‘Tis the season for enjoying greenery indoors as we gather together for the holiday. We tend to think about the “boughs of holly” and “Christmas tree” as holiday plants, but we also are probably shoulder-to-shoulder with houseplants huddling inside for the winter! This is the time of year we might think about the last time we fertilized them or repotted them, and might fall victim to the myriad of quick-tip, how-to videos that make “this thing” or “that thing” seem like “THE thing” to solve all of your problems. One of these is epsom salts and it is not necessarily the silver (or magnesium) bullet for plant growth you think it is, but there are ways it can be helpful!

What is Epsom salt?

Epsom salt is a mineral called magnesium sulfate. People use it in baths for sore muscles and to help them relax. It's also good for your skin and might help you sleep better. Epsom salt can be good for plants too.

Magnesium is one of the six macronutrients plants need in order to grow. Magnesium is the building block of chlorophyll, the main chemical that takes photons from the sun and converts them into chemical energy. Epsom salt also has sulfur (the sulfate part!), which helps plants make proteins.

You can use Epsom salt on all sorts of plants, but it's especially good for things like roses, tomatoes, peppers, and acid-loving plants like azaleas and rhododendrons. Those plants tend to have a higher need for magnesium. Acid-loving plants tend to live in areas where there's less magnesium in the soil. Adding a little extra with Epsom salt can help them stay healthy and green.

Things to Know Before You Reach for the Epsom Salt Carton

For Houseplants

Houseplants are usually growing in pots in soilless media. Think of these pots as little islands for your plants! In the grand scheme of gardening, they are growing in a very small place, so adding too much magnesium can actually cause problems. You could end up with something called magnesium toxicity, which basically means the plant's getting too much of a good thing.

Epsom salt can be thought of as fertilizer, and many houseplant fertilizers already contain magnesium and sulfur. If you are not sure, read your fertilizer label! It has to tell you how much magnesium and sulfur is in the solution. Follow fertilizer label recommendations when applying fertilizers to your houseplants. Add only a small amount of epsom salts to your potted plants. A recommendation for reference would be about 1 teaspoon for a 3-gallon pot! More is not better! It's always better to start with less and add more later if needed, since it's easier to correct a magnesium deficiency than it is to fix magnesium toxicity.

Different potting mixes can have an impact on how plants take up magnesium. For example, if you use a lot of peat moss, it can make the magnesium less available to your plants. Coconut coir, on the other hand, tends to allow the magnesium to be more accessible to the plant's roots. Think of it like a sponge – coir can soak up the magnesium and hold onto it, making it easier for the plant to use when it needs it. Peat moss is a little different. It can still hold magnesium, but it doesn't release it to the plant quite as easily due to its lower pH.

For Outdoor Plants

Any time you want to apply a fertilizer to your outdoor plants you should start with a soil test! In North Carolina soil testing is a service offered to the general public through the NCDA&CS’s Agronomic Service. It is a free service from April 1 to Thanksgiving, and there is a $4 charge per sample between December and March. You can get soil sample boxes at your local N.C. Cooperative Extension office year-round, and also get service year-round helping you with how to take a sample and how to interpret your results. All for free! So, before you go putting epsom salts in your garden, please do a soil test! There could already be more than enough magnesium available to your plants.One exception could be a raised vegetable garden or a veggie garden in pots. Magnesium-loving vegetables, like tomatoes, could benefit from a sprinkling of epsom salts, especially after a heavy rain, which can flush fertility out of your garden soil or compost. I have to emphasize “sprinkling” here! It is better to add too little than too much! And what exactly is “too much” will depend on the size of the area or box you are growing your vegetables in. Better to err on the side of caution.

How does Epsom salt compare to dolomitic lime?

There is some misinformation on the Internet that epsom salts can “help balance the soil’s pH”. This is not true. There may be some confusion here. Both Epsom salt and dolomitic lime can be beneficial for your garden, but they work in different ways. Epsom salt is primarily a source of magnesium, which is important for plants to make chlorophyll. Dolomitic lime, on the other hand, provides both calcium and magnesium, and it also helps raise the pH of your soil if it's too acidic. So, if your soil is low in magnesium, Epsom salt can be a good choice. But if you need both calcium and magnesium, or if you need to raise your soil's pH, dolomitic lime would be a better option.

Armed with this information, you should be ready to effectively manage the fertility of your media or soil to benefit your plants! We know that magnesium is important for plants, and we know that Epsom salt is a good source of it. Epsom salt can be great for outdoor plants, but it's easy to overdo it with indoor plants in pots. It's always better to start with less and add more later if needed, since it's easier to correct a magnesium deficiency than it is to fix magnesium toxicity.

Amanda Bratcher is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.