Seaweed is the best garden amendment, hands down
My wife and I have been living in a beach house for the last two weeks, and have been blessed with an incredible run of perfect summer weather. The exclamation point came with the passing of tropical storm Hanna on Saturday whose winds piled fresh seaweed high on the beach — a literal windfall for this organic gardener. I collected sacks of it. Any gardener living near the Northeast or Northwest coast, where kelp grows, ought to consider collecting some. It’s the best seaweed for the garden. It’s a trick I learned from my grandparents which helps me grow enormous vegetables. Here’s the gist of my method, from an article I wrote for Back Home magazine this summer:
Feed bags or burlap sacks work very well for collecting. Plastic bags are a tempting alternative, both for the smell- and ooze-factors in a vehicle, but cannot stand the wet-weight of very much seaweed. However, if you’ve got very sturdy bags or a lot of reused shopping bags, those will work fine.
Here in New England I collect only what I can use of the brown seaweeds like kelp, rockweed, and knotted wrack, which tend to be larger on a volume basis than the green or red varieties, and more sturdy. I try to avoid any seaweed that does not look fresh — once exposed to the elements for a few days, it begins a cycle of decay which rapidly degrades its value in the garden. It also becomes a wildly fragrant home to a whole host of beach-bound animals, from sand fleas to spiders, so it’s just best to leave this little habitat in place for those critters, and thus keep your vehicle smelling comparatively fresh.
Rules about collecting seaweed vary by state and also may differ from town to town — some require a permit. It’s definitely a good idea to check whether there are local regulations before gathering by contacting the local town government.
One of the best things about the harvesting process is the conversations with fellow beach-walkers. I’ve had more chats with strangers at the beach while collecting seaweed than during any other activity, including diving and fishing. People are always curious about what I have in mind for it, and many of these conversations have revealed the questioners to be avid gardeners themselves. A number of these folks have remembered to me parents or grandparents who used seaweed in their gardens, too, and hadn’t thought of it in many years.
Dried seaweed powders and liquid extracts are commonly sold in garden centers and seed catalogs, and for good reason. They’re often derived from kelp, one of the world’s fastest growing plants (up to a half-meter a day), and besides being full of necessary nutrients, it also contains growth hormones (auxins, gibberellins, and cytokinins) which are readily taken up by plants and put directly to use.
Besides aiding soil nutrition, seaweed also stimulates soil bacteria while increasing soil structure, aeration, and moisture retention. Additional purported effects on plants are improved seed germination, increased nutritional value, more extensive root systems, and greater resistance to pests like nematodes.
You can easily make your own powder or liquid amendment with seaweed — all it takes is drying and pulverizing on the one hand, or on the other, making a tea from seaweed fronds soaked in fresh water for a couple days. Some prefer to add seaweed directly to the garden, turning it in with a fork and letting the soil do the rest. This is best done after the weed has been thoroughly dried, though, as fresh it attracts the attention of critters with good noses like raccoons and dogs who will dig it up. And the bulkier seaweeds become quite slimy during decomposition, making encounters with it in the garden less than pleasant.
Composting seaweed, on the other hand, allows the gardener to deliver the goodness of it in a more inert form, so this is my preferred method of incorporation.
The first thing I do is to soak the seaweed in a barrel for an hour to remove any of the residual salt from it, then dump the water, refill it, and add more. There are gardeners more experienced than I who will say that rinsing is unnecessary, that the salt content of seaweed and the amount of salt clinging to its suface is negligible. My intuition says that these sea salts might also be beneficial to the garden, being that their constituents are quite various and rich in minerals other than just sodium, but I usually err on the side of caution. The one verifiable benefit to soaking is that it removes the pounds of sand which clings to the weed, something which I like to avoid adding to my raised beds. Some gardens of course can benefit from the addition of some fine sand. I keep the sand aside to add to potting mixes and the like.
Once the seaweed is rinsed, I create a compost pile with equal parts seaweed and straw, in layers as high as needed: four inches of straw, two inches of seaweed, four more inches of straw, two more inches of seaweed, and so forth, and capping the whole pile with straw.
Seaweed is not exceedingly high in neither nitrogen nor carbon, so it doesn’t matter too much what it’s mixed with in the compost, but it’s important that whatever is mixed in with the seaweed will bring an airy-ness to the pile. On its own the seaweed bogs down quickly into an ooze impermeable to gas exchange, leading to a long, smelly, anaerobic digestion.
To finish I always cover the pile with chicken wire and stake that down securely to keep critters and their noses out. If they get into the pile, they don’t seem to eat anything, but their clawing disturbs a nicely constructed compost heap rapidly.
It doesn’t take very long for the new compost to ripen, depending on the usual atmospheric factors, and is perfect for direct addition to the garden and as a constituent in seed-starting and potting mixes.
So if you’re planning a trip to the shore, remember to pack something in which to carry some seaweed home. If you’re fortunate enough to happen on a wealth of fresh kelp, rockweed, or the like, then bag a bunch and join the increasing ranks of seaweed-savvy gardeners.