Winter always comes and goes every year, so many ask me about freezing their extra potting soil on the porch or in the garage. What will it do? Does freezing sterilize potting soil? What’s the best way to store potting soil in winter?
Generally, potting soil does not freeze unless it is wet. Freezing the soil at 0°F or -18°C will not effectively sterilize it. Only high temperatures (over 140ºF or 60ºC) can kill soil-bound organisms such as fungi, mold, nematodes, gnats, and other plant pests. Finally, potting soil is best stored in waterproof, resealable bags or containers.
Do you have more questions about using, storing, sterilizing, reviving, or reusing potting soil? Here are seven tested ways to make the best of what you have. Read on.
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Potting soil can freeze only when it has moisture or liquid. However, dry potting soil can become so cold that any organisms living in it can either go dormant or even die.
When we say that soil is frozen, it’s the water or moisture content that’s frozen, not the soil. In fact, dry soil is freeze-resistant at whatever temperature.
In short, frozen soil or permafrost actually refer to the frozen water within the soil.
If you left your potting soil outside in winter and it’s frozen solid, you can bring it inside and let it thaw beside the woodstove. One bag of frozen potting soil should be completely thawed in only one day.
To store potting soil for the winter, any container that keeps the contents dry will work, such as covered bins or garbage cans. Even resealable and extra-thick plastic bags will do.
To prevent potting soil from freezing in winter, keep it dry in containers or bags that are waterproof and resealable. As mentioned, the soil will not freeze even at the lowest temperature if it does not contain any moisture or water content.
The best way to store potting soil so that you can use it for indoor gardening even in winter is to store it in airtight containers such as those used for food storage. Other options include metal, tin, or stainless steel food tins used to store flour and other baking materials.
If you’re storing a large amount of potting soil, try out large, airtight pet food bins that come with wheels and even a scoop, such as this one on Amazon.
You could also store potting soil in extra-large, extra-thick, heavy-duty ziplock bags such as this one on Amazon. Simply pour potting soil into the ziplock bag, press the seal securely, and label the bag with important information, such as date of storage, additives included, and the like.
Indoor gardeners with limited space (such as those who live in tiny apartments or compact housing units) often need to hide leftover potting soil in creative spaces such as a storage bench.
Potting soil does not last long when stored in wood or ceramic containers that often have pores that allow moisture to enter. Even a glass or metal container should only be used if it is watertight and has a sealed cover.
When the temperature goes below freezing (32°F), any water, liquid, or moisture in potting soil can turn into ice. When this happens, the soil expands, the pot or container may crack, and it’s very hard to use frozen potting soil for indoor gardening.
If you have tropical houseplants on the porch or outside the window, bring them indoors before temperatures go below 45°F. Plants that are native to cold-weather areas may recover from the cold shock after several months. Others may not.
A light frost (29-32°F) may cause minimal damage, but a severe frost can kill plants, particularly young plants, and new transplants. If, for some reason, you can’t bring in your potted plants, wrap the pots to trap heat around the roots: use old blankets, bubble wrap, burlap, or geotextile blankets. You can also check out a freeze frost protection cover for potted plants.
If you have perennials that naturally die in winter and revive in the spring, plastic pots filled with potting soil can stay outside in winter. If you’re using clay pots, bear in mind that when winter rain or snow fills the pot, freezing temperatures can cause the water to expand and the pots to crack.
Always let your potting soil dry before storage, so that issues such as freezing, mold, and mildew can be avoided.
Frozen potting soil can be used for many herbs, houseplants, and flowers. But to prevent root shock, let the potting soil thaw to room temperature. Indoor gardeners who do some planting in the cold season typically bring potting soil indoors and let it thaw before they use it.
Cold potting soil may shock the roots of tropical houseplants and hothouse herbs. However, there are many plants that don’t mind the cold.
Among the most frost-tolerant herbs, our favorite kitchen flavors come from perennials such as:
- Thyme (Thymus vulgaris)
- Fennel (Foeniculum vulgare)
- Chives (Allium schoenoprasum)
- Mint (Mentha)
- Sage (Salvia officinalis)
- Rosemary (Salvia rosmarinus)
- Winter savory (Satureja montana)
- Tarragon (Artemisia dracunculus)
To know more, read our guide on cold-tolerant herbs.
Favorite houseplants that thrive in chilly weather include:
- ZZ plant (Zamioculcas zamiifolia)
- Clivia (Clivia miniata)
- Jade (Crassula ovata)
- Snake plant (Dracaena trifasciata)
- Maidenhair fern (Adiantum)
- Philodendron (Philodendron)
In addition to that, you can also enjoy the colors of blooms indoors, even if it’s chilly outside.
Flowers that don’t mind cold potting soil include
- Wax plant (Hoya)
- Moth orchid (Phalaenopsis)
- Coneflower (Echinacea)
- Christmas cactus (Schlumbergera)
Exposing infected potting soil to freezing temperatures (0°F or -18°C), will not kill bugs, fungi, molds, larvas, gnats, nematodes, and other pests. Rather, these bugs and microorganisms will only go dormant and become active again when the temperature is warmer.
If you’re growing herbs, fruits, and vegetables indoors for your food, you want to sterilize potting soil in a chemical-free way – so no bleach or stuff like that, all right? Your options include baking or exposing potting soil to high heat. Solar heat will do, too.
When using garden soil or reusing potting soil, sterilize them to prevent garden pests, plant diseases, fungal spores, and other unfriendly bacteria from infesting your indoor garden.
Ordinary garden soil can compact and make drainage difficult for potted plants. Likewise, pure potting soil should be sifted clean of debris and enriched with soil containing organic compost, for best results.
You can kill bacteria in potting soil by steaming or by nuking it. Leave open the top of a clean zip-top, plastic bag with about two pounds of moist soil in it. Place it in the center of a microwave oven and heat it on high until the middle the moisture in the soil begins to escape.
Actually, potting soil doesn’t expire. However, quality can deteriorate when potting soil is stored for more than six months. For instance, don’t use potting soil when it smells, compacted, or there’s mold, fungus, or insects in it.
The fact is, even after several years, organic potting soil can still be refreshed, enriched, and used. The quality of unused potting soil degrades after about six months. On the other hand, it is best to replace used potting soil every other year.
Bear in mind that the peat moss in potting soil can decompose, particularly if there’s moisture. Completely dry potting soil should be OK to use, no matter how old it is, provided it doesn’t smell bad.
If your old potting soil looks and feels dense and compacted, it may be suffering from decomposed peat moss. Unopened bags of soil can last for about a year or two but peat moss and other organic materials continue to break down even if the bag remains sealed.
If a bag of potting soil is opened or not, if it’s stored under direct sunlight or in shade, if it’s in a damp place or where it gets rained on, these are some of the ifs that can degrade the quality of potting soil.
Potting soil can make you sick. Cases of legionellosis (Legionnaire’s disease) due to Legionella longbeachae infection have been found in Scotland, Japan, Australia, Europe, New Zealand, and the USA, particularly among those who directly handle potting soil or compost. As of this writing, there is no vaccine available for this disease.
You can make your own potting soil if you want. All you need is 1 part sterile garden soil or topsoil, 1 part compost, 1 part peat moss or coconut coir, and 1 part perlite or vermiculite.
What is organic potting soil? It’s made from organic matter (mulch, compost, animal manure such as worm castings, guano, and the like) with no chemical additives or pesticides. Organic mixtures also contain acids that help the roots of a plant soak up more water, becoming more permeable, and this helps them take in more water and nutrients from the soil.
What is potting soil? It’s a mix of peat moss to keep moisture longer, compost, and potentially vermiculite to keep the soil light and airy and facilitate the drainage of water.
Can I keep soil in pots over winter? It is not recommended to leave a planter outside during cold winter days especially if made of ceramic. Indeed, water tends to increase its volume when it freezes causing ceramic pots to crack under such pressure, especially in case of a sudden night temperature drop.
Can I mix garden soil and potting mix? Potting soil can be mixed with garden soil for particular cases such as raised beds, but it’s not a good mix for containers. Learn more about these different types of soil and how to use them in various types of gardens.
Should I use potting soil or a potting mix? Use a potting mix if you’re into container gardening. For large-scale gardening or raised bed gardening, use potting soil.
Before you go, here’s a summary of the top takeaways:
Dry potting soil won’t freeze: It’s wet or moist soil that freezes. Dry soil won’t freeze but it will be cold and crumbly. The best way to store dry potting soil in winter is in waterproof, resealable containers, preferably one with a scoop.
You can use frozen potting soil: Cold potting soil can be used. However, let it thaw or heat it up to at least room temperature before planting, so as not to shock the roots of plants.
Freezing does not sterilize potting soil: At best, nematodes, worms, and parasites will go dormant in low temperatures. However, they’ll revive when the potting soil gets warmer. The best non-chemical way to sterilize soil is by using high heat, such as steaming, baking, or microwaving.
There’s no soil in potting soil. Commercial potting soil mix does not contain dirt or topsoil. Many gardeners add some garden soil to help potting soil hold water and liquid fertilizer longer.
Potting soil can go bad: Don’t use potting soil that smells, one that has crawlies in it, or one with debris such as sticks, stones, and dead plant matter.
And that’s it. You’ve now reviewed some tested tips and tricks about potting soil in winter. Happy gardening!
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- “A cluster of Legionnaires’ disease caused by Legionella longbeachae linked to potting compost in Scotland, 2008-2009” by S. J. Pravinkumar et al in Eurosurveillance
- “Distribution of Legionella longbeachae and other legionellae in Japanese potting soils” by M. Koide et al in Journal of infection and chemotherapy
- “Eradication of Phytophthora ramorum and other pathogens from potting medium or soil by treatment with aerated steam or fumigation with metam sodium” by R. G. Linderman & E. A. Davis in Horticultural Technology
- “Legionellosis” by World Health Organization (WHO)
- “Management of vegetable diseases in home gardens” by W. M. Colt et al, University of Idaho College of Agriculture
- “Overwintering Potted Plants” by Brooklyn Botanic Garden
- “Soil sanitation procedures for the home gardener” by R. M. Davidson & A. D. Davison in Plant Diseases
- “The Dirt on Dirt: What is potting soil?” in Science of Gardening
- “What to do with planters, pots in the winter” by D. Robson in The Patriot Ledger
- “Topsoil vs. potting soil” by S. Boehme and M. S. Boehme in The Coloradoan