Overwintering potted perennials--how warm is too warm?
Hi, I need to protect a bunch of potted perennials from the cold this winter. I have an unheated shed attached under the house that is insulated, and due to residual heat from the house the temperature is over 50 degrees during the day. Is that too warm for potted perennials? How warm is too warm? BTW, it is pitch black in this shed as there are no windows.
Montgomery County Maryland
This may depend on what the specific plants are; can you list their names?
Often trees, shrubs, and perennials needing to stay potted during the winter are "heeled-in," where they are "planted" pot and all in the ground in order to stay as insulated (and yet cold and dormant) as possible. In the spring, they can then be lifted out and moved to their permanent location. Are these in decorative containers long-term, or simply in nursery pots awaiting planting? The best location for them would be outdoors, in the ground, and since our soils have not frozen yet, they can still be planted. If you need to keep them potted, the shed temperatures may be too warm based on what you describe. Do you know how much warmer than 50 degrees the shed gets, and how cold it gets overnight (on freezing nights)? Sometimes overwintered perennials that stay too warm simply break dormancy and sprout too early; in that case, possible pest (such as aphid) outbreaks could occur. They would also need light, air circulation, and more frequent watering if dormancy is broken and growth resumes. They would also have lost some cold-hardiness at that point, so a drastic temperature swing from a late freeze could cause damage.
Thanks for your answer. The problem I'm facing is that I don't have a place that I can "plant" the pots in the ground over winter. And I've never measured on a warmer day than today (and today is pretty cold), so that would imply that the shed in fact rises above the 55 degrees I measured today. I don't know how cold it gets overnight. Is there any other method I can use besides "heeling in" or storing in an overly-warm shed that would be a happy medium for my situation?
Ok. If planting and heeling-in arent options, then probably overwintering the group outside with added protection around the roots is best. This way, they are "on schedule" to break dormancy when they should and will also dry out less often. (Do check potted plants for water now and then in winter, though, as dry conditions can reduce their hardiness.) It sounds as if the shed would be too warm to ensure unbroken dormancy all winter. You can insulate the roots by mounding-up material around the pots; the larger the mass, the slower it will change temperature or reach extreme temperatures. (Rapid temperature changes are perhaps as damaging as outright cold; gradual changes are better.) Mulch, sawdust, straw bales, bagged soil...anything you can "plant" the bunch in above-ground will help. During thaws or particularly warm spells, check the pots for water as you normally would during the growing season, except make sure not to use water that's too warm.
Plant nurseries will often overwinter inventory this way (though on a larger scale, certainly), and winter damage is typically minimal or a non-issue. (Although it depends on the species involved - some need more protection than others.)