children, as they can participate as well as an adult, or they can simply enjoy the beach while we gather the seaweed. As we fill up our sacks, our thoughts drift to the many benefits this will bring our garden.

  • Saves water, keeps soil moist at ground level

    The purpose of any mulch is to keep garden soil from drying out at the surface. And by preventing moisture from evaporating, mulch reduces the need for watering. The practice of mulching is essential in areas where conditions are hot and dry.

  • Eliminates the need to weed
    Mulch covers the soil and blocks new weeds from sprouting. Because the soil beneath the mulch remains moist, any weeds which do manage to sprout through the mulch are easy to pick.
  • Repels slugs and other pests

    Slugs are immediately repelled by two things – salt and sharp-edged materials. Seaweed has a natural salt content which repels slugs, and within a few days of application it dries and becomes quite crispy. Slugs do not like “crispy” surfaces, as the sharp salty edges cut into the soft body tissue. While some mulches may provide hiding spots for slugs, earwigs and other pests, seaweed mulch does not have this disadvantage.

  • Enriches the soil
    Seaweed is a broad spectrum fertilizer that is rich in beneficial trace minerals and hormones that stimulate plant growth. Seaweed is high in carbohydrates which are essential building blocks in growing plants, and low in cellulose so it breaks down readily. Seaweed shares no diseases with land plants.
  • Boosts lethargic plants
    Seaweed fertilizer contains an abundance of fully chelated (ready to use) micro-nutrients which can be readily absorbed by plants without any further chemical decomposition needed.
  • Helps lighten the soil
    Compacted soil can benefit as seaweed mulch breaks down. As the material becomes incorporated into the soil, aeration is improved and the soil becomes more crumbly and moist.
  • Does not contain weed seeds, unlike bark mulch
    Two years ago we used commercial bark mulch to cover our garden pathways for the purpose of blocking weeds. Today, these pathways are sprouting horsetail, an invasive weed which can be difficult to eradicate. Seaweed does not bring any foreign weed seeds into your garden.
  • It’s free!

But what about salt? Is this a problem?
We have been using seaweed as mulch for many years and have not seen any adverse effect, such as a salt overload in the soil. In our region we have plentiful rain. If you are concerned about salt, seaweed can be spread out over the driveway and rinsed with a hose. Of course this is not an issue if you are using freshwater lake weed.

Gathering seaweed for use in the garden

Lindsay gathering seaweed on the beach

  • Gather ‘mid-beach’
    Seaweed is often found scattered on the beach from the water’s edge to the highest point of recent high tides. The seaweed ‘mid-beach’ is drier than seaweed at the tide line and therefore lighter to carry. It also has fewer bugs than the seaweed high up on the beach, and is a little more pleasant to gather.
  • Use fine, broken up seaweed
    Look for patches of seaweed that are smaller in leaf size as this will be easier to apply as mulch. Set large kelp fronds aside – the wide pieces are difficult to form around plants in the garden beds. The kelp can be used to make ‘kelp tea’ and used as a foliar spray to deter insect pests.
  • Use onion sacks or woven poly bags or buckets
    We like onion sacks for gathering seaweed because they are lightweight, the water drains out easily, and they are easy to grip. They are also small enough that we don’t overload ourselves with heavy sacks. Woven poly bags are great if you can find them – ask at your whole foods store because these bags are used to ship whole grains. Don’t use plastic garbage bags as they are too difficult to handle. Your hands get slimy when picking seaweed and this transfers to the bag making it slippery.
  • Limit your impact by picking lightly from several areas
    Each patch of seaweed provides food and shelter for many small marine species. Pick no more than a third of the seaweed from any patch, and move on to another spot.

Applying Seaweed to Your Garden Beds

  • Apply seaweed within 36 hours of gathering
    Seaweed breaks down quickly, especially when in an enclosed sack. If you leave the seaweed in the sacks too long, it gets sludgy and slimy, and is more difficult to spread evenly over the soil.
  • Apply thickly, at least 4 – 6” deep
    Spread the seaweed thickly and evenly around the garden beds to cover any exposed soil. You may think you’re adding too much, but in a few days you will think otherwise.

    seaweed mulch around plants
    In this picture you can see the bare spots showing how much the seaweed has shrunk after the first application. Only a few days earlier, the seaweed was 4 – 6” deep with no soil showing through.

  • Reapply in one week, another 4 – 6” deep
    Seaweed shrinks when it dries. Even a generous layer of seaweed will dry in a few hot days to expose much of the soil. The seaweed will become very stiff and crispy. Ideally, you should reapply in a week or so, and this second application will dry out but still provide good coverage for your soil. Once we make the second application, our beds are usually well-mulched for at least 4 – 6 weeks even in hot, dry weather.
  • Leave a clear space around plant stems
    Once you have spread the seaweed around the plants, push it back just a little from the plant stems so they are not in direct contact with the wet seaweed. Once it dries, the seaweed will naturally ‘shrink’ away from the stems, but it’s a good habit with any mulch to keep plant stems clear.
  • It’s OK to mix seaweed with other mulch
    We use whatever mulch we can get, and it doesn’t matter if you mix several varieties of mulch on a garden bed. For example, our tomatoes are mulched with dried grass clippings (straw) and topped with seaweed. In the fall we’ll add maple leaves. Mix and match, it matters not.
  • Don’t use seaweed to cover garden pathways
    Some people use seaweed as a pathway mulch but this is a waste of good seaweed, which is better used on the garden soil for the reasons listed above. If seaweed is used on pathways, it quickly thins out to allow weeds to get through. And the seaweed will become very slippery underfoot after a rain.

Other uses for seaweed in your garden

  • Save kelp for making kelp tea
    You can put kelp, or any seaweed, into a bucket or large glass jar and fill with water. Leave this in the sun, covered, for a few days and your ‘tea’ will be brewed. Use this as a foliar spray to deter insect pests, or apply directly to the soil around seedlings. Bear in mind that this concoction will smell bad, so be sure to store it downwind!
  • Use seaweed for sheet composting
    If you have a good supply of seaweed, it is ideal for composting directly on a garden bed which is being prepared for planting. In our garden, we use seaweed to suppress green manure which is grown in the bed as a fertilizer and soil conditioner. The problem with growing green manure directly in a raised garden bed is digging in the finished green manure prior to planting without putting stress on the sides of the bed. Our solution is to ‘smother’ the green manure with seaweed for several weeks. This breaks down the green manure, with minimal disturbance to the raised beds or the soil organisms.

    seaweed in raised garden bed
    In this picture, the bed on the right has ‘green manure’ fully grown and ready to dig into the soil. The bed on the left was identical before we added seaweed to smother the green manure, speeding its breakdown and making it easier to incorporate into the soil.

  • Use seaweed as a supplement for chicken feed
    If you have chickens, seaweed has a hidden benefit. Simply drop the sack (it must be porous, like an onion sack or woven poly bag) on the ground in the chicken yard. The next day, roll the sack over with your foot. You will likely see a cloud of insects emerge, freshly hatched sand fleas and other little bugs. The chickens will feast on these bugs! You can repeat this for a few days, then put the seaweed, which will now be sludgy, into the compost.
  • Put seaweed in the compost as well as in the garden beds
    When gathering seaweed for the garden, save a couple bags for the compost. This will help build and condition your compost with trace nutrients. While the instinct is to use your precious seaweed directly on the garden beds, you will get equal value from the enriched compost in a few months. This picture shows seaweed being mixed into compost.

seaweed in compost pile

I realize that seaweed is not available to gardeners living away from the coasts, and I was a little reluctant to even write this article. But each region of the country has different advantages and disadvantages for the gardener, and making the most of what is locally available is essential for those looking for sustainable solutions for gardening.