Cold hardiness of potted plants left outdoors in winter

Lee Poulsen
Mon, 08 Oct 2012 17:45:26 PDT
Back when I lived and gardened in Austin, Texas (Zone 8B, long term average of the coldest annual winter minimum was 18°F/-8°C. Austin is now Zone 9A), there were many years when any plant whose aboveground parts are killed by temperatures significantly colder than freezing would freeze to the ground. If such an occurrence caused the entire plant to die, then it wouldn't return in the spring. One example of this is Bougainvillea, which never survived to grow another year whenever we had a winter night that was near or below our average coldest night. So back then people would grow them in pots and bring them in in the winter. Or replant each spring.

However, I learned that there were a fair number of plants that I now know as geophytes, that were not supposed to be hardy in Zone 8B, that nevertheless came back each spring no matter how bad a freeze we had during the winter. One such plant was bananas. No we rarely saw fruit on bananas unless we had an unusually warm winter. But no one ever replanted them. They just came back each year. The surprise to me was that Hippeastrums did just fine in the ground. Even though every book I could find at the time declared that they were hardy only to Zone 9 or 9B. (Each half zone is a difference of 5°F.) On the other hand, I was always puzzled by most gardening books of the time talking about "when the ground freezes" to some depth or another. The ground never froze in Austin, not even in the coldest, once-in-a-century freezes when it got down into the single digits °F. (There were two of those during the 1980s…)

Given that, I suspect that some geophytes native to central Texas were probably not very hardy if they were grown in pots kept aboveground all year round fully exposed to the elements, since they never experience being completely (or partially) frozen when growing in the ground.

--Lee Poulsen
Pasadena, California, USA - USDA Zone 10a
Latitude 34°N, Altitude 1150 ft/350 m

On Oct 8, 2012, at 4:44 PM, penstemon wrote:

>> The duration of the freeze is irrelevant: one night's freezing solid of a
> pot will do the damage.
> I probably missed most of the discussion. The reason why bulbs are not very 
> hardy in pots is that the bulbs themselves are not very hardy. This may 
> sound counterintuitive, but a bulb, being buried in the ground, has no 
> adaptive reason to develop hardiness to cold. Most bulbs are not hardy at 
> temperatures approaching -10C.
> The smaller the volume of soil, the easier it is for cold to penetrate. I've 
> killed hundreds of tiny calochortus bulbs this way, until it dawned on me 
> what it was I was doing to the poor little bulbs.
> There is an excellent discussion of this in The New Plantsman, September 
> 1994. "Frost hardiness of bulbs".
> Bob Nold
> Denver, Colorado Z6 

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