Ceanothus species Oregon grape Ocean spray
Baptisia australis ; Echinacea purpurea ; Hemerocallis ‘ Stella d ’ Oro ’; Nepeta × faassenii ; Platycodon grandiflorus ; Sedum ‘ Autumn Joy ’.
~~~ QUESTION : 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 ?
ANSWER : 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 the plants 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 algal 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 notes that seaweed has been used as fertilizer for centuries in coastal regions .
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 ( https :// forums . botanicalgarden . ubc . ca ) 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 .
It is worth noting that you must have a license to harvest seaweed from Washington beaches ( see www . dnr . wa . gov / seaweed ). Harvesting is not permitted everywhere , and where it is allowed , there is a ten-pound wet-weight limit . 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 .
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 . More sustainable mulch options include free wood chips from a local arborist . m
Rebecca Alexander is the manager of Reference and Technical Services at the Miller Library , located in the UW Botanic Gardens ’ Center for Urban Horticulture ( 3501 NE 41st Street , Seattle ). She is also a contributing editor to the “ Bulletin .”
RESOURCES
Mouritsen , Ole . “ Seaweeds : Edible , Available & Sustainable .” University of Chicago Press , 2013 .
Tenenbaum , Frances . “ Gardening at the Shore .” Timber Press , 2006 .
Washington State University Extension Shore Stewards . “ Spotlight on Shoreline Planting .” 2015 . https :// s3 . wp . wsu . edu / uploads / sites / 2144 / 2015 / 03 / Spotlight-on- Shoreline-Planting . pd
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