Our typical last day of frost in Boulder, Colorado is Memorial Day (late May.) There are many years where there are many warm sunny days with and without freezing nights leading up to the long awaited "last day of frost". These sunny days will heat the water significantly and keep the plants and earth inside the wow from freezing.
I tend to get anxious in the spring to get the garden started and have set up my wows early enough where the water inside has frozen solid. I seem to remember reading that the wind is more damaging to plants than the cold temperatures so I figure the frozen wows still provide some protection to the plants inside.
Here are some pictures from March 2009. That year we had a spring snowstorm the official report was 15 inches (38cm) see: http://www.denverpost.com/ci_12000857
These pictures are from the days after the snowstorm and 2 full months before our official last day of frost. The peas were planted Feburary 5, 2009 and the garlic was planted the week after Halloween 2008.
This is the day after the storm
March 27, 2009
2 days after storm
March 28, 2009
3 days after storm
March 29, 2009
Peas inside Wall of Water
March 29, 2009While peas are pretty cold hardy the wows also protect the plants from getting smashed by the snow. Our spring, and fall, snows tend to be wet and heavy. My garlic got squished by the snow but sprang back after a few days of sun.
March 29, 2009