Seawater, Is It Ever Ok To Use On Plants?


As an off grid homesteader, you’re always looking for ways to use the natural resources around you to enhance your garden.   If you are lucky enough to live by the ocean, I bet you’ve wondered if seawater would be ok to use somehow.

Full strength seawater is harmful to most plants except for a few, but dilute the seawater and you unlock a whole host of benefits for the plants which include disease suppression, increased microbial life, tastier fruits, healthier soil and faster growth.

To get the most benefit of using seawater in your garden, you have to know how to use it properly.  Here is how seawater should and should not be used for your plants on your tropical homestead.


Related: If you are interested in using seawater for plants, then you have to check out Korean Natural Farming (KNF). Korean Natural Farming utilizes inputs such as fermented seawater (among others) to increase the health of your plants. Get your copy of the book that explains it all!

JADAM Organic Farming


Composition of Seawater

Before we get into what seawater does to plants, let’s talk about seawater, the vastest resource on earth.

Seawater is a mixture of 96.5% water, 2.5% salt, and 1% other substances, including other chemical elements and atmospheric gases. 

Although seawater behaves almost exactly the same as freshwater some of its differences include higher density, high viscosity, higher boiling point, and lower freezing point.

Seawater As A Fertilizer

Because seawater is so complex, it can be used carefully on plants to increase the much-needed minerals, yields, and flavor of crops.

Many of the chemicals found in seawater are good for plants as a fertilizer. When diluted, seawater provides many nutrients and micronutrients, which cannot be found in other fertilizers. While there are seawater fertilizers, which you can purchase, here is how to make your own.

How to Use Diluted Seawater as Fertilizer 

According to researchers at the University of Hawaii’s College of Tropical Agriculture and Human Resources, you can make your own diluted seawater fertilizer following this recipe. 

  1. Collect Seawater.

Collect seawater near the shoreline, no more than 1 to 2 inches below the surface. Avoid collecting seawater near rivers, fishing boats, or shipping lanes to minimize pollution. 

  1. Let it Sit.

Place the collected seawater in a bowl and leave it to sit, uncovered, for a day. This lets evaporation, aeration, concentration of solids, and the prevention of some airborne microorganisms from growing.

  1. Dilute.

After 24 hours have passed, dilute the seawater with freshwater. For every part seawater, dilute with 30 parts fresh water. This translates to roughly four ounces of seawater to one gallon of fresh water.

  1. Water.

With a watering can, sprayer, or irrigation system, water your plants with the diluted seawater solution in the morning or evening before it gets hot.

  1. Store.

Keep the diluted seawater in a clean glass jar out of direct sunlight. Use your diluted seawater as soon as possible. Discard when it starts to smell, or when a white haze or mold appears.

Use diluted seawater on your plants every year to every month.

For enhanced benefits of using seawater on your plants, ferment it and increase microbial life dramatically.  I have been using fermented seawater on my coconut trees and have had some amazing results growing coconuts miles from the shoreline.

Related: Fermented Seawater For The Home Garden

Benefits of Using Diluted Seawater as Fertilizer:

  • Stimulate the growth of beneficial microorganisms
  • Suppress disease
  • Increase fruit and vegetable yields
  • Speed up growth
  • Increase flavor of fruits and vegetables
  • Improve soil health
  • Improve overall tolerance of plants

What Happens If You Use Full Strength Seawater

As you can see, seawater in small quantities benefits plants, but you may be here to learn if seawater in larger qualities is okay for your plants. 

The short answer is no, watering plants with seawater or over-exposure to seawater is not okay for most plants. Extended exposure to the salt from seawater can permanently destroy your plants.

Here is how seawater interacts with most plants.

Leaves

The saltwater on plants can cause burns to the foliage or inhibit photosynthesis, but generally, some saltwater on the leaves will not negatively affect your plants long term. 

Roots

Salty, or saline soil, can have even worse effects on your plants. 

The salt can prevent moisture absorption of the roots, severely dehydrating the plant. If the soil is very saline, the salt might even pull moisture out of the plant into the ground – the opposite effect of what you want when you water.

If the plant is receiving freshwater, it may be able to cope for a short period of time. But after a while, the plant may develop salt poisoning. Salt poisoning prevents processes used in the conversion of chemicals to nutrients and sugars from the plant.

Effects Seawater Has On Photosynthesis

Photosynthesis, that long word you likely learned in high school biology, can be inhibited by saltwater. The process of photosynthesis is how plants convert sunlight to chemical energy they use to survive. 

Osmosis

Plants absorb water through osmosis, where molecules move from an area of high concentration to low concentration. Plant roots have a higher concentration of molecules than soil so water moves to the roots. Water easily makes this transfer, but chemicals and salt take longer. 

When salt water falls into the soil, the roots of the plant will try to absorb it like normal water but instead will be blocked from osmosis, leaving the plant dehydrated. If the soil is heavily saline, negative osmosis of the plant could occur, with the soil absorbing water from the plant.

Stomata

Stomata are tiny openings on the leaves of plants that play a vital role in photosynthesis and gaseous exchange of the outside air with the internal branched system of air within the plant. 

Stomata are part of the respiratory system of the plant, allowing carbon dioxide in and excess oxygen out. In the presence of too much salt, they may fail to open, trapping excess nutrients and preventing new absorptions.

What Plants Are Tolerant To Seawater

Although seawater is generally not okay for plants in large quantities, sometimes exposure cannot be avoided. Choosing salt-tolerant plants is your best bet in regions near the ocean.

The amount of seawater a plant can absorb safely depends on its type. Some plants have evolved to live in seawater, such as seaweed. Other plants have evolved to be highly sea-water tolerant, deriving nutrients from saline soil and properties that help them cope with seawater spray. Others can withstand seawater, while some are highly sensitive to it in any form. 

You might not be able to completely avoid seawater getting on your plants, so it is best to look into salt-tolerant flowers and foliage.

Halophytes, salt-loving plants, generally grow in coastal salt marshes and along with bodies of saltwater. 

High salt-tolerant plants will generally handle saline soil and some seawater spray, but should still not be planted in or along with seawater.

Moderately salt-tolerant plants can handle moderately saline soil and the occasional seawater spray. 

Low salt-tolerant plants should not be planted in saline soil or where they might be exposed to seawater. Although the rare seawater mist might not kill the plant, they need to be given extra care if they are planted in such an environment to minimize stress and dehydration.

One thing to keep in mind when looking through seawater-hardy-plant lists is some plants are tolerant of salty soil but not direct seawater. 

Here is a list of select flora for your garden near seawater that can withstand both saline soil and some seawater.

Plants 

  • Sand Live Oak. Moderately tolerant. USDA gardening zones 7 through 10.
  • Champaca Magnolia. Moderately tolerant. Zones 10 through 12.
  • Pineapple Guava. Moderately tolerant. Zones 8 through 11.
  • Daylily. Moderately tolerant. Zones 3 through 9.
  • Yarrow. Moderately tolerant. Zones 7 through 9.
  • Muhly Grass. Highly tolerant and great in zones 6 through 10.
  • Rosemary. Highly tolerant. Zones 7 through 10.
  • Virginia creeper. Moderately tolerant. Zones 3 through 10.

Vegetables 

High Tolerance

  • Asparagus. Can withstand flooding.
  • Beets.
  • Bell Peppers.
  • Broccoli.
  • Cabbage.
  • Quinoa. Excellent salt-tolerant grain grown all over the world and researched heavily because of its health and environment versatility.
  • Kale.
  • Loquats.
  • Spinach.
  • Tomatoes.

 Moderate Tolerance

  • Carrots.
  • Cauliflower.
  • Lettuce.
  • Peas.
  • Potatoes.
  • Squash.
  • Sweet Corn.
  • Soybeans.

Low Tolerance

  • Beans. 
  • Celery.
  • Cucumbers.
  • Radishes.

Seawater-Loving Plants

Any plant that can live in or near saltwater has developed a system of quickly excreting the salt from the water when it comes in contact with saline water. Here are just a few halophytes.

  • Succulents (purslane). Zones 4 through 11.
  • Sea Lavender. Zones 4 through 11.
  • Marram Grass. Zones 4 through 10b.
  • Yerba Mate. Zones 9 through 11.
  • Coconuts.  Zones 10 through 11.
  • Naupakapaka. Zones 9 through 11

How do these plants use seawater effectively?

Each salt-tolerant plant deals with salt differently.

In plants like quinoa and sea lavender, salt is absorbed into bladder-like cells on its leaves, leaving the rest of the moisture salt-free and able to be absorbed by the plant. 

In other plants like succulents, the cells can absorb enough water to dilute the seawater it comes in contact with.

Some salt-resistant plants have roots that grow deep enough into the soil to bypass the salient part. Others have structures that protrude from the root into the air above ground.

Final Thoughts

While many plants are okay with some seawater, choosing a plant with salt tolerance is important. Until we find an efficient way to desalinate seawater, freshwater is what all living things need to stay alive. 

Although a small dose of minerals in seawater is great for your plants, make sure your salt-tolerant plants are getting plenty of fresh water and care to withstand the effects of a coastal environment.

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Sean Jennings

Sean has been living simply Off-Grid in Hawai'i for over 18 years. He lives debt free on Hawai'i Island with his family and over 40 chickens. When he's not tinkering around the homestead, he's off exploring the shorelines for fish & surf.

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