Response to frost varies: wilting foliage of Japanese anemony plants next to un-affected leaves of Epimedium.
When the air temperature drops below 0C (32 F), interesting things begin to happen in the leaves: ice crystals begin to form in the leaf tissues. First, they form in the spaces between the cells and in the cell walls. This is where the concentration of dissolved substances is lower than inside the cells themselves and the freezing point* is therefore higher. This causes the water from within the cells to seep out and to condense on the growing ice masses. With less liquid water in the tissues, the leaves wilt.
As water seeps out, the increasing concentration of organic substances within the cells further lowers the freezing point, which is good because it prevents the formation of ice crystals in the living protoplasm. At the same time, however, the loss of water increasingly dehydrates the cells, and this in itself can be damaging to the living tissue.
Some plants are better than others at surviving this difficult situation. The ice crystals can damage the cells, especially when they are large. However, this threat is reduced in frost-hardy plants by the presence of special proteins that bind to the surface of the ice crystals and prevent them from growing larger. Also, the properties of the cell membranes would have altered in a way that enables them to function at low temperatures.
Hellebores wilting at -1C.
It would take more than -1C to have an effect on daffodils.
When thawing occurs, particularly if this is gradual, the ice crystals melt, water goes back into the cells and the leaves regain their vigour. This is how it works in hardy plants.
In non-hardy plants, damage to the cell membranes would have occurred, water does not re-enter the cells completely and normal metabolism cannot resume. The leaves stay wilted and change colour as they die.
Hardy plants take time to prepare for winter. In the autumn they undergo the process of natural ‘hardening-off’, induced by the shortening days and lowering temperatures. The results can be amazing. Some woody plants have been found to survive temperatures as low as liquid nitrogen (-196C) when in the dormant, winter-hardy state. And yet, when actively growing, the same plants can be killed at around -3C, only just below freezing.
* Freezing point – the temperature at which the given substance or tissue freezes. Not everything freezes at 0C (32F).
Reference: “Science and the Garden” D.S. Ingram, D. Vince-Prue, P.J. Gregory (RHS, 2002)