- Author: Ben Faber
Deciduous plants need a certain period of dormancy during which they accumulate hours of cold. This is a very practical process on their part, since most dormant plants grow in cold climates. Differing areas have different patterns of cold. Some areas have very distinct ending points to the winter and if the plant leafs out, it will grow and be fine. In many winter areas, the end point is not distinct. Spring may come on with glorious weather, but then there might a cold snap in the spring and all that new growth is frozen back. Plants that have survived in these spring frost areas will thrive. Those that have not adapted to the erratic spring weather will die.
Chilling hours are the number of hours needed to take the plant into a period where it is assured of not getting hit by the cold. The are different ways of calculating this and the plant has an internal mechanism that has developed in that area to make sure it comes into a period where there is less likely-hood of a late freeze occurring.
So what has happened is that humans have moved plants around and moved them out of the areas where they are best adapted. It looks better, tastes better, has the characteristics that people want. So we move plants that require a lot chilling into areas that don't have much cold weather. And we move plants that require low chilling into areas that have cold winters. In the latter case, one knows right away that a mistake has been made because the plant flowers too early and gets hit by the late spring cold.
In the former case where plants requiring high chill are moved into warmer winter areas some odd things happen. For example the plant never leafs out in the spring. Then one knows there is not enough cold in that area to grow the plant successfully. Delayed flowering and leaf out are the most common for plants growing in marginally cold areas. This may not be noticeable in most winters when a normal winter occurs.
But this winter has been different in California. It was way too warm. And in many areas fruit trees requiring more chill than happened, just have not flowered or leafed out. Areas along the coast has been the worst hit, but it has happened in the Sierra foothills and the San Joaquin Valley. These were trees that were out of place and it is sad to see. The only thing that can be done at this point is hope for colder weather next year. But maybe that is not something one should hope for too much.
Effects on lack of adequate chilling on apricot - delayed and uneven bloom.