Carlos J. Tapia T.Agronomist M. Sc.Cherry production specialist. Technical Director at Avium and Co-founder of SmartCherry.
The main objective in this transition stage is to have the beginning of physiological dormancy around May 1st.
Physiological dormancy can be recognized when the tree reaches at least 50% of fallen leaves, always considering that a yellow leaf is considered fallen leaf. This ensures an acclimatized and “hardened” plant to withstand low temperatures and frost events during the dormant period, and also, for its part, the accumulation of cold hours is maximized and thus they can be counted early in the season.
What should be taken into account so that plants can acclimatize quickly and enter a dormant period optimally? Several aspects that we will learn about below.
Post-harvest nutrition:
Regarding post-harvest nutrition and fertilization management, the months of January and February are considered the most important in terms of orchard recovery. This means that, as well as preventing thermal stress, soil and foliar fertilization based on the results are vital to ensure reserves and start a new season after the dormant period.
March is already recognized as a transition month to the beginning of the winter dormancy stage, so it is important to generate a nutritional signal to the plant and not continue fertilizing beyond March 10, so as not to over-stimulate the plant or maintain a vegetative state until very late in the season. This considers both soil fertilization and post-harvest foliar programs.
Irrigation programming and management:
Atmospheric demand and water consumption of the plant decrease towards the end of February and beginning of March. Demand is expressed in evaporation and not in ambient temperature. This is important to understand, because although there are high temperatures at this time, they last only a few hours during the day and are not comparable to the same maximum temperatures in midsummer.
At the same time, the nights and mornings have much more marked low temperatures and high humidity than in the months of January and February. The above is the basis for the decrease in the frequency of irrigation towards the month of March. It is very important to use tools that help make objective decisions for this: test pits, humidity sensors, tensiometers, etc.
The change in irrigation frequency (not time) under normal conditions should decrease between 30% and 50% of that established in summer by the third week of March.
In general, irrigation should be suspended towards the end of March. This sends a signal to the plant and initiates the lignification process of wood and fruit centers so that the plant can better withstand low winter temperatures. However, in soil conditions with lower water retention, such as sandy soils and those with the presence of stones, the end of the irrigation season should be postponed until April 10.
In special situations, watering may be started again at the beginning of a very dry autumn or winter, but never before physiological dormancy is recognized, represented as at least 50% of fallen leaves (yellow leaves are considered fallen leaves).
Use of Molybdenum (Mo) as a complement to plant acclimatization:
There is much talk about the action of Mo as an enhancer of leaf fall or as a causal agent of the physiological entry of plants. However, this component acts through the nutritional pathway, moderating and enhancing the action of some nutrients, such as distributing the different nitrogen forms that are in the plants, in favor of reserves, and the action of some phytohormones such as ethylene and abscisic acid, the latter causing the natural fall of leaves in autumn.
Mo applications via foliar spray during the month of March have proven to be effective in the acclimatization process, mainly in young plants; this, especially in colder areas, promotes the hardening and lignification of plants, in order to improve adaptation to the leaf fall process and to better withstand low winter temperatures, favoring the condition of the plants from a phytosanitary point of view.
A plant adapted to its environment, often by using foliar moisture, may have a greater chance of carrying out the leaf fall process naturally and normally as long as there are environmental signals that allow it. In this case, a constant drop in ambient temperatures.
There are many commercial options of Mo formulated for foliar application on the market. The concentrations for use according to the manufacturer must always be respected.
ANDstrategy to complement leaf fall:
The specific consideration is that physiological dormancy should ideally be present at the beginning of May. The first cultural signal that should be given to the plant to start entering dormancy is to stop watering towards the end of March; this is complemented by ceasing soil and foliar fertilization towards the end of February. After this, the state of yellowing and/or leaf fall should be evaluated (yellow leaf = fallen leaf).
This evaluation analysis should be done around April 15-20, as it is a reasonable time for the plants to show changes based on what was done as a primary strategy. If by that date there is no sign of leaf fall, a chemical strategy should be applied to ensure their fall.
Tests have been carried out to allow a rapid response to throwing away leaves if necessary, the most effective being the least “natural” one, which is through the “poisoning” of tissues.
In this case, it is recognized that the sulphate ion (SO4), present in some nutrients, causes poisoning of green tissues when used in high concentrations, which is the way to stop the work of the leaves and cause abscission. However, the mixture with urea has enhanced the decomposition effect, causing a rapid fall.
Hence the mixture of a “de” Sulfate (Zn Sulfate being the most commonly used) plus Urea, as a complement to leaf shedding in a last resort terminal strategy. The concentrations for use of both formulated products are between 1% and 2%, always mixed. Up to two applications could be made at intervals of 7 to 10 days if necessary.
Cover should be used according to the volume of the canopy and always prioritising reaching the last shoot in height.
