Perhaps some potting soil is leftover in your outdoor planters after bringing the plants inside for winter. Or maybe you discovered a bag you forgot somewhere that had to endure the harsh winter season. In either case, you may wonder if potting soil can stay outside in the winter and still be okay to use come spring.
You can leave potting soil outside in the winter, as the cold temperatures will not cause harm. However, you should protect your potting soil from moisture to avoid contamination and bacteria growth. Potting soil should be stored in closed bags or airtight containers if left outside over the winter.
Whether you want to store potting soil outside over the winter, or you accidentally left it out and want to know if it is reusable, then keep reading. In this post, I’ll cover what you need to know about leaving potting soil outside in the winter. I’ll also explain the adverse effects of cold temperatures on soil and what you must do to avoid them.
How To Protect Potting Soil Stored Outside in the Winter
Leftover potting soil can be left outside for winter, yet you must store the mix properly to protect it from moisture and contamination. This issue is particularly true if it has organic ingredients.
Moisture leads to problems with mold and mildew, and troublesome organisms such as fungi or pests can infest open bags of soil. Unfortunately, either will result in contaminated, unusable potting soil come planting season.
To avoid these problems and maintain soil integrity, I recommend that you store leftover potting in sterilized containers with airtight lids to keep out moisture and unwanted contaminants.
You can keep the soil in its bag and seal it with tape to store it in the container, or you can empty the bag and pour the soil directly into the container. You may even want to seal the container lid with tape to provide extra protection.
If you have unopened bags of potting soil sitting out in your yard (especially if they are large, heavy bags that you don’t want to try to move indoors), they should be fine to sit through the winter. If you have an open bag, inspect it to ensure it is free of visible pests, and if so, reseal the bag with clear packaging tape.
How To Sterilize Containers for Storing Potting Soil
The container you use to store your leftover potting soil must be sterile and free from any ruinous organisms, as these could potentially overrun the soil and cause harm to your plants down the road.
To sterilize your storage containers, follow these steps:
- Fill the container with a 9:1 water to bleach solution.
- Place the lid into the solution, leaving the container to soak for at least 10 minutes.
- Remove the lid, empty the container, and leave both to air dry.
- If needed, wipe the container and lid with a lint-free cloth or paper towel to ensure they are completely dry before placing your potting soil inside.
Can I Reuse the Potting Soil in My Containers Left Outside Over Winter?
You can reuse the potting soil in your containers left outside over winter with minimal risk. Yet, you should add some fresh potting soil and fertilizer to help restore depleted nutrients. You should also test the potting soil to ensure its nutritional value is appropriate before use.
Still, there is a chance that invisible pathogens have taken residence in your potting soil, leaving new plants at risk of disease. For this reason, it is best to avoid combining multiple pots of leftover soil and revive each pot of soil individually. This separation will help to eliminate the spread of contamination should you have any issues.
Be sure to stir and sift through the old potting soil to check for bugs and remove old roots and other nuisances. Then use a general NPK fertilizer to condition your mix and ensure your plants have all the nutrients they need to grow and thrive.
Also, remember that you can safely reuse diseased potting soil after thorough sterilization and reconditioning. Yet, it might be best to throw it away and begin anew.
This issue is especially pertinent if you are using the soil for any fruit or vegetable plants from which you intend to eat later on. In that case, it’s best to use fresh, healthy potting soil. Otherwise, it can significantly impact plant development and growth, potentially leaving you with inedible food.
Test Potting Soil Before Use
It is best to use a soil test to determine what nutrients need replenishing in your potting soil. Testing ensures you provide the correct nutrients and amounts your particular plants need.
A kit such as the Luster Leaf Soil Test Kit (available on Amazon.com) is excellent to have on hand for quick, easy soil tests. This inexpensive kit includes 40 tests that use a simple capsule system, producing results with near-lab accuracy at an affordable price. It also has a chart of NPK and pH levels for over 100 different plants for your reference, as well as instructions on adjusting soil conditions.
Does Potting Soil Go Bad?
Potting soil does go bad in that it loses effectiveness over time. This deterioration over time is why potting soil has expiration dates. However, you can usually condition expired soil and still use it for your plants. Use bags of potting soil for maximum benefits within six months of opening.
After a while, the organic matter in potting soil gradually breaks down, resulting in changes in nutrient levels, moisture content, and texture. Yet, this can often be corrected and the lifespan of the soil extended by adding fertilizers and other amendments.
Still, it is always ideal to use fresh potting soil as soon as possible, whenever possible. If you notice mold, check out this article: Your Bag of Potting Soil is Moldy: Is it Still Usable?
The Effects of Cold Temperatures on Soil
While winter temperatures won’t cause harm to your potting soil, ecologists have studied the effects temperature has on the soil. Their studies confirm that soil temperature plays a significant role in ensuring proper plant growth. For instance, too cold a soil can lead to plants having problems with root growth and ion uptake.
Temperature also affects soil composition and how microbial organisms function, in general. The most significant issues observed in cold soil include:
- Inability to transport nutrients
- Depletion of essential nutrients
- Improper water content
- Irregular organic carbon storage
- Cold affects environments needed for chemical reactions.
In addition, a plant’s ability to absorb mineral nutrients depends on numerous biological, physical, and chemical processes affected by temperature.
Soil Temperature and Root Rot
There is also a direct correlation between soil temperature and root rot development in certain plants. In particular, the temperature affects the level of severity in which the disease infects the plant.
A study conducted by Oregon State University determined the infection rates found in cereal crops for two common varieties of root rot, Rhizoctonia solani AG-8 and Rhizopus oryzae. The findings range from moderate to severe cases based on temperature differences in the soil.
In particular, studies show that R. solani causes more severe root rot in wheat during winter seasons when the soil is between 6-19°C / 43-66°F. The studies show that temperature considerably impacts soil and ecosystems worldwide, potentially threatening crop productivity.
Ensure Potting Soil Is Dry and Warm Before Using
The data from these and other studies conclude that soil temperature is critical to successful gardening. Indeed, cold temperatures will similarly affect many organic ingredients in potting soil. Due to this, it is essential to ensure that you give any potting soil left outside in the wintertime to thaw and warm up a bit before you use it.
While you can leave potting soil outside for the winter, you should store the mix properly to protect it from moisture and contamination. Also, you can usually reuse any potting soil left in containers over the winter if it is free from diseases. However, remember that you will need to add fresh potting soil and fertilizer to enhance nutrient levels.