LACK OF COLD
The perennials plants of temperate or cold climates have a peculiarity. They need cold (chill) during the winter to complete their annual cycle.
Some scientists have studied the question, from the late '40s, giving rise to several methods of calculation. They were then trying to understand the physiology of crop plants, especially woody plants, which live for several years and whose vegetative cycle start in spring, is strongly influenced by the conditions of winter.
The most commonly used method is the one developed by the American Weinberger and published in 1950. It is based on a simple calculation of the accumulation of hours of temperatures below 45ºF (7,2ºC), considered the temperature threshold below which the plant is susceptible to cold for its physiological needs.
Later, other methods have been proposed to combine the hours of cold with heat hours, considering that the effect of a low minimum temperature is partially offset if the maximum temperature of the day is high. It gave rise to various models (Utah, Crossa-Reynaud, Erez, Bidabe, etc.).
Whatever the used model, its comparison to reference is what enables an acceptable interpretation. But that is not the issue. The use of this data permits the classification of varieties of the same species according to their adaptation. This allows the farmer to choose crops and varieties best suited to its production area.
Let's consider an example that I know well, the Peachtree.
The chill behavior of fruit species has been studied mainly in the US, so that the words used often come from American language.
We usually class varieties of peach, numerous (which include nectarines, nectarines, flat peaches, white flesh, yellow flesh, etc.), in 3 categories of cold needs (chill), high-chill varieties, which need more 650/700 hours, low-chill varieties, which need less than 350/400 hours to complete their cycle, and medium-chill between both.
This range of needs is quite natural, and reflects a small part of the enormous genetic diversity of the species. These features, identified in the native areas of the species (China), have been used by geneticists in their breeding programs (see my article on this subject http://culturagriculture.blogspot.com.es/2014/01/3-selection-mutation-breeding-gmos.html) to create commercial varieties adapted to different types of climates.
What happens if a plant suffers a lack of cold?
In winter, when the plant is in dormancy, it actually has a physiological activity, very discreet, but fundamental. Plant organs (wood buds and flower buds) complete their internal development thanks to the cold.
A lack of cold disrupts this phase. The plant is unable to complete its internal development. Theoretically, it can't wake up from its winter dormancy. Yet the plant reacts not only to the chill but also to the photoperiod, which is the length of day and night.
When the days begin to lengthen, so from late December in the Northern Hemisphere, plants "know" that spring is approaching and it is time for them to wake up. Yet, they have slept very badly, because of the lack of cold, and the awakening will be chaotic.
In areas like here, in Andalusia, where the problem of the lack of cold is usual, the choice of adapted varieties and crops is generally well assumed by farmers. It is even usually a prerequisite to be able to produce annually and ensure the sustainability of the farm.
But this year, with the particularly mild winter that lived the Northern Hemisphere, many farmers in areas where this phenomenon does not usually occur, could have some surprises this spring.
In Morocco, where the lack of cold is very common, it's normal to observe large flowering disparities.
Farmers will have to keep an eye on the bud break of perennial crops, because it is very likely that the buds begin their cycle unusually early. On some crops, very susceptible to certain diseases, as it is precisely the case of peach, of whom one of the major diseases, leaf curl, settles down from the bud break, it will probably be necessary to anticipate the protection to avoid bad surprises later. We must also remember that insects, animals, fungi, bacteria, respond to the same stimuli as seasonal plants. So if a plant has its bud break very early, its usual diseases and pests will be problematic very early too.
Lack of cold causes a huge gap between the presence of fruit and the leaf development.
Then, it is likely that until harvest (specially spread out under these conditions), farmers find many and varied physiological abnormalities. Specifically, flowering, which in the case of peach tree, usually lasts between 10 and 15 days, and may drag on longer than one month, floral defects may be numerous (flowers without ovary, abortion of the pistil, fall of buds before they open, etc.). Wood buds will have difficulties to start, and will do it with a big time lag, causing the young fruits are not properly fed by lack of photosynthesis. Agricultural losses can also be situated on a lack of fruit size and a lack of quality.
Moreover, this anarchic but very early budding, could substantially increase the sensitivity to spring frosts, should they occur. "A warm Christmas means a cold Easter", popular wisdom doesn't make a mistake there.
It's usual to find fruits nearby maturity beside others barely out of the flower, with several weeks of shifting maturity.
Finally, important for fruit growers, it is common in these circumstances, that the first fruits, much more advanced than the later ones, cause significant physiological falls, by emphasizing the effects of competition between largest and smallest fruits. It is therefore advisable to delay decisions thinning, and do it with caution.
Over time, the plant will eventually balance, but the farmer will suffer serious consequences of lack of production and lack of quality.
In short, a too mild winter can be a serious problem, both in agriculture and in gardens. Plants may be sufficiently disrupted, and the risks of frost in spring are high, with potentially serious consequences.The cold wave that is coming now should put the record straight, except for plants that have already begun their budding process, which makes them more vulnerable to the cold.