Many people seem to have problems during the winter with their citrus trees. There is a phenomenon that affects citrus grown in containers know as Winter Leaf Drop (WLD), which is what happens to the trees during storage in winter quarters. With the appearance of fall and a decrease in temperatures, the trees are brought indoors to protect them from freezes and frost damage.
Unfortunately, the most common recommendation freely given for storing the trees indoors is as follows:
"Give the tree as much light as possible, keep the medium moist, but never over water. Let the medium dry out more than in summer. Hold the tree at temperatures around 41 - 50 F (5 - 10 C)."
If you have followed this advice and have had no problems, you are lucky because most people who did so had problems. The trees start to shed leaves, leaf after leaf drops, and in spring, after the last frosts, when the trees are placed outside again, many trees have few or no leaves remaining on the limbs, twigs and branches. The cleavage is often between the leaf petiole and the leaf blade, the petiole remaining on the tree. Often the dropped leaf shows no chlorotic patterns or any other discoloration. Our first thought is that Winter Leaf Drop was influenced by low-light conditions during winter times. But even with extra illumination Winter Leaf Drop continued. Often, after bright, sunny mid winter days, more leaves are shed than after longer periods of less bright light. So we tried to find out what might be the cause of Winter Leaf Drop. We chose five lemon seedling trees, all about one foot high. The seedling trees were placed in different locations with different conditions:
1. The first seedling tree was placed in a cool and bright location at a south facing window, at temperatures around 43 -50 F (6-10 C).
2.The second seedling tree was placed in the same room, but at a north facing window.
3. For the third tree, we chose a temperate room with temperatures around 59 F (15 C).
4. The fourth tree was placed at the same temperatures but on a south facing window.
5. The last plant was placed in a warm room of 70 F (21 C) at a south facing window with extra illumination.
All trees were irrigated as needed, just to keep the root ball moist, but not wet and not allowed to dry out. Only #5 was irrigated more regularly and fed evenly. After winter we found that #1 shed nearly all its leaves. #2 dropped some leaves only, and the others had only lost some (fewer than five) or no leaves. We found light may NOT be the factor causing WLD, so we thought about temperature. After a long discussion with citrus experts in Florida and Israel, it was found that temperature will cause WLD. The temperature tables from the book Biology of Citrus show that citrus stops root growth and root function if the soil temperature drops below 54.5 F (12.5 C). Leaf activity will be reduced if the temperatures drops below 64 F (18 C). Leaf activity means the full process of water evaporation for leaf surface cooling, energy transformation (photosynthesis) and starch reduction for building amino acids and other compounds for forcing plant growth and cell development. Citrus controls its leaf temperature by evaporating water from the leaf blade. This reduces the temperature even during hot periods and will maintain the leaf temperature at the optimum levels between 77 F (25 C) and 95 F (35 C). But even on cold days the sunlight can heat up the leaf surface quite quickly to levels beyond the critical temperature of 54.5 F (12.5 C). Photosynthesis itself works better in cooler conditions with high light radiation than in the warmer periods of the day, so most of the photosynthetic starch production is done in the morning before noon and less water is evaporated than during the afternoon. Optimum leaf temperature for photosynthetic activity for most plants ranges from 50 F (10 C) up to 90 F (32 C). Photosynthesis itself needs carbon dioxide, light and water to transform the carbon dioxide into starch and oxygen. During the night the starch will be oxidized to provide energy needed for plant growth and development. The whole process is called breathing. Water and nutrients for the breathing process must be taken up by the roots. Oxygen and carbon dioxide will be delivered from the air around the plant, taken up by the leaf surface (and to some extent by other green parts of the plant) so leaf and root activity must run in a balance to provide the best plant performance for growth, flowering and fruit development. If a citrus tree is stored at temperatures below 54 F (12 C) but gathers enough light for photosynthesis, this balance is broken. The leaf activity requires water, which the roots cannot deliver. The tree stops evaporation and water will be unavailable for cooling the leaf surface on bright days, so the tree reduces active leaf area by leaf abscission. This seems to be the best theory about what causes WLD.
Partial or complete defoliation was never critical if the root ball was kept a little more on the dry side, but if it was too wet, a quick root decline developed even if Poncirus trifoliata was used as a root stock. Most of the trees recovered quite will in spring (if the roots stayed healthy and a heavy bloom was set). But in recovering the whole canopy, often the trees used up much of their starch reserves in the stock, which did not fully refill during the short summer times. After some years, many trees suffered, growth was stopped and the trees died because all of the starch had been depleted. So what to do about WLD? Irrigation during wintertime seems to be a recommended practice to slow down WLD. Irrigation with warm water 77-90 F (25-32 C) supports the root function, even the water uptake, so WLD will slow down. Irrigation reduces the plant stress during cold winter time and is therefore recommended.
Keeping the trees in a room with high humidity seems also to slow down WLD but cannot prevent it. Also a place more in the shade, to minimize leaf activity, slows down WLD. Keeping the root temperature below 64 F (18 C) but at or above 59 F (15 C) seems to work best for stopping WLD. The plant functions are minimized, but water and nutrient uptake for leaf activity is high enough to support the breathing process and leaf surface cooling by water evaporation. If WLD persist, force the root temperature higher, around 70 F (21 C) this should stop leaf drop. Sometimes during the winter, fruits dry out on the tree and drop if the tree is stressed too much. So for fruit development and fruit maturity, higher temperatures and good leaf activity should be maintained. Irrigation with a nutrient solution should be done even in winter.
Hope this helps - Millet
