Winter Desiccation Injury of Rhododendron
John R. Havis
University of Massachusetts, Amherst, Mass.
A certain amount of winter injury is seen frequently on rhododendrons grown in the colder regions. Cold hardiness ratings do not always agree with the responses observed. We have obtained evidence that foliar dehydration, or desiccation is frequently the cause of winter injury, especially the so-called winter burn. We agree with other observers who have attributed this damage to foliar dehydration and that it results from plants being exposed to winter sun and wind while root., are in frozen soil. Our objectives are to measure the degree of dehydration that brings about injury and hopefully, find ways of reducing the damage.
Symptoms of Desiccation
Most rhododendron leaves droop and curl when they freeze, usually at 27° or 28°F. The leaves curl more tightly as the temperature falls. The same drooping and curling takes place when the leaves become dehydrated, as illustrated in Fig 1. Whether or not such leaves recover without damage depends on the degree of dehydration and the inherent ability of the cultivar to tolerate desiccation.
Fig. 1. Wilted rhododendron plants with roots in
frozen soil, water deficit of about 45%.
Fig. 2. Similar plants, but roots in cold but not
frozen soil, water deficit of about 3.5%. Both
photographs were taken when the air temperature
was above freezing.
The leaf damage symptoms of desiccation usually do not appear until water has become available again to the leaves. Part of the leaf nearest the petiole and on both sides of the midrib may appear normal, but the leaf tip and margins fade to tan or brown and are brittle. A black zone sometimes separates the healthy and dry areas. Fig. 3 illustrates these symptoms. Leaf injury from low temperature causes blackening of the entire leaf, or the area affected, but without tan or brown dried outer edges.
Environment Favoring Desiccation
In severe climates, the soil normally freezes in the entire root zone of rhododendron plants. There is essentially no water available to plants from frozen soil. One might also expect greatly reduced water uptake from cold soil, just above freezing. This is true of many herbaceous plants, but cold hardened rhododendron plants appear to be able to extract ample water from cold, but unfrozen soil. We have held the entire root mass at between 32° F and 33° F for several days and found that the leaves retained their normal water content. Similar plants dehydrated rapidly when the soil was held frozen for the same length of time. The water in most soils freezes between 31° and 32° F.
The freezing point of stems and large roots is approximately 28° or 29° F. We have found that water will flow through stems that are held at 29° to 30° F as long as the stems do not freeze. If a plant is not shaken, it is possible to lower the stem temperature to about 25° F without freezing, and water continues to move up the stem. If the stem is shaken, it freezes, and water movement stops. If the plant is shaken by wind as the temperature falls, freezing takes place at near 28° to 29° F. The fact that soil water freezes at near 32° F, but water movement can occur in stems at temperatures three or four degrees below 32° strongly suggests that conditions could occur in early winter when plants are able to obtain water even though the surface inch or so of the soil is frozen.
Once stems freeze and water movement stops, water does not move again until the stem temperature rises to above 32° F. The stem temperature is often determined by contact with soil, mulch or snow. These materials may prevent a stem from thawing completely on a warm day which is favorable for rapid loss of leaf moisture. We believe that maintenance of frozen stems as well as frozen soil contributes to desiccation injury of rhododendrons.
Tolerance to Winter Desiccation
The degree of leaf dehydration can be expressed in terms of water deficit, which is the percentage loss of the water held by the fully turgid leaf. This can be understood by visualizing a glass of water. If we take a glass that is full of water and spill out 25% of the water, we can say that it has a water deficit of 25%, because it has lost 25% of the total water that it can hold. Likewise, a leaf that has lost 25% of the total water that it can hold is said to have a water deficit of 25%. This is a convenient term for comparing the amount of dehydration of different species having various leaf sizes.
Fully mature leaves of plants with adequate water supply have water deficits of 1 or 2% in early morning. At mid-day of a warm sunny day, water deficits are usually about 10%. A day or two without available water produces leaf water deficits of 20 to 25%. The rate of increase depends, of course, on the conditions of the environment. If the leaves of R. catawbiense 'Grandiflorum' reach water deficits of about 50%, injury symptoms as described earlier usually appear after water again becomes available. We say, therefore, that the critical water deficit is about 50% for that cultivar. The critical water deficits for 'Boule de Neige' is about 60%, and for R. carolinianum (PJM hybrids) is about 70%. Thus, R. catawbiense 'Grandiflorum' is much more likely to suffer desiccation damage than are the other two.
If a rhododendron plant is allowed to develop a water deficit of 20 to 30%, and it is then given water, the leaves recover to the normal full water content within 24 hours. However, if it is allowed to dry until near its critical water deficit before being given water, several days are required for water to move into the leaves. If a branch is removed from such a plant and placed in water, the water moves into the leaves of the branch in a few hours. This suggests that a severe water deficit in the plant causes a marked resistance to water uptake in the root.
When using cut branches in water, leaves of both the species R. carolinianum and the 'PJM' hybrid recovered from water deficits up to 70%. Using entire plants, however, the species clone usually failed to recover from water deficits of 50% to 60%, apparently due to increased resistance of roots to water absorption. The 'PJM' hybrid was able to take up water and recover from dehydration of up to 70% of its moisture. This is a demonstration of the striking desiccation hardiness of this cultivar.
It has been suggested that a relationship exists between resistance to freezing injury and resistance to desiccation injury. We do not accept this as being entirely true, because we have found that fully mature leaves of R. catawbiense 'Grandiflorum' have the same critical water deficit for injury in midsummer, when they have no tolerance to freezing, as in mid winter, when they are hardy to -40° F. At the moment we have no unusual suggestions for reducing winter desiccation injury. Planting sites in cold regions should involve protection from winter sun and winds. Sensitive plants often benefit from special sites where the soil does not remain frozen for long periods. Experiments other than those mentioned have shown that the so-called "anti-transpirant" sprays do not give effective protection against desiccation injury of rhododendron.