Cold hardiness of potted plants left outdoors in winter

Rodger Whitlock
Sat, 06 Oct 2012 11:31:10 PDT
The earth itself is a source of heat. Not very high grade heat, but heat 
nonetheless. At depth, this leads to interesting phenomena like melted rock. 
Hear the surface, the effect is usually much less dramatic, but definitely 
real. Even where there is permafrost, if you go deep enough, you get to warmth.

This is one reason that potted plants survive winter better if plunged in soil 
or sand. Not organic mulch - it's an insulator.

Another important factor is how dry the soil is. My experience is that potted 
bulbs left out in our winter rains don't survive a hard freeze where their 
siblings kept under cover and on the dry side come through just fine.

Those living in places with very cold winters (and, yes, Montreal has very cold 
winters, though not as bone chilling as Ottawa's!), if you don't have a cold 
frost-free place to keep your tenderish bulbs inside, a coldframe with pots 
plunged in a sand bed underneath may give your darling geophytes their best 
chance of survival. During cold snaps, lay a few single sheets of newspaper 
directly over the plants and put anything you can find on top of your coldframe 
- old burlap sacks, old bed linens, leaves, snow, conifer boughs, nearly 
anything that helps insulate the frame.

If you can sink your frame into the ground, like a pit greenhouse, so much the 

Rodger Whitlock
Victoria, British Columbia, Canada
Z. 7-8, cool Mediterranean climate

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