For this reason, forsythia may fail to flower after an extremely cold winter, or may only flower on the lowest branches - those that were covered with snow. Regular readers of this column will know that the main factor that kills a plant in winter is the formation of ice inside its cells. The hardiest of trees are those which dehydrate their cells to the point where ice formation inside of them is simply impossible. Semi-hardy plants like forsythia have adopted a somewhat less reliable, but no less remarkable strategy of winter survival called supercooling. To understand it we need to consider a basic principle of ice formation.
Liquid water consists of many closely packed water molecules arranged in a haphazard manner.
If you visualize individual water molecules as bricks, a glass of water would be somewhat like a barrel into which bricks had been randomly tossed - they're close enough to be touching, but in no particular order. When water's randomly organized molecules become aligned and locked together in an precise and regular pattern, you have ice. An ice crystal could therefore be visualized as a brick walls, and from the plant's point of view, it's large enough to do some damage. A single bacterium, a speck of dust, or some other very tiny object is necessary to provide the initial template upon which the crystal begins. Once the first two molecules have locked together, the necessary pattern has been established, and the crystal growth continues as other molecules lock on.