Blog Post Borrowed (with permission) from Jason Reeves, Research Horticulturist at the University of Tennessee.
With the upcoming single digit temperatures predicted, you may want to think about protecting your outside potted plants. I typically think of plants as being one full zone less hardy when in a pot as opposed to being planted in the ground. Let’s take Loropetalum for example. It is a zone 7 (0 to 10 ˚) plant. If the temperatures drop between 10 – 20 ˚ (Zone 6) for any length of time, I would be concerned with it setting above ground. A lot of hardiness has to do with the length of time that the temperature is held at such. One night at 10˚ will likely do little to no harm but a few days at or below 10˚ could be the kiss of death.
Hardiness Zones are as below:
Zone 5 is -20 to -10˚; Zone 6 is -10 to 0˚; Zone 7 is 0 to 10 ˚; Zone 8 is 10 – 20˚
There are several things you can do to help protect plants with questionable hardiness. The most obvious is to move them indoors. An enclosed garage usually does the trick. You can bring them into a heated space, but it is best for them to remain dormant so don’t leave them in for more than a few days. Other options include digging a hole in the ground and planting pot and all, but that’s probably not an option today or tomorrow. Raking leaves or pulling mulch around the pots is another option. You can even pile leaves over the top for added protection. If the plant is a conifer (needled evergreen), don’t leave the raked leaves over the foliage for an extended time (several weeks) or you stand a chance of causing damage to the foliage.
Any plant that is borderline hardy as well as plants that are considered hardy once established, but were fall planted (in the ground) would also benefit from some added protection of leaves and mulch. Plants I would be most concerned with include loropetalums, crapemytles, gardenias, edgeworthias, less hardy cultivars of Encore azaleas, variegated chinaberry, ‘Florida Sunshine’ Illicium and purple muhly grass.
Contrary to what many people think, most all plants, particularly those that hold their foliage during the winter need to be well watered before the soil freezes. Once the soil freezes the plant cannot take up moisture but the foliage continues to need water and will desiccate in the winter wind. Once the soil freezes the plant cannot take up moisture but the foliage continues to need water and will desiccate in the winter wind.
Another thing to keep in mind is that as temperatures begin to rise , the freeze/thaw cycles do damage to the root systems. It is best if plants held in pots can be placed in a shaded location to temper the thawing process.