Actually, this is both true and a myth. Salt really does make a great weed killer (herbicide), as it will kill just about anything that grows, but is so toxic it simply can’t be recommended in most garden settings.
Salting any type of planting will kill plants for months, years, even decades: a sort of scorched earth policy for plants of all sorts, leaving the ground absolutely barren for ages. The Romans are said to have salted the earth at Carthage in 146 BC* in order to destroy any chance of that civilization rebuilding. It’s that efficient at killing plants!
*That, in fact, probably never happened: salt was too valuable a commodity at the time to waste spreading thickly over fields! But it would have worked had they done it.
Even the much-hated herbicide RoundUp (glyphosate) pales in comparison to salt when it comes to environmental damage. At least glyphosate does decompose; salt never does.
How Salt Works
Salt kills plants by osmosis. Where there is more salt outside of the plant than inside, it will draw the water out of nearby plant cells, causing leaves (if applied by spraying) or roots (if watered in) to dry out and die. If you spray salty water on most plants, the leaves and possibly stems will soon turn brown, but they’ll probably soon put out new growth, as salt is a contact herbicide: it doesn’t travel through the plants’ vascular system and therefore only kills the tissues it touches. If it is watered into the soil, though, and kills the roots, that will kill the whole plant.
Salt also kills most soil organisms, both good and bad, including bacteria, fungus, insects, earthworms and slugs.
The Poison Is in the Dose
Of course, we humans use table salt all the time and we’re fine with it. So are plants… when the salt is highly diluted. In fact, both plants and animals (including humans) need both sodium and chlorine (table salt is sodium chloride: NaCl) for healthy growth … but only in small amounts.
Take a look at how many plants grow on the edges of bodies of water with high salt concentrations, like the Dead Sea or Great Salt Lake. There aren’t any, are there? Neither body of water has any fish either: they’re too salty.
How much salt is too much? I hesitate to recommend ever using salt as a herbicide, so in fact the dose matters little. Even if you apply salt to kill weeds on a patio, sidewalk or parking lot where no growth at all for several years would be a benefit, remember that the salt applied will eventually be diluted by rainfall and soak into the water table or run off into nearby bodies of water. If everyone started doing this, it could eventually render the water locally unusable. Already, the use of road salt is causing salt levels in ground water and nearly bodies of water to skyrocket near many cities, a disaster for the environment; gardeners blithely salting weeds would just make things worse.
Where You Never Want Plants to Grow
I simply don’t recommend using salt as a herbicide, especially in or near gardens, lawns, other plantings or where there are tree roots. If you insist on using salt to keep weeds down in pavement or in cracks in a driveway or sidewalk, places where you never want to see plants grow, make sure to at least keep well away from anything growing. If, under your conditions, you’re sure that the salt will not reach the water table, you can spread a thin layer of rock salt (de-icing salt) in between bricks, pavers or stones to keep weeds away for years. Or dilute table salt or rock salt in water (about 3 parts salt per 1 part water), mixing until it dissolves, then (carefully) pour it on. But you’re playing with fire!
Be forewarned that salt is also highly destructive to concrete and paving stones, so by solving one problem, you might be causing another.
You’ve accidentally spilled salt on plantings? Soils can take years to recover if left on their own, but if you water regularly, this will gradually dilute the salt causing it to flow elsewhere.
I don’t recommend using salt to kill plants. Salt may be cheap, efficient at killing plants and even organic (hey, it’s a natural product!), but I feel the potential damage to the environment is just too great!
Learn to tolerate a few weeds rather than kill them. Then everyone will be better off!