Do gardenias need Epsom salts?
If you type ‘gardenias’ and ‘Epsom salts’ into Google, you’ll find a range of gardeners keen to share their love affair with this cheap and available plant tonic.
Dissolving some of the white crystals in a watering can, then dowsing the root zone with it is supposed to bring yellow-leaved gardenias back to life, especially in springtime.
The thinking behind this practice is that the yellow leaves are a sign of magnesium deficiency. Adding Epsom salts, or magnesium sulfate, is supposed to replenish the missing magnesium, perking up the gardenia in the process.
Some gardeners recommend a one-off dosing, others a feed at regular intervals.
The Epsom Salt Council – a body that promotes Epsom salts for a manner of health, beauty and gardening reasons – recommends application every 2-4 weeks for acid-loving shrubs.
But not everyone agrees.
Don Burke, an Australian TV gardener, says the yellow leaves aren’t a sign of magnesium deficiency at all but a sign that the plant has suffered in winter.
“The problem is caused by cold damage, rather than a mineral deficiency,” he writes in The Complete Burke’s Backyard: The Ultimate Book of Fact Sheets.
“So forget the Epsom salts. Instead, give your gardenias a good feed in spring, using some aged chicken or duck manure.”
Associate Professor Linda Chalker-Scott, a US urban horticulturist at Washington State University, also doubts the power of Epsom salts.
When she reviewed the scientific evidence for applying it to garden plants, she didn’t find anything robust enough to recommend it.
Any evidence she did find was from studies on commercially grown plants, none of them gardenias, that were suffering from known magnesium deficiency. Nothing on use in the home garden, nor on plants whose magnesium status was unknown.
She also says that claims it makes plants grow bushier don’t stand up.
Could it do harm?
Isn’t it worth giving it a try anyway? Surely, thousands of gardeners can’t be wrong.
She says that as Epsom salts is so easily soluble in water, it’s easy to overdo a dosing and cause run-off. She concludes:
“It is irresponsible to advise gardeners and other plant enthusiasts to apply Epsom salts, or any chemical, without regard to soil conditions, plant needs, and environmental health.”
Even if the plant is magnesium deficient, adding Epsom salts won’t always fix things, she says. This might be the case with soils heavily leached by rainfall or suffering from too much potassium.
What might help, she says, is boosting soil nitrogen, which better allows some plants to take up magnesium from the soil.
So, along with Don Burke, that’s two recommendations to give your plants a nitrogen boost with a good feed and ditch the Epsom salts.
Image: Flickr/Angela Anderson-Cobb