I noticed a Seattle P-Patch garden that was using milfoil from Lake Washington as a mulch in the vegetable beds. That made me wonder about using other aquatic plants as mulch, such as seaweed. Would this be beneficial to the plants? Or would it add salt to the soil and cause problems?
Milfoil (Eurasian watermilfoil, or Myriophyllum spicatum) is a Class B noxious weed in Washington State, and it is on the quarantine list. I am not sure whether moving milfoil dredged from the lake into a garden as mulch violates the quarantine’s prohibition on ‘transport of plants,’ but presumably it had died back before being spread on the beds. When the plants decay, they do impart nutrients (potentially beneficial to the soil, but a detriment to the lake because they cause algae growth), but Lake Washington is not a pristine body of water, and I would be somewhat concerned about pollutants.
As for using seaweed as mulch in the garden, the book Seaweeds: Edible, Available & Sustainable by Ole Mouritsen (University of Chicago Press, 2013) notes that seaweed has been used as fertilizer for centuries in coastal regions. “In France and on Iceland, this practice goes back at least as far as the 14th century.” In Scotland and Ireland particularly, scraps of seaweed that wash ashore have been added to soil to form raised beds for potatoes and other crops. Such beds hold moisture well, but there is a concern about soil salinity (harmful to earthworms and some plants) and pollutants from contaminated water, so it is best to wash the seaweed in rainwater before use. Plants that were originally shore plants, like asparagus, cabbage, and celery are more salt-tolerant.
There is an enlightening discussion on the University of British Columbia Botanical Garden forum about using seaweed in the garden. A biologist urges rinsing the seaweed at the beach to free any creatures that might be attached to it. Even desiccated seaweed higher up on the beach harbors living things that will not survive if you unwittingly transport them with the plants you are collecting. Additionally, there may be seeds and roots of other plants you might not want to introduce into your garden.
It Is worth noting that you must have a license to harvest seaweed from Washington beaches; it is not permitted everywhere, and where it is allowed there is a ten-pound wet-weight limit. There are specific guidelines on what tools to use, and how to leave behind the base of the plant so it can continue to flourish. Be mindful that seaweed is an integral part of a complex ecosystem, and you do not want to disrupt habitat and food sources when gathering plants to use as mulch. Also heed any notices posted about pollutants that may have been released in the water where you are harvesting.
All of this being said, it does not make much sense to collect seaweed for mulch unless it is ‘in your own backyard,’ that is to say, you live near the beach. There are more sustainable mulch options (feed a compost pile with materials already in your garden, and use that as mulch; obtain free wood chips from a local arborist) that do not come with so many environmental factors to consider.