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Can You Really Reuse Potting and Seedling Mix?

Timothy Boyer Ph.D.'s picture
Reusing potting soil can lead to mold contamination.

Looking to save money with your gardening? Here’s a practical tip based on gardening experience that answers the question of whether you really can—or should—reuse potting and seedling mix.

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A Problem on the South 40

I have a ritual when the mornings are warmer. I release my were-dogs to wake up the neighborhood and terrorize the squirrels; I make my coffee and ease into the day by watching the commotion; and then I take stock of the “South 40” to see what gardening needs to be done that day.

The “South 40” is not as glamorous as it sounds. It’s just my moniker for my urban plots, potted plants, seedling flats and lab bench gardening experiments. But it is home away from the farm.

That said, I found something new growing overnight in one of my flats this morning—fluffy, snow-like mold. Yay! The answer to another gardening question I had been researching: whether gardeners can or should reuse seedling and potting mix.

My Flat Is a Flop with Mold

Seedling Mold Problems

A closeup of the mold problem

One of the problems of seed starting is controlling temperature and moisture. Especially if you live in a clime like Ohio where if you get tired of the weather, “Just give it a few minutes and it’ll change” gardening reality. While some of my flats have done well this Spring, a few have been resistant to producing seedlings, while others have produced…but took longer than they should have, due to our schizoid weather.

Seedling Germination Sins: You Reap What You Sow

This flat of fluffy, snow-like growth came off of one my slow-to-germinate seed flats filled with a variety of common herb seeds. I have others that are doing better, but wanted to watch this one to see what happens. Yesterday, it was sprinkled with growing stubborn herbs; sparse but healthy appearing. This morning, like magic, they were covered with a filamentous snow of death. I took a few photos and moved on with a new resolve…hence this article.

I look to save money where I can with gardening as indicated in past articles:

Saving Money on Garden Soil Testing

Free Gardening Program You Can Do With Your Children

Low Cost Solutions for Filling Your Raised Bed Garden with Soil

How to Start a Quick, Simple and Inexpensive Garden in Just One Day

Saving Money with Repurpose Gardening: Confessions of a Spirit Raccoon

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Today’s Big Money-Saving Question

And one money-saving gardening question that I see asked online, but rarely given a satisfying answer to is “whether or not you can really reuse potting soil and seedling mix to save on money?” And the best answer is: It depends.

It depends on the aforementioned seedling problems of temperature and moisture control—especially when using commercial seedling flats with plastic domes that function like miniature hothouses. It depends on how long your seedling flat is behind schedule on producing seedlings. And, it depends on how contaminated your growing space is with mold.

In other words, it’s a multifactorial problem that some gardeners experience when re-using potting soil and seedling mixes. Sometimes you can get away with it, and sometimes you won’t. And that’s why you get conflicting and unsatisfying answers online.

The point to all this is that mold is ubiquitous. In other words, it’s everywhere. And given the right conditions long enough, you will see mold pop up everywhere. Think of that forgotten fruit in the back shelf of your fridge.

This is why commercial seedling mixes like vermiculite and others are advertised and sold as being “sterile”—it provides not only a good moisture retaining porous “soil” for starting and growing plants, but its sterility also provides new plants a fighting chance in a world filled with fungi.

So, you’ve grown some seedlings in a flat and have transplanted them. And, now you are ready to start another flat and are tempted to re-use the same flat of seedling soil. It might work for you, but the odds are against it and the money saved is easily lost with a subsequent bad flat with lost seeds and time. I recommend that re-using seedling and potting mixes are a poor way to attempt to save money. It’s not impossible, but it’s not optimal for good growth.

Caveats to the Answer to Today’s Big Money Saving Question

While my answer to today’s big money-saving question may raise some objections, here are alternative approaches that some gardeners use that might allow re-using seedling and potting soil mixes:

• Rise the used soil with boiling water to kill the fungi and then air-dry before using for replanting.

• Bake the used soil in a metal pan in an oven to kill any fungi and then reuse for planting.

• Microwave the used soil to kill any fungi and then reuse for planting.

An Important Reason Why Not to Reuse Potting and Seedling Soil

One final point—and it’s an important one—on why not to reuse potting and seedling soil: it’s just like any laboratory situation and your garden is no different—you don’t not want to risk cross-contaminating your other flats or pots with unnecessary exposure to more mold than normal. The best use of old seedling and potting mix is just to recycle it in your compost pile.

If you have any mold-related gardening advice or experience, please let us know in the comments section below.

Timothy Boyer has a Ph.D. in Molecular and Cellular Biology from the University of Arizona. For 20+ years he has been employed as a freelance health and science writer. Today, with a background in farming and an avid home gardener, Timothy continues writing about science with a focus on the connection between plant biology and gardening for healthy living. For continual updates about plants and health, you can also follow Timothy on Twitter at TimBoyerWrites.

Images courtesy of Pixabay and the author

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