Long ago, gardeners who lived near the ocean learned that seaweed was good for their plants. Exactly how it works is difficult to pin down, but scientists have found in seaweeds a veritable soup of plant-growth stimulants, vitamins, chelating agents, trace minerals, enzymes, and amino acids, all of which influence the growth of plants in different ways.
Robert Parnes in his classic Organic & Inorganic Fertilizers (Woods End Agricultural Institute, 1986; $40) says, "Perhaps the most important merit of seaweed is its content of assimilable organic materials, in particular the growth hormones."
Seaweeds contain small amounts of nitrogen, phosphorus, and potassium. On a dry-weight basis, seaweeds contain up to 1.2 percent nitrogen, 0.2 to 1.3 percent phosphorus, and 2.8 to 10 percent potassium.
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.
When gardeners talk about using seaweed on their plants, they are usually referring to a brown algae, specifically the one known as knotweed or rockweed (Ascophyllum nodosum). It's common off the coast of Norway but also grows along the American coast from northern Maine to Canada and throughout northern seas. The seaweed industry had an early start in Norway where seaweed supply was abundant, hence the "Norway" on the labels of many seaweed products.
The seaweed product that's been around longest (40 years) is Maxicrop. It is normally sold as a concentrated dry powder that you mix with water and apply to plants as a spray. But it is also available as a liquid concentrate, as are most other seaweeds.
Most liquid seaweed fertilizers are extractions manufactured by hydrolysis, and most of the basic research done with seaweeds (by T. L. Senn, formerly of Clemson University) used this form. You might read about liquid seaweed products that are "cold-pressed" or "enzymatically digested", but little research exists regarding these materials. Seaweed is also available as a dry meal intended for adding to soil. Liquid concentrates cost more than the powders, but they are much easier to mix with water.
Another seaweed fertilizer is derived from California bull kelp (Nereocystis luetkeana). It is offered only by Peaceful Valley Farm Supply, whose catalog claims this fertilizer is more potent than ascophyllum-based products, but I haven't seen any research to back up such claims.
I recommend using seaweed extracts on plants where irregularly yellowed foliage suggests a micronutrient deficiency. Diagnosing such hunger signs is very tricky for most home gardeners. The surest way to know--a leaf-tissue analysis--is expensive, which is why applying the broad spectrum of micronutrients found in seaweed extracts to your plants may be an efficient remedy.
A deficiency of micronutrients in the soil rarely causes a deficiency in the plants. Micronutrient deficiencies in the plants are commonly caused by soil that has poor structure, is poorly drained, is cold, or has a pH that is too high or too low. In these situations, the tonic of a seaweed spray can help the most.
I've tried seaweed sprays on yellowing citrus plants with only limited success. Although seaweed sprays have a natural iron chelate (a chemical that aids iron absorption), the response of citrus to synthetic chelates is often noticeably quicker than to seaweed.
Apply seaweed meal to soil, or apply extracts as a liquid soil drench or onto leaves. Nutrients applied directly to leaves are absorbed and then pass into the plant's circulatory system. Once there, the nutrients are distributed throughout the plants' tissues.
Most liquid extracts require dilution in the range of 1 to 2-1/2 ounces per gallon of water. To mix powders (1/4 teaspoon per gallon) easily, treat them like a gravy. First mix a small amount of water with the powder to make a thick, smooth paste and dissolve lumps. Then, add the paste to the full volume of water. Apply seaweed meals at the rate of 1 to 3 pounds per 100 square feet.
Follow the dilution rates and spraying intervals recommended on the container. Don't assume that using more than label-directed amounts of a benign material like seaweed can't hurt. Studies have shown that at greater than recommended concentrations, seaweed sprays can actually retard the growth of plants.
Foliar seaweed sprays are best applied to plants with a pressurized tank or backpack-type sprayer. These devices are much better than a hose-end or siphon-type sprayer because they can spray a very fine mist, which allows more solution to stay on the leaves rather than drip to the ground. There is also evidence that a fine mist allows more rapid or efficient absorption of the seaweed in the water.
Be sure to thoroughly cover the undersides of leaves as well as the tops. Then carefully clean the sprayer, so the nozzle isn't clogged next time.
When to spray. Leaves take a "siesta" in the midday heat and won't readily absorb the spray, so the best time to spray is early morning or evening. If your plants display any signs of disease, spray only in the morning, so the leaves will dry quickly and reduce the infection's spread.
Foliar sprays are absorbed relatively quickly, in 1 to 24 hours. Should it rain within a day, respray. Likewise, don't irrigate plants from above until a day after spraying.
Using a spreader-sticker (a soaplike material that makes sprays spread over and adhere to plants) will improve the efficiency of the seaweed spray. Most spreader-stickers are made from petroleum products, though some gardeners use soy oil, safflower oil, or liquid soap (1/4 teaspoon per gallon) as a natural spreader-sticker.
Composted seaweed. Your plants can also benefit from seaweed's many nutrients if you collect the material yourself and add it to your compost. Gardeners who live close enough to an ocean may find such a venture practical. However, before collecting seaweed yourself, check with local coastal authorities to be sure it's legal, and rinse away salt with fresh water.
Are seaweed sprays beneficial? I'll never forget my experience 15 years ago. I restored a wan, yellow-leaved persimmon tree to green vigor. I did it with a foliar application of seaweed, and, afterward, became a believer in the miracle powers of these ocean plants. Since then, however, results have been mixed. Though seaweed may not always produce the desired results, the cost of an application is relatively low. Though no soil or plant additive will ever replace good gardening practices, only seaweed provides so many key plant micronutrients and growth enhancers in a quickly available form.
Robert Kourik is a horticultural consultant and writer who lives in Occidental, California.