Chilling requirements and frost risk

Temperatures at dormancy and bloom are key factors that make almond production possible or not. Fruit trees require a minimum number of chilling hours (bellow 7ºC) during dormancy in order to blossom properly. This cold period is short for almond trees, which demand around 300 hours per year. Almond trees blossom in late winter and they are very sensitive to frost at this time. The balance between chilling hours and risk of frost is the main reason why almond production is not possible in many places.

average tempsLa Mancha is located in an interior plateau 650 m above sea level in central Spain. Its climate is classified as Mediterranean continental. The weather is rather extreme, with long severe winters and long hot summers. Spring is short. Mild temperatures stabilize only in autumn. Regarding almond production, the required cold hours are guaranteed. However, there is high risk of frost at blossom, which is the main limitation for almond production in this region.

With average temperatures during December and January between 2 to 6 ºC, almond trees are ready to blossom as soon as average temperatures rise for a period of several weeks. Depending on the year, that happens from early February to mid March. Average temperatures were very low during early February in 2015, so bloom started in early March for some varieties and mid March for others.

Low temperaturesThe data provided by the nearest meteorological station show that temperatures go up and down during late winter and early spring. Cold spells are normal during February and March. In 2015, minimum temperatures dropped bellow 0ºC in early March, but in other years, that was normal even in early April. Nothing can be done to prevent that, but before setting up an almond orchard there are two decisions that must be taken to lower this risk of frost damage.

The first one is location. March frost is due mainly to irradiation. Frost appears in the morning if the wind calms down and the skies are clear; an inversion layer is formed, cold air accumulates in low areas and warmer air remains in the slopes. Setting up an orchard in a low field increases risks. In the draw you can find a representation of the location of the meteorological station which provides the data of the graphs and the location of a new almond orchard. Minimum temperatures in the orchard may be higher during irradiation episodes, and frost may just appear in the low area.Draw

The second decision is the variety. There are varieties that bloom late, such as Filippo Cea and Lauranne. They blossom some days or weeks later than traditional varieties, which can be critical to escape frost some years. These varieties are also self-compatible and do not require cross pollination. That is important because, even if no frost appears, low temperatures make the work of pollinating insects hard.

All in all, climate in La Mancha is not ideal for almond production. If I were a professional farmer, I would stick to the traditional vineyards. Perhaps the self-compatible varieties that bloom late could make a difference. I bet on this specialty crop because it does not demand too much care and I do not earn a living out of farming. I do this with low investment and low running costs. In any case, agriculture has proved to be a risky business. The last graph shows how a dangerous frost appeared in late March, 2017, when Lauranne trees were at full bloom.Helada 25 marzo 2017

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