Citrus resistance levels depend on species and variety.
Table on page Hardy Citrus page shows observed resistance levels for mature trees.
During the first few years of growth, the trees are not yet fully established. They will only reach the indicated level of hardiness after three to four years in the ground.
This is the most dangerous period in the event of an intense cold snap or heavy snowfall.
Non-woven fabric (NWF) is commercially available in rolls up to 10m wide. It is commonly used by horticulturists in the spring as a forcing veil for vegetables. It is a synthetic fabric, for which there is no natural alternative yet. It allows light, rain and air to filter through while providing thermal gain.
In theory, a simple veil can save two or three degrees.
However, this is not borne out by our practical experience. With minimum-maximum thermometers installed under the veil and outside, we observed the same minimum temperatures (-7 degrees C at night). Maximum temperatures, on the other hand, were much higher under the sail (+10/12 degrees).
On sunny winter days, the temperature under the canopy can exceed 18 degrees or more. Under these conditions, the tree risks waking up too early. And if the sap starts to circulate, the slightest frost at night can dry out and kill the tree.
On this basis, our recommendation is to install the sails only in the event of a very intense cold snap, and to leave them on during the day only if the weather is overcast and the temperature remains negative.
We strongly advise against leaving the sails in place for several weeks or months. The tree would almost certainly be subjected to excessively high temperatures.
If you decide to install a veil anyway, be ready to remove it during the day, as soon as the sun shines.
To install, place the TNT veil in a bell shape above the tree on three or four sticks. The veil must not pinch the foliage so as not to burn it.
A square of around 5 x 5m can be used to create the bell on sticks 1.5 to 1.8m high. In windy conditions, the ends need to be secured to the ground with stones.
A grammage of 17gr to 19gr per m² is generally sufficient. Two sails can be applied one on top of the other.
Plant mulching generates a relative heat bubble. A thick mulch will provide a few degrees of heat gain at the base of the tree and under the crown. See the Watering chapter for full details on mulching methods.
Mulching must be maintained and renewed regularly. If mulching with dry hay, renew twice a year around June and November.
Mulching should be installed or renewed twice a year, before frost (November) and in late spring (June).
Dry hay, or hay already in the process of decomposing, is ideal, to a thickness of 20-30cm and a radius of 50cm in a circle around the tree.
Protective sails can êon trees less than three years old from December to February, in the event of an intense or long-lasting cold snap. After these exceptional events, it is advisable to remove the sails as soon as possible. Do not leave the sails on during sunny days.
Installing fixed sails throughout the winter is not recommended, as it presents a number of risks.
Firstly, the accentuation of temperature differences between day and night, especially on sunny winter days, can cause irreparable thermal shock.
Secondly, the early warming of trees under canopies can trigger an early and rapid vegetative awakening (as early as January), making them extremely vulnerable to the slightest frost.
These negative effects are more pronounced with thicker sails.
In practice, it is impossible to install TNT sails on mature trees.
Citrus trees have flexible branches and can withstand a few inches of snow without damage. Beyond that, it is advisable to shake the branches to lighten them and prevent them from breaking.
For young trees of certain varieties, it is possible to tie the branches to stakes to limit breakage in the event of snow. For an orange tree, for example, use 5 to 10 stakes.
Stakes are not suitable for other varieties, such as mandarins.
Yuzu and lemon trees are more resistant to snow, except in the case of exceptional snowfalls (40cm or more).
If the tree is covered in ice (freezing rain), it will have more of a protective effect, maintaining a temperature of around -2 C on the branches. Leaves can be covered in ice and resist, except generally those at the end of branches, which will then turn yellow with the thaw.
Young shoots can suffer and burn in moderate frosts, from -1 or -2 degrees.
The last fall growth, from October to November depending on the weather, will be particularly vulnerable. The shoots will bend and then mend, turning brown and then black.
This is of no consequence to the tree, as the wood of the trunk and main branches provides the strength.
However, all damaged ends should be pruned at the end of winter, in the second half of March. In April and May the pruned secondary branches will regenerate quickly. Growth will even be accelerated, provided the tree is well fed in early spring.
In the event of severe frost and major damage, with whole branches and leaves suddenly drying out, wait until spring to take action.
In March, prune the branches, starting from the tip and observing whether the cut wood is brown (dry) or green/yellow (alive). Work your way up the trunk until you find the living part of the tree.
If the trunk is dry and brown all the way to the rootstock, the plant is lost.
In some cases, the rootstock alone may survive and start up again the following spring. It can be recognized by its straight, thorny branches with three-lobed leaves.
