Mani Skaria, Ph.D.
Founder & CEO of US Citrus, LLC
Professor Emeritus, Texas A&M University-Kingsville
Sorry other citrus-producing states and countries for the above question.
Obviously, we pay the price – our high heat, many 3-digit temperature days.
A fine and a balanced blend of tartness, sweetness, acidity, juiciness, color, and flavor are very appealing to consumers of grapefruit. The Rio Grande Valley of Texas is known for best tasting and red-flesh grapefruits in the world. Recent examples of Rio Red and Star Ruby grapefruits were developed by Dr. Richard A. Henz of Texas A&M University-Kingsville Citrus Center in Weslaco, Texas has become the backbone of the grapefruit industries in many places like south Texas, South Africa, and Israel. Prior to Rio Red and Star Ruby, Texas produced another red flesh grapefruit called the Ruby Red.
Weslaco, Texas is located at latitude 26 degrees north of the equator and 97 degrees west, means it is only 1,800 miles north of the equator. Citrus is a subtropical climate group, with grapefruit as a heat-loving member of the family. In general, citrus trees show little growth below 13oC (=55oF).
Annual heat unit is a measure of heat plants need and for proper growth and fruit development. This measure is used well in vegetables, and fruit trees. Crops have upper and lower temperature thresholds, outside of which the physiological functions are not normal.
The ideal temperature for grapefruit is above 60 degrees Fahrenheit. South Texas temperature can reach three-digit numbers some days. Grapefruit trees along the coastal areas maintain warm and stable temperature. In cooler areas, full sunlight and a south-facing planting help grapefruit tree stay warm.
Grapefruit produces lots of fruit inside. The large size of a grapefruit tree brings shading effect to inside fruit. Together with internal fruit and warm condition, grapefruit tree produces higher quality fruit in places such as south Texas.
Heat units are calculated as a function of:
1) the number of days in a 30-day month
2) taking 13oC (=55oF) as the temperature minimum for citrus physiology
3) average monthly temperature of the region
4) and the deviation from the minimum times 30 (days) give the heat units.
Following are some of the calculated heat units in various places in tropical and subtropical conditions.
Location Region Heat Units (Calculated)
South Texas Subtropical 3,900
Orlando, Florida Subtropical 3,465
Riverside, CA Mediterranean 1,700
Valencia, Spain Mediterranean 1,626
Wakayama, Japan Maritime, cool 1,951
Palmira, Columbia Tropical 3,918
In extreme temperature conditions, shading from other trees or bark whitewash would reduce sunburn impact. In such conditions, the root system is expected to be kept moist always.
Another heat measure is called the heat index (HI) which is an indicator combining the temperature and relative humidity as a measure of perceived temperature on the human body. It is well-explained with the chart taken from the National Weather Service of the United States of America.
Among all the citrus varieties, grapefruit is the only one with an origin in a tropical island – the Caribbean island, the Barbados.