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  • Carolyn Elgar

Seaweed - A Mystery and a Miracle for Plants

Carolyn Elgar

Master Rosarian, Orange County Rose Society

Roses & You, June 2020

Want to give your rose garden a real treat? Give it a dose of seaweed and it will benefit in many ways, including some we don't know much about. Your rose bushes will flourish and reward you with increased healthy growth.

Seaweed has been used for food and fertilizer for hundreds of years. Particularly in areas with easy access to ocean beaches, such as Japan and China, seaweed was spread throughout the garden as a compost or manure. In Ireland, people started collecting it in 1200 AD. It has been used for many purposes - medicine, food, or in decorative, ceremonial items (Ancient Hawaiians used it in their leis.). The benefits of using seaweed in the garden were officially recognized and confirmed by research in the 1950s when people started formulating extracts that could be used for organic farming. Since that time, much has been discovered and written about the many benefits of seaweed applications, especially when used in a liquid form. Even recently we have discovered new compounds in seaweed that increase plant health.


The trace elements, growth hormones, vitamins, and micronutrients in seaweed, or kelp, increase growth and plant health so much that most researchers consider it to be a biostimulant for plants. Biostimulates are not considered fertilizers; they have compounds that make fertilizers more effective and increase plant vigor. They are somewhat of a mystery because the totality of what they do for plants is still not completely understood and is specific to the source. Humic acid is another biostimulate for plants.

Biostimulates are defined by what they do more than by what they are, since the category includes a diversity of substances. As the name suggests, they stimulate growth, but they do much more. Stress tolerance is perhaps the most important benefit of biostimulants - tolerance of drought, heat, UV light, and even diseases. (Biostimulanting turfgrasses, Zhang and Schmidt)

As a biostimulant, seaweed contains known elements as well as compounds that are unique to their source. In the soil, these compounds improve water retention and soil aeration, and microflora increase. The trace elements in seaweed provide some fertilization of minor elements; it does not contain significant amounts of major nutrients, such as nitrogen, potassium,or phosphorus. The elements that affect plant metabolism and disease resistance are what interests researchers and they are still learning about these processes.

...the current status reflects an increasingly sophisticated knowledge of metabolic compounds with direct effects on plant metabolism... or by interacting with pathogens... Journal of Applied Phycology"

How to use

Seaweed is available in liquid, powder, and meal forms. The liquid form is composed of extracts processed from the algae. These extracts are also available as a powder that can be reconstituted by adding it to water. Finally, dried and ground seaweed is sold as a meal that can be worked into the soil. The liquid forms are used more often than the dry forms, in drenches and foliar sprays.

Spraying liquid seaweed or kelp on your rose bushes' leaves will deliver all of its beneficial compounds directly to the plant. Kelp meal in the soil may not be as effective due to problems such as bad drainage or poor soil health. As a result, foliar application is the most recommended way to use the product; in addition to immediate nourishment, disease resistance is enhanced because of the elements absorbed by the leaves.

There is some controversy about the effect of processing on the compounds contained in liquid seaweed. Most garden products that contain seaweed use Norwegian kelp, the brown algae Ascophyllum nodosum. The extraction process is usually done by hydrolysis - some kind of solvent is combined with the algae in a soak to harvest the nutrients. Sometimes heat is used in this process, which may diminish the effectiveness of the hormones in the liquid. For this reason, some companies use cold press or enzymatically digested processes where no heat is involved.

Kelp liquid can also be used as a drench. In this form it takes less time to go through the plants' roots and it is mild enough that is can be used frequently, around every three weeks.

Foliar application may be the preferred application method, but the challenge of dealing with phototoxicity, or leaf burn, could discourage the gardener from using this approach. Follow the dilution directions on the package, spray early in the day and not when the temperature is over 75 degrees, and use a spreader/sticker additive to keep the spray from drying too quickly on the leaves. Again, because it is mild, you can reapply every three weeks or less often, if that's your preference. Spray the tops and undersides of the leaves until the solution is dripping off.

The take away here is that seaweed has many benefits for your rose plants, including some of them that we don't understand. Its compounds increase the effectiveness of your fertilizing program and the energy level of your bushes. It improves plant health and may increase disease resistance. Lacking major nutrients and providing trace elements, it won't overload your plants with too much of something or burn their roots. Your roses will appreciate the extra love.

Kelp, photo courtesy Carolyn Elgar

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