In this paper the word 'ultratropic' is arbitarily taken to refer to any fruit plant that can be injured or killed by a temperature of 0° C. The two most frequently referred to tropical fruits, as far as cold injury, are the breadfruit, Artocarpus altilis, and the mangosteen, Garcinia mangostana. The United States Department of Agriculture on more than one occasion shipped potted mangosteens to Key West on a trial basis. Their reasoning was that this island city, located nearer to Havana, Cuba than Miami, Florida, was the most southern on the Continental United States. If this Garcinia was to survive this was the most suitable location as to temperatures. Unfortunately, all of the plants died because the mangosteens could not tolerate the highly alkaline soils of the Florida Keys. It was not realized how important soil acidity was for success in cultivating certain ultratropical fruit. Previously this had been ignored with all attention focussed on temperature alone.
My Bal Harbour experimental fruit grove is located between the Atlantic Ocean and Biscayne Bay. To acquire a low pH soil. I trucked in black acid sandy loam. This is locally referred to as 'Hammock Sand'. I have had three mangosteens fruit here even though the trees are growing nearly 200 miles north of Key West. None of these Garcinias received cold protection during our winter months, as the area is surrounded by salt water and seldom experiences frost. While this tropical rainforest tree can survive as long as temperatures do not drop below 0°C, the same degree of coldness will kill a breadfruit down to the ground. Fortunately, it normally sends suckers up from the roots with the return of warmer spring weather. This Artocarpus does just fine on high pH calcareous soils such as those existing on our Florida Keys.
Being able to grow ultratropical fruit successfully, even in the warmest parts of the state depends largely on our South Florida weather. If the trees require too long to bear, say ten to fifteen years, it is likely they will be visited by a tropical storm or one of those infrequent frigid Arctic air masses prior to reaching maturity. It is therefore suggested prospective growers obtain grafted or vegetatively propagated plants that produce much earlier than seedlings. Also, if and when they do fruit, the plant will usually be much smaller and easier to protect from the elements than a seed-grown one of equal age.
DATE: November 1995
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