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At the sight of a rhododendron in winter, inexperienced hobby gardeners often think that something is wrong with the evergreen flowering shrub. The leaves curl up when cold and appear dried at first glance. The same goes for bamboo and many other evergreen plants that go into the winter with full foliage.
The curling of the foliage, however, is a normal adaptation to frosty temperatures and dry easterly winds: by the leaf edges bulging downward, the plant protects itself from excessive water losses. The stomata lying on the undersides of the leaves, over which the bulk of the transpiration takes place, are better protected from the drying wind in this position.
Incidentally, the leaves bend on their own as soon as the water pressure in the vacuoles - the central water reservoirs of the plant cells - sinks. But this also has another effect: when the water content decreases, the concentration of minerals and sugars dissolved in the cell sap increases at the same time. They act like the winter salt, as they lower the freezing point of the solution and make the leaves more resistant to frost damage. Only when the liquid in the cells freezes and expands, the leaf tissue is damaged.
From the rolled up sheet to frost damage
The natural frost protection of the evergreen leaves has its limits: If it is very cold for a long time and the sun warms the leaves at the same time, the so-called freeze-drying threatens. The warm sunlight stimulates evaporation, but at the same time the pathways of the shoots as well as the roots are still frozen and can not transport or absorb water. If this condition persists for a long time, the rolled-up leaves first turn brown and later also the younger shoots - this causes the typical frost damage, which you then have to cut out of the bushes with the pruning shears in the spring.
By the way, the different species of bamboo are a bit more flexible in heavy frost than most evergreen shrubs: they shed most of their leaves when the weather gets too critical, and then simply re-emerge in spring.
The damage pattern of Phytophthora root fungus is similar in rhododendron to the protective mechanism in heavy frost
Frost damage or Phytophthora?
Root fungi of the genus Phytophthora cause a damage pattern in rhododendron, which is very similar to a typical frost damage. The fungi clog the track so individual branches are cut off from the water supply. As a result, the leaves also roll due to lack of water, then turn brown and die. Often the damage affects entire branches or branches and is thus much more pronounced than a normal frost damage. A key differentiator is the season in which the damage occurs: If you notice the brown, rolled leaves until winter or spring, a frost damage is more likely than a fungal attack. However, if the damage does not occur until the summer, Phytophthora may be the cause, especially in rhododendron.