Here is the garden on 22 March. I have just taken off the mulch that covers the rows and put it in the walkways between them. The upper layer (2-3 cm) of soil is frozen but will thaw out in the next few days. At this time of year exposed soil thaws quickly but the upper crust will often re-freeze at night. Very cold weather can drive the frost deeper into the ground. Repeated expansion/relaxation of this layer, called frost heaving, can break plants where the stem and root join. This can be a problem in alfalfa fields, especially if they were cut late in the fall and did not produce enough growth to insulate the soil around the stem.
We don't have anything in our garden that can be damaged by frost heaving. We use mulch to reduces winter frost depth and keep soil organisms active. However, this benefit is somewhat offset by delayed thawing of the upper soil layer.
In past years I have measured soil temperatures during the winter and spring. You can find this information on my personal website (http://warrenbuckles.com/). Look in the section 'Warren and Wendy Garden' and find the article titled 'W & W Garden Soil Temperatures - how hot is our dirt?'
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