Northland
A few times a year, a beach-load of storm-tossed seaweed clothes our coast, and gardeners gather the rich resource.
Wendy Laurenson on the many reasons to collect seaweed.
This treasure from the gardens under the sea is perfect for nourishing our gardens in the ground, and mother nature has already done the production, the harvest and the delivery.
Some gardeners wash their seaweed haul, or chop or trench it. I lay it directly on the garden. The trailing necklaces of finer species are easy to weave around small plants, and the bigger clumps sit comfortably beside more robust plants. Then I leave them alone.
This rich resource comes preloaded with a veritable feast of nutrients perfect for soil biology and plant health.
While seaweed’s NPK (nitrogen, phosphorous, potassium) rating is relatively low at 1-0-4, it is one of the most highly mineralised plants on the planet and contains almost every micronutrient (over 60 trace elements) in an immediately available form.
Natural plant hormones are the tiny-mighties in seaweeds. They are present in very small amounts but impact plants and their environment in a big way. Each has a specific role in plant health, including boosting yields, initiating bud movement and growth, and improving resistance to frost, disease, and stress conditions. Seaweed extracts can also extend the shelf life of fruits and veges, lengthen the life of cut flowers, and increase the nutrient uptake and protein content of pasture crops.
Seaweeds absorb nutrients by filtering seawater directly through their surface tissues to concentrate them in their plant structures. So unlike our land plants, they don’t need roots for nutrient uptake (the “root” on some species is simply their anchor or “holdfast”). They do, however, need the right light and conditions suited to their particular species.
We have over 1000 species of seaweed in New Zealand. Some are subtidal, living their lives fully under the sea, but most of the smorgasbord that washes up here are brown inter-tidal seaweeds ripped from their reef anchorage.
The simplest way to use seaweed is as a mulch.
Seaweed keeps the soil more moist than other options because it is so absorbent, swelling back to its pliable state whenever it gets wet and staying that way for a few days.
If the seaweed layer is a couple of clumps thick it is a great weed suppressant, with the added bonus of no weed seeds, and it doesn’t blow away.
Seaweed mulch can also deter slugs and snails because of the salt and surface texture, and I notice rabbits and birds leave seaweed-wrapped seedlings alone too.
The benefits begin as soon as the seaweed hits the soil, but eventually it gets crisp, then decomposes, enriching the soil with organic matter, minerals and nutrients. It adds plenty of carbohydrates which plants use as building blocks, and it promotes bacterial activity. The alginates (part of seaweed’s gelatinous stuff) help improve soil structure and aeration, plus they retain moisture well.
Decomposition will be faster if the seaweed is chopped or cut up with a mower, or it can be added directly to the compost heap.
Liquid seaweed application is even better than mulch for immediate plant health and nutrient availability. A few clumps of seaweed soaked in a bucket or barrel of water does the trick. The longer it is left to marinate, the stronger the brew will be, but I start using it after just a couple of weeks. Liquid seaweed can also be used to promote rooting – put cuttings in liquid seaweed mixed with water until roots develop.
Seaweed soaked in water underlies the Irish and Scottish tradition of seaweed baths which are reputed to have health benefits including for skin problems and arthritis. Fresh seaweed is briefly steamed and soaked in a bath of heated seawater to release its extracts, then you relax and unwind in the nourishing waters.
Most of our seaweeds are edible, but the best known for food here is porphyra, called karengo by Māori and nori in Japan.
The gelatinous agar and alginates in the cell walls of many seaweed varieties also have a specific food use as gelling agents for stabilising icecream, and thickening sauces and soups.
Agar extraction from Pterocladia lucida seaweed has been used as a culture medium in petri dishes for decades, and as a base for tissue-cultured plants. In recent years, several companies have been using our native Ecklonia radiata kelp for making products for the horticulture, apiculture and agriculture sectors.
One species of New Zealand Carophyllum seaweed shows promise as a natural food preservative, and very recently a New Zealand aquaculture company has started investigating a red seaweed with potential to reduce agricultural gas emissions when added to stock feed.
Meanwhile, there’s another fresh delivery of this rich resource on the beach this morning.