Potted gardens are a great alternative to backyard gardening, especially if space is constrained. But what do you do when winter comes? Re-using the soil for the next season may seem like a no-brainer. But there are few things to keep in mind when preparing your potting soil for next season.
Before explaining what to do with your potting soil, it is worth reviewing the ideal potting soil mixture needed for a healthy potted garden.
Potting soil mixture
Standard soil is actually not a great growing medium for potted plants. There are many potting soil products that are designed specifically for potting gardens that are much better suited than standard garden soil. Potting garden soil should be:
- Porous – This is THE critical factor for potting mixes. Potted plants need a light growing medium that drains well and has ample airflow. Perlite and/or vermiculite are the white granules seen in standard potting mixes that help with drainage and airflow. Porous growing mediums are also lighter and the pots can be moved around with ease. If soil is too dense, water does not flow fast enough and can lead to soil compaction. This can inhibit root growth. Furthermore, if water cannot drain fast enough, it can lead to root rot. When water drains, it pulls out the various soil gases that are created by plants and microbes and replaces it with fresh air. This is called as gas exchange.
- Nutritious for plants – compost material (i.e. plant food) is critical for potting mix because plant roots have less range for accessing nutrients in a pot. Adding compost to you potting mix will ensure your plants can access nutrients and flourish. Worm casting are a highly recommended source of nutrients for potting mixes.
A good potting medium is coir – which is made from the fibrous material extracted from between the inner and outer shells of a coconut. Coir is more porous than peat moss and has less of an environmental impact as well.
What to do with potting mix when winter approaches
Soil contains water, and as we all know, water expands when it freezes. If soil is left inside pots over the winter, it can damage pots, especially if pots are ceramic or clay-based. For this reason, potting soil should be removed from the pots if possible. Doing so will also give you a chance to remove old roots and weeds that can be added to a compost pile.
What if you don’t have space to dump out potting soil? Alternatively, you can store the soil in your pots over the winter and plan to freshen up the mix in the spring. This is also more doable with plastic pots that have less risk of breakage from expanding soil.
Keep in mind the following points below before preparing your container garden before winter sets in.
The Tomato Rule
Pots that have been used to grow tomatoes should not be used again to grow tomatoes for at least two years. Make sure you know which pots have grown tomatoes and that soil can be used to grow peppers, beans, salad greens or anything else.
The reason for this rule is to avoid the risk of blight, or other issues that can damage tomato plants. Tomatoes are especially prone to disease and pathogens.
Mixing soil – what to keep in mind
Mixing soil from pots is safe to do. But remember the tomato rule. Soil from pots that have not grown tomatoes can be mixed together and used for new tomato plants. And soils that have grown tomatoes can be used for anything other than tomatoes. Mixing soil can actually be beneficial to even out nutrient or mineral imbalances.
When you mix the soil, check for “fluffiness” and density. Porous soil with ample airflow and drainage is extremely important for container gardening, as previously mentioned. Mixing soil is the perfect opportunity to check for this critical factor and allows you to fluff up the soil yourself.
After 2 years, potting mix will lose much of its structure and will need to be replaced, or amended with new mix and compost. For potting soil that you have deemed unfit for another season, simply apply it to an in-ground garden.
If you’ve noticed one of your potted plants was diseased, DO NOT reuse the soil. Pathogens and other issues can remain in the soil and affect new plants in the future.
Another sign that soil should be replaced or amended is if you notice a white build-up of white crust on the surface. This white crust is actually salt build-up that stems from watering with tap water. This salt build-up can also slow down plant growth. Using rainwater instead of tap or well water can mitigated this issue.
Leaving soil outside over winter
Insect carry-over is a risk that can be mitigated by leaving soils outside. Potted soil is more exposed to the cold weather compared to in-ground soils as it is not insulated as much. Cold air can kill of some of these pests. However, it is recommended to check through your potting mix in the spring. Larvae, grub and insect eggs may have burrowed into or entered your pot and could feed on or damage plant roots in the spring.
Container gardening is a great option for space-constrained gardeners. However, attending your soil is required to ensure the health of the next season. Some specific considerations have been outlined in this article that should give you the confidence to re-use soil the best way – to save resources and optimize the materials you already have available.
By Nate Van Beilen