Letter from Lander Clifton
No doubt this year's cold spell had many a rare fruit grower worried stiff at the thought of possible tree loss and damage caused by the unusually low temperatures experienced in most parts of North Queensland. I thought it may be helpful to share my experiences and make a few suggestions to help our trees through winters in years to come.
Firstly, I think it is a good idea to purchase a set of thermometers (one for max. and one one for min. temperatures), particularly if your trees live in the country, at high altitudes, or in areas where there is a lack of natural vegetation, i.e. mature forest. The coldest temperatures during winter occur when the skies are clear and without cloud cover, as heat absorbed by the ground during the day is allowed to radiate back out into space. The secret of getting your trees through these hard times lies very much in retaining this heat radiating from the ground, as well as the heat being constantly generated by the processes taking place within the trees themselves. To do this, we covered our Durians (ranging from 5 to 12 feet in height) with old mosquito nets which were supported by a centrally positioned pole, either sunk into the ground or tied to the trunk. Before the trees were covered, several dozen banana leaves and/or sheets of newspaper were placed amongst the foliage and branches.
One of the trees was left without attention, to compare our results. It was a tree about 10 foot in height and it lost every leaf and from 6 to 10 inches of all its recent stem growth. The rest of the trees all made it through with barely a mark, even though they were all enjoying a healthy burst of new growth at the time. This growth continued practically unaffected once temperature began to rise again. The covers were left on for several weeks during which time our temperature recording went down to 6°C. Care should be taken when removing the nets to prevent stem breakage.
The soil's water content is also a key factor. As water retains considerable heat absorbed during the daytime, it is a good idea to irrigate regularly during the cold period. This also ensures a good sap flow within the tree and a good heat distribution throughout the plant. The processes taking place within the plant at night are largely heat-generating in nature and known as respiration. Oxygen from the atmosphere is absorbed and used to burn sugars providing the energy needed in order to construct new cell growth. Oxygen (O2) + Sugar --> Carbon Dioxide (CO2) + ENERGY.
This heat generated from respiration is vital to the trees' ability to cope with low temperature stress. The respiration processes scan be stepped up to some degree by the application of a foliage spray such as liquid seaweed or fish emulsion.
Hopefully these suggestions will be of some use to members.
Letter from William F. Whitman
CAN ULTRA-TROPICALS BE GROWN IN AREAS VISITED BY FROST? In your R.F.C.A. Newsletter No.28 under "Letters to the Editor", Paul Recher wrote of his inability to grow Pometia pinnata and other cold-sensitive fruits in his "absolutely frost-free" area. My Florida-grown Fijian longan (Pometia pinnata) has reached an estimated 14m (45 ft.) in height with a 41cm (16 in.) trunk diameter in spite of numerous frosts during the life of the tree. Last Christmas Eve a severe winter cold front swept over our state killing off an estimated one-third of Florida's commercial citrus grove acreage.
My fruit orchard in the Greater Miami area experienced frost, the same as it has on other occasions over the years. The Fijian longan along with other fruit trees such as langsat, santol, South American sapote, rambutan and mangosteen shed all or most of their leaves until spring, when replacement growth commenced. In spite of this, most of them bore fruit this past summer. It appears that many ultra-tropical fruit trees, upon reaching a certain size can be partially or completely defoliated by cold and remain dormant through winter until the return of continued warm weather without extensive damage.
To accomplish this, the writer uses a small greenhouse with heat and humidity control. Here cold-sensitive fruit plants are grown until they reach a height of 1 m (3 ft.) to 2 m (6 ft.) prior to spring planting them in the grove. By the time winter returns, they should be even larger and better able to tolerate unfavorably cold weather. If these same trees were set out in the ground while in the juvenile stage, few, if any, would survive our occasionally cold Florida winters.
Under these conditions, small, young tender tropical plants tend to shed their leaves with each cold spell and put out new growth during the warmer weather in between. After three or four times of this alternate leaf dropping and regrowth, the plant usually is so weakened that it dies. Larger, more mature trees seem to acquire the ability to remain dormant without extensive leafing out again until spring returns. If you don't have a heated greenhouse, possibly a friend does or maybe a commercial hot house grower will let you slip a few small plants in? It can mean the difference between success and failure for those of you working with ultra-tropicals in the cooler winter locations.
DATE: November 1984
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