Table of contents
- Winter hardiness zone serves as a guide
- Other factors at a glance
- Winter protection optimizes chances of survival in the bed
- Potassium lowers the freezing point in cell water
- Conclusion
The general classification as a hardy plant down to -10 degrees Celsius can only be used as a rough benchmark for the olive tree. In fact, various factors have a significant influence on the degree of frost hardiness of your olive tree. These include the regional location of the garden as a macro location, the local microclimate, the age and the constitution of the plant. So that you can objectively assess how much cold your Olea europaea can actually withstand, we will examine all influencing factors in detail below. Benefit from our tips on how to optimize the natural winter hardiness of your olive.
Winter hardiness zone serves as a guide
If you choose to plant your olive tree or winter it outdoors, there can be no doubt that it will survive the cold, safe and sound. A close look at the location of the garden and the winter climate that prevails there serves as an important clue. Since the experiences of the past one or two winters are too uncertain as a basis for decision-making, experienced hobby gardeners fall back on historical weather data. Following the example of the USA, Europe was divided into geographical hardiness zones in the early 1980s.
Within the scale from Z1 to Z10, each zone covers a temperature range of 5.5 degrees Celsius. Z1 stands for a wintry minimum temperature of below – 45.5 degrees Celsius and Z10 for – 1.1 degrees Celsius to + 4.4 degrees Celsius. The area from Z5 to Z8 that is interesting for Central Europe is further differentiated into semi-zones a and b.
The details at a glance:
- Z5a: -28.8°C to -23.4°C
- Z5b: -26.0°C to -23.4°C
- Z6a: -23.3°C to -20.6°C
- Z6b: -20.5°C to -17.8°C
- Z7a: -17.7°C to -15.0°C
- Z7b: -14.9°C to -12.3°C
- Z8a: -12.2ºC to -9.5ºC
- Z8b: -9.4°C to -6.7°C
On the associated map, colored according to winter hardiness zones, it can be seen that the central habitats of the olive tree are within zone Z8 (- 6.7 to 12.2). Interestingly, regions in western North Rhine-Westphalia, in the Rhine Valley or along the Moselle are also included in these mild winter hardiness zones. If your garden is in these or similar locations, you can assume that your olive tree can spend the winter outdoors.
Macro-location and micro-climate mark the boundaries
The assignment to a suitable winter hardiness zone is only one criterion if you want to assess the frost resistance of your olive tree. The table can only act on the basis of empirical average values, from which the actual course of the winter can deviate significantly. After all, a location within Z8 or the western edge of Z7 is a good starting point for an informed decision. In addition, the microclimate within your garden is relevant. These factors signal that your olive tree is also winter-proof within the next coldest zone Z7a and Z7b:
- Location within a basin
- Location on a southern slope
- Places in niches, corners or protected by dry stone walls
A detailed view is required here, especially outside of mild winter zones of Z8 and higher. An olive tree, braving severe frost on a sheltered terraced slope, perishes in a drafty corner a few yards away. So it helps a lot if you know exactly what conditions are in your garden and where the snow melts early or where the ground stays frozen well into spring. However, relying on a mild microclimate and treating an olive tree in hardiness zone Z6 and colder as a hardy plant is just wishful thinking.
Tip:
The degree of its frost hardiness loses relevance on the olive tree when it comes to the upbringing and maintenance cut. In the spring, trim the crown into shape only when weather forecasters aren't expecting freezing temperatures any longer.
Other factors at a glance
In a garden within the right hardiness zone and with the appropriate microclimate, essential criteria are met to cultivate an olive tree as a hardy plant. Because it is a prized exotic species, you can play it safe when assessing its frost tolerance by considering the following factors:
- Young trees grown in regional nurseries are more frost-resistant than adult, imported specimens
- A location with gradually falling temperatures in autumn and winter hardens the plant optimally
- Regions with very early and very late frosts significantly reduce winter hardiness
In addition, there are many hundreds of olive varieties that are more or less hardy by nature due to their origin. Scientists from the Universities of Madrid and Cordoba have carried out detailed investigations from this point of view. Some varieties could emerge as explicitly winter-hardy. These include Cornicabra (hardy to -13 degrees Celsius), Arbequina (hardy to -11.8 degrees Celsius), Hojblanca (hardy to -9.9 degrees Celsius) and Empeltre (hardy to -9.5 degrees Celsius).
Winter protection optimizes chances of survival in the bed
An olive tree north of the Alps is not only confronted with frosty temperatures. In addition, the wet and cold winter weather affects its winter hardiness in a way that the Mediterranean plant is not familiar with. By equipping a planted olive tree in the bed with the following winter protection, it is better prepared for the hardships of the cold season:
- Cover the tree pit with a high layer of fall leaves and brushwood
- Cover crown and trunk with breathable, translucent fleece
- Tie the winter coat together to make it windproof
Foil of any kind is unsuitable as winter protection. No air exchange can take place underneath, whereupon condensation forms. The most stable hardiness comes to naught when an olive tree dies from rot and mold.
Tip:
In the tub, the root ball is less hardy than an olive tree in the bed due to its exposed position. By wrapping the pot several times with bubble wrap and placing it on a block of wood, the plant can withstand cold temperatures of up to – 10 degrees Celsius in a wind-protected location.
Potassium lowers the freezing point in cell water
Preventive measures, such as winter protection in beds and tubs, cushion the imponderables of external weather influences on winter hardiness. In addition, you can arm an olive tree from the inside out for capricious winter weather. You can do this by fertilizing with potassium in the fall. Among other things, this nutrient is able to strengthen the cell walls. In addition, potassium in cell water lowers the freezing point. With products such as Patentkali or Thomaskali, the nutrient supply is modified accordingly in August/September. Comfrey manure acts as an organic source of potassium in ornamental gardens that are cultivated in a way that is close to nature.
Conclusion
Subsuming the winter hardiness of an olive tree to the winter hardiness zone Z8 and a minimum temperature of -10 degrees Celsius does not do justice to the complex issue. In order to be able to make a well-founded assessment of how much cold the precious exotic can actually withstand, the factors explained here should be included in the decision. Macro-location, microclimate, weather conditions and olive variety are important criteria that have a decisive influence on frost resistance. Furthermore, there is the option of winter protection measures, which are more extensive in the tub than in the bed. Furthermore, you can positively influence the minimum temperature by arming your olive tree with potassium fertilizer in autumn for the hardships of winter.
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