Which pests can survive in potting soil over winter?

(Question)

I was wondering which pests (leaf miner, aphids, thrips, white flies, etc.) can survive in used potting soil over winter outside in a container?  Roots and this years plant will be removed.  I’m hoping to reuse the soil next year for container gardening and it would nice if any existing pests are killed off.

Also, should the container be covered?  I plan to store the potting soil in a smart pot outdoors.

(Answer)

It is perfectly fine to keep using some of your potting soil for a couple of years as long as the current plant material is pest free.  I would not be too concerned about new plant pests moving into the pots throughout winter as this is the dormant season for plants and pests.  If however, you currently have issues with a pest that overwinters in the soil, I would recommend replacing the entire soil amount of very small containers, and about the top third of larger containers.  As there are so many host plant specific pests and also diseases out there, I do recommend you contact us again when a specific problem arises.

Regarding the pests you mention above:

Of the leaf miners, there are some kinds that do overwinter in the soil and others that don’t.  Here, it depends on the plants you grow and the leaf miner that can potentially affect them.

Aphid eggs overwinter in host plants.

Thrips do not overwinter in soil in Ontario.

Whiteflies who complete their lifecycle in host plants, do not tolerate temperatures below -6 degrees C.

Potting soil is usually a so called soilless mix, which most of the time consists of peat, vermiculite/perlite, and different types of wood products such as bark. Typically a slow release fertilizer is added in too, but this is used up after a certain amount of time, certainly after one growing season.  However, the organic materials in your potting soil will decompose and compact so you will have to add new material.  You will also have to add nutrients when using the soil over several years as it becomes less fertile.  To renew your potting soil you can either mix in new potting mix, compost, or vermi-compost (worm).  The latter two have the advantage of also containing the necessary plant nutrients, which would otherwise have to be added in form a liquid, or slow release fertilizer.

There is no real need to cover the soil in your smart pots over the winter as there will be very little in the way of nutrient leaching from the soilless mix.