Garden Advice: Kelp Belongs in the Ocean

The products sold to improve the health of our landscapes seem endless; I just wish more of them actually worked. I recently received an advertising postcard in the mail. At first glance, it appears that they are selling a valuable product. After all, who doesn’t want to “improve the health and vigor” of their trees and shrubs? But then I reread the claims and some red flags went up.

First of all, it’s hard to guarantee such vague terms “ho-hum” and “show-stopping.” What if my plants merely went from so-so to fair-to-middling? Then there’s the endorsement, in the orange-beige box. The application “helped” the newly planted younger trees. How so? In which way were they helped? And what about the line “it seems like they pop into life – blooms, new leaves, color and vivacity”? Don’t deciduous trees normally get new leaves and flowers in the spring?

There was more information online. According to the website, ArborKelp “is a biostimulant, that promotes root growth and heightens stress tolerance in plants.” You can read the entire claim on their website.

Kelp forestIn case you didn’t grow up at the beach, kelp is a brown algae that grows in cold ocean waters around the world. It’s the basis for an incredible ecosystem, from the fish darting among the floating fronds to the myriad invertebrates hiding in the tangled “holdfast”—the structure that secures the plant to the ocean bottom.  So, my first concern was, where does this kelp come from? How is it harvested? Is it sustainable?

Lots of companies manufacture and sell kelp fertilizers, and there are lots of extravagant claims. The question is, do they work? I started searching for some scientific studies to support these assertions. I specifically wanted unbiased sources. They were very hard to find.

First, I eliminated any website that was selling a kelp product. Then I skipped those that made claims but didn’t cite any references to back up those claims.

I ended up with two articles with opposing conclusions. It was a good reminder that well-designed, repeated studies are essential before we can have any degree of confidence in the results.

First is an article on The National Gardening Association website by popular horticulturist Robert Kourik. He writes:

Several university studies have shown that seaweed can produce dramatic results in plants: geraniums produced more flowers per plant; grapes were sweeter; gladiolus corms grew larger; and cucumber yields increased 40 percent and the fruits suffered less often from softening and rotting. Improved yields after seaweed treatments were measured in potatoes, sweet corn, peppers, tomatoes, apples, strawberries, okra, and oranges. Better frost tolerance, increased seed germination, and greater capacity to absorb trace elements were other documented benefits for plants.

I would very much like to see these university studies, but the article provides no links.

On the other hand, there’s “The Myth of Curative Kelp,” by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University. Chalker-Scott writes:

— Seaweed extracts contain plant growth regulators which, like traditional rooting products, can stimulate root growth in cuttings and transplants
— Seaweed extracts have no reliable effect on plant production or resistance to disease and environmental stress, especially in field conditions
— Variations in plant materials and environmental conditions are greater determinants of plant health than applications of seaweed extract
— Seaweed extracts for landscape use represent a poor use of natural resources, especially those from environmentally sensitive coastal ecosystems

Should we be applying kelp to our gardens? I have decided to forego the expense, preferring to enjoy my kelp forests intact on my next diving trip.





3 thoughts on “Garden Advice: Kelp Belongs in the Ocean

  1. This product is best used to stimulate root growth in new plantings. When you transplant a tree the roots are damaged to some degree. Applying a higher nitrogen product stimulates growth of foliage, which diverts energy from root growth. ArborKelp is a root stimulating product without the higher nitrogen. It therefore is a good idea for newly moved trees, but not necessarily something you would use regularly on your gardens. It’s a great idea for the right application.

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