Demystifying the Overwintering of Potted Mums
Hello Cooperative Extension Expert, I started looking online how to overwinter potted mums and ended up being very confused by the different approaches. Could you please tell me what method works best? Thank you. Best regards, Germaine Timmermans
Prince George's County Maryland
Nurseries sell potted most fall-blooming Chrysanthemums as a seasonal plant as they are borderline winter-hardy in our area. This could be due to temperatures or to soil conditions, as they require good drainage to avert root-rot - something our heavier clay soils (in most of central MD) are lacking. The most insulated place to keep them over winter is in the ground, whether still potted or planted without the pot in a more permanent location. Full sun exposure and well-drained soil are important in choosing a location for them to thrive next year. Overwintering potted plants above-ground is riskier, not only because the roots will reach colder temperatures being uninsulated, but because the small volume of soil in the pot will change temperature faster than the ground. Rapid temperature changes can be equally damaging to outright cold. They can also dry out more frequently than in-ground plants, and letting a container get too dry over winter can also lead to damage. There are late-blooming mums (sometimes in a genus other than Chrysanthemum) also available at nurseries in the fall; these are hardier, especially when planted in the ground.
If planting in the ground is not an option, the goal would be to increase insulation around the root ball by surrounding it with more material - either mulch, straw bales, contained dried leaves, or even "planting" it in a larger container that's filled with soil or mulch. Otherwise, storage in a cool shed, garage, or other protected space that's not heated would at least allow them to remain dormant while avoiding the most drastic temperatures and temperature swings. Here too, attention to watering is important - they should not stay wet, but they should also not be allowed to dry out completely.
You're welcome. Gardeners have tried both techniques - cutting-back in early winter and leaving stems until spring - and the latter may confer more winter hardiness to the plants. (While this may apply more to in-ground plants exposed to the weather, it shouldn't hurt in your situation.) We would recommend putting off cutting them back until growth resumes in spring, at which point you can see how far back the stems may have died, and prune to where the leaves are emerging.
Thank you - happy holidays to you as well!