Forsythia

GardenLine | Trees and Shrubs | Forsythia

Brian Baldwin

Of the flowering shrubs available to prairie gardeners, forsythia (Forsythia ovata) holds a special place in our hearts because it provides the very first springtime flowers. If you should see a shrub covered in bright yellow bell-like flowers in the next few weeks, it is almost certainly forsythia.

While early flowers grant forsythia great appeal, these flowers are not risk-free. Although their branches are fully hardy to prairie winters, forsythia flower buds are hardy only to -38°C. 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.

Supercooling involves cooling pure water below 0°C without the formation of ice. 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.

Normally, when pure water is chilled to 0°C, the first two molecules (bricks in the wall) will lock together using some microscopic impurity in the water as a template. 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. This template is known as the "ice nucleator." Once the first two molecules have locked together, the necessary pattern has been established, and the crystal growth continues as other molecules lock on.

For plants, the trick to supercooling is maintaining the water in the cells in such an ultra-pure state that no nucleators are present. With no impurities present to act as templates, the first two water molecules will not connect and the first microscopic ice speck will not form. If nucleation can be avoided, water can remain liquid at remarkably low temperatures. In fact, absolutely pure water will remain liquid until its temperature has dropped to -38.1°C.

If the internal temperature of a forsythia's buds never quite drop to forty below, the plants will flower in spring. This has also been suggested as a reason far more plant species are hardy in areas that never quite reach -40°C.

`Northern Gold' is the only prairie-hardy forsythia cultivar. Saskatoon residents who would like to see this plant blooming should keep an eye on the corner of the City Hall grounds directly across from the Frances Morrison Library where a cluster of these shrubs should be flowering soon - if the winter of '96 wasn't too much for them.

Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.