What to Know Before Reusing Old Potting Mix

While reusing potting mix is common, it can harbor disease and impact plant health. Use our tips to determine when it’s time for a refresh.

We also have this handy soil calculator to figure out how much soil, potting mix, or compost you need for your garden beds and containers.

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Reusing Potting Mix

It’s no secret that many of us reuse potting mix each year without any known issues.

And why wouldn’t you?

The cost of potting mix can add up and it seems wasteful to toss it out after each growing season.

And yes, it’s great when it works but there can be risks—some serious and others mild.

While you can’t know for certain if your potting mix is safe to reuse, there are clues to guide your decision.

I’ve written this using outdoor container growing as the example. Houseplants typically require more caution since the indoor spread of insects or disease may ruin an entire plant collection.

Let’s go over the key points so you can get started.

To reuse or not reuse—that is the question!

Contents

• Potting Mix 101
• How to Reuse Potting Mix
• Frequently Asked Questions
• Resources

Potting Mix 101

While garden terms vary around the globe, I’m talking about products sold as potting mix in Canada and the United States.

What is potting mix?

Potting mix is a soilless medium used for growing plants in containers. “Soilless” means it does not contain soil.

Ingredients are typically some combination of organic matter and minerals including:

• peat moss or coconut coir
• perlite or vermiculite for moisture retention
• decomposed wood fiber or bark

While not truly sterile, commercial blends should be free of pathogens and seeds.

Potting mix is best for containers because it’s much lighter and fluffier than soil, allowing air and water to readily reach plant roots.

You may see “potting mix” and “potting soil” used interchangeably. Always check the product label to be sure the product you’re buying is soilless, safe for food crops, and made for container growing.

I mention safe for food crops, because if this is your default, there are fewer concerns about tossing leftovers in your garden or compost bin.

What is soil?

Soil is a mixture of minerals, dead and living organisms, air, and water.

Topsoil is the upper 5 to 10 inches of soil.

You’ll also hear soil commonly referred to as earth or dirt.

This shows how to do a basic soil composition test at home.

Should I reuse potting mix?

If your potting mix appears to be in good condition with no sign of disease, it is probably ok to reuse it—but there is no way to be certain.

Unless we send samples to a lab for testing—and no lab is going to check for every possible pathogen—you just have to use your best judgement and go from there.

Visually, it should look similar to how it does in the package, without any strange things growing in it.

Scent-wise, sometimes “off” potting mix has an unpleasant odor. While you should never intentionally smell it without a protective mask, but you may notice an odd smell without trying.

So, what can go wrong?

Worst case scenario, the potting mix spreads disease causing future plants to struggle or die.

Another possibility—and more likely—is your used potting mix is depleted and new plants won’t have the conditions they need.

If mine seems somewhat depleted but otherwise ok, I combine old with new and add compost or fertilizer to make up for lost nutrients.

How to Reuse Potting Mix

If you can afford it, the top choice is always to start fresh with new potting mix. You can always save the old stuff as filler.

For those of us who want to save money—or perhaps cut down on plastic waste (all those bags add up!)—the half-new solution is a good compromise.

The Half-New Solution

If your old potting mix seems fine, consider combining old with new to save money while still keeping the plants happy.

• One-part old potting mix
• One-part new potting mix
• Slow-release, organic fertilizer (per product instructions) or finished compost

You can either combine the old and new together, or, for deeper pots, use the old stuff to fill the base (where plant roots won’t reach) and put the good stuff up top.

Frequently Asked Questions

Can I use soil from the garden instead of potting mix?

No. Soil from your garden is not recommended because it lacks the traits for successful container growing. Overall, potted plants growing in potting mix fare much better.

What happens to potting mix during a growing season?

How potting mix changes during the growing season depends entirely on what you’re growing.

Fast-growing flowering annuals gradually fill their container with roots, use up available nutrients, and compact the potting mix. By the end of the season, the potting mix is depleted.

Shallow-rooted herbs may just use the top few inches and leave the rest in good condition.

Either way, once you empty your containers, you can assess the used potting mix and decide from there.

Is it ok to reuse potting mix that has old plant roots in it?

Fine, thread-like plant roots (disease-free) are not an issue but do tease them apart with gloved hands and combine them thoroughly with the potting mix so new plant roots can grow freely. Like any organic matter, these old roots will gradually decompose. Remove any larger or chunky bits to keep everything light and fluffy.

What’s the best way to get rid of old potting mix?

If the potting mix was labelled food safe (safe for organic food growing), does not contain artificial additives, and you’re confident it is disease-free, you can discard it in a garden bed or compost bin.

If you’re unsure about disease, hot compost piles will kill off unwanted bacteria and pathogens once temperatures reach around 160°F (71°C).

Some gardeners bake old potting mix at 180°F | 82°C for 30 minutes but I can’t imagine it’s worth the time or energy (literally).

Don’t worry about perlite or vermiculite—those are minerals mined from the earth. If you don’t like the look of the little white bits, you always bury them.

You can also use old potting mix as filler in large flowerpots or raised beds. Add it to the base of the container and use new potting mix higher up where plant roots will grow. I keep a bin of old potting mix on hand for this very purpose.

This has more tips on what to use to fill tall raised beds.

Is it ok to keep old potting mix in containers over the winter?

The best practice is to discard old potting mix.

If your annuals have finished up, empty the pots, clean them (see below), and put them in storage. Assess any leftover potting mix and store what you like for future use in a clean, air-tight container.

If you are growing long-living annuals or perennials that are overwintered in pots, it’s still ideal to repot them at the end of the growing season. This will help reduce unwanted bacteria, viruses, fungi, and pathogens that may be lurking. It’s also a good time to prune the roots and switch to a larger pot if necessary.

It’s your decision whether you use all-new potting mix or some combination of old and new.

I do this with semi-hardy potted fruit trees that I am storing in a garage for the winter.

How do you disinfect flowerpots?

• Empty the containers
• Wash with mild soap and water and rinse
• Disinfect by submerging container in 4 teaspoons bleach per quart of water for at least one minute
• Rinse and allow to dry thoroughly before storing.

Resources

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~Melissa the Empress of Dirt ♛