Why Does Salt Act As a Weed Killer?

Cheap and easy to apply, salt can be an excellent weedkiller, especially in difficult-to-weed places such as gravel walkways. Sodium chloride, the technical term for salt, affects plants in a number of ways, depending on whether it's dry or in solution, the concentration of the solution, and the part of the plant affected.

Techniques For Using Salt

The easiest way to use salt as a weedkiller is to shake or throw it over an area, as evenly as possible. Don't use it on soil containing roots of favorite trees or shrubs since they'll be affected also. You can put a teaspoonful on individual weeds such as dandelions, though the deep roots may require several applications. If your weeds are growing out of cracks in cement, you may find that dissolving salt in a small quantity of water is especially useful.

How Does Dry Salt Work?

Salt crystals applied to directly to fragile plant parts, such as leaves, pull water out of the cells by a process called osmosis. Plant cells are mostly water surrounded by a cell membrane that allows water molecules as well as nutrients and other compounds to pass through. When the salt crystals are wet slightly, by dew or a light rain, water from the cells is pulled toward this very salty solution, shrinking the cells, dehydrating them, eventually killing them. Shake salt on a lettuce leaf and you'll quickly see this reaction.

How Does a Salt Solution Work?

Scattering salt on the ground may kill the leaves, but roots will be unaffected until the salt dissolves and sinks down to their level. You may choose to dissolve the salt in water or simply wait for the rain to do the work. Of course, a cloudburst may wash it away completely so a combination of your own solution and a dry weekend may be a better bet. The smallest roots may be killed on contact by a strong solution, but the effect on larger ones is more complex. They will have difficulty taking water out of the soil because of the increased osmotic pressure, the greater concentration of salt in the soil. This causes all the symptoms of drought because they are unable to take in water even when the soil is wet. They may take ions of sodium and chloride into their tissues, causing yellowing of the leaves or other reactions to that toxicity. Some plants are more tolerant than others, just as some are more tolerant of drought, so results may vary from weed to weed.

Keywords: Salinity, Weedkillers, Salt

About this Author

Over the past 30 years, Mara Grey has sold plants in nurseries, designed gardens and volunteered as a Master Gardener. She is the author of "The Lazy Gardener" and "The Complete Idiot's Guide to Flower Gardening" and has a Bachelor of Science in botany.