How to use seaweed to fertilize tomato plants?

Seaweed is among the best gifts nature makes available for the gardener. 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

• 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.

• 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.