Don’t bother.

Using Epsom Salts in the garden is a waste of time and money: may my grandparents (and renowned scientific journals like The Mirror) forgive me for contradicting them like that.

Since the dawn of time, the Epsom Salt industry has had one overarching vision for mankind’s destiny: to consume even more product by dumping Epsom Salts all over their garden.
It’s not enough for Jessica Biel to chuck a baby’s worth of the stuff in her bath, no: it has to go on everything.

It would be like if we, Ashridge Nurseries, worked out a way to sell more plants by convincing people to throw the plants they buy from us on their bonfire.
Even that would be less cheeky, because at least you would get a bonfire out of it.

Do Plants Benefit from Epsom Salts as a Fertiliser?

Epsom Salt is pure Magnesium Sulphate.
It contains no Nitrogen, Phosporous, or Potassium (the N-P-K values that you see on any fertiliser packet) at all, so as a standard fertiliser, it’s useless.

Plants do need Magnesium for making chlorophyll, it’s one of the 9 macronutrients essential for healthy plant growth.

However, it’s rare for a garden topsoil in the UK to be Magnesium deficient. Serious nutrient deficiency tends to happen in three places:

• The potting soil of plants that have been in the same pot for a long time
• Soils with extremes of high or low pH, and/or very sandy & free draining: these conditions are rare in British gardens
• Cultivated fields that have been ploughed for a long time: if your new garden was previously a field before you converted it, thorough soil testing is a must

Any potting soil that you buy from the garden centre, or compost that you make at home, will be a perfectly good source of Magnesium.

If you are really stuck with terribly poor garden soil, then inorganic fertilisers in general, used moderately in combination with regular soil tests, are a real world fix, and can pull great and flowers out of the worst soils.
But inorganic fertilisers have a lot of costs, mostly lasting a short while, and causing soil imbalances in the long term if overused.
The “natural”, low-cost solution for a home gardener is to improve your soil with organic matter (both rotted compost or manure, and mulch) so that soil life, which is the engine of soil fertility, can thrive.

Remember, a gardener is above all feeding the soil, rather than the plantsthemselves (with container grown plants, it’s a bit harder to feed the soil in the pot without repotting the whole plant, but you can still change the top layer of potting soil/mulch and water with a compost tea to replenish the organic matter being lost each time the pot is flushed through with water).

A well-fed soil should never need additional fertiliser, and adding fertiliser to rich soil is a waste of money that could be harmful to sensitive plants.

Do Epsom Salts Kill Pests?

No. Bugs don’t like being sprayed with it and many will scurry off at the moment of contact, but that’s all. They will be back.

It doesn’t do anything to fungus, either.

But My Grandmother Said it Prevents Blossom End Rot in my Tomatoes!

I know, we’re in the same boat. I certainly won’t be the one to tell her otherwise, and I don’t expect you to either.
This is not a hill to die on, grandmothers are too precious to argue with, but the pedantry isn’t over:

Blossom End Rot in tomatoes is caused by Calcium deficiency.
Epsom Salts contain zero Calcium, and too much in the soil will actually inhibit Calcium absorption.

OK, But What About Roses? Surely They Love a Salty Treat?

Sorry, but roses are plants too, and there is nothing special about how they grow that means that they need special fertiliser.
The American Rose Society is a massive party pooper on the whole Epsom Salt and Roses parade:

“There is no published, scientific research on Epsom salts effects upon roses. The origin of these “research-based” recommendations is unclear.”

Linda Chalker-Scott, Ph.D

Fine, Should I Ever Use Epsom Salts in the Garden?

It is wise to only use a fertiliser if soil tests confirm that your soil is indeed deficient in a given nutrient, and you are struggling to grow certain plants.
Soil tests should rule out any need for Epsom Salts in most gardens, and where a Magnesium deficiency exists, there are other long term remedies, mainly involving incorporating organic matter and appropriate rock dust.

Epsom Salts are the kind of tool that an advanced gardener who likes to tinker with fine details might use, perhaps when growing plants in soilless potting mix.
A typical gardener should not be casually using them.