Grapefruit (Citrus paradisi) usually take a full year to ripen on the tree, including long and hot summers, to develop sweetness. The Texas grapefruit variety 'Rio Red' is no exception. This variety has a deep red fruit flesh and lacks seeds, especially nice for halves at breakfast or making juice easily. It also produces more dependably than the similar variety 'Star Ruby'. Grow 'Rio Red' in U.S. Department of Agriculture hardiness zones 9a to 12, where winter frosts are light and infrequent at most.
The grapefruit variety 'Rio Red' was released by Texas A&I University (now Texas A&M at Kirksville) in 1984. Ruby Red grapefruit seedlings were exposed to radiation and the buds grafted onto other grapefruit stock for evaluation. These new branches tested by Texas A&I University originally were labeled as A&I-1-48S.
Slowly growing to 20 to 25 feet tall and wide, the Red Rio grapefruit tree has smooth gray bark with large deep green leaves with occasional thorns on the thinnest branches and twigs. The springtime flowers are white, up to 2 inches in diameter and fragrant. The developing fruits have a pale yellow skin with irregular blushes of pink-red. Mature fruits are 5 to 6 inches in diameter. The fruit interior is a deep salmon-red, known as a "Texas red," and lacks or has greatly reduced numbers of immature seeds. This grapefruit variety bears heavily and has a tendency to produce slightly larger crops in alternating years.
Grow Rio Red grapefruit trees in a fertile, well-draining soil that receives 36 to 44 inches of rain or irrigation annually. Soil pH should be as close to neutral as possible, but the range of 6.5 to 7.5 is ideal. Evenly moist soils across the year allow for strong growth, flowering and subsequent fruit development. Long, hot summers create sweeter-tasting fruits; premature picking of fruits before about 10 to 12 months' time makes them taste more bitter. Cold winters thicken the fruit rind, while humid weather makes rinds thinner and increases the amount of juice in fruits.
The elongated apex of a grapefruit is called a "sheepnose," although any misshaped fruit earns the name. This condition often plagues the fruits on the Rio Red grapefruit tree. Disagreement exists as to the precise underlying cause of sheepnosing, but some explanations include above-average temperatures or the result of fertilization, excessive nitrogen and watering after a prolonged drought when fewer fruits grow on the branches, as commented in Texas Citrus and Subtropical Fruits' Valley Citrus Notes. Sometimes grapefruits in bundled clusters on trees seem to sheepnose more so than solitary fruits. Regardless, when compared with other varieties, sheepnosing is much more common on Rio Red trees.
The hybrid cross of 'Rio Red' with 'Star Ruby' yields another Texas red grapefruit variety called 'Rio Star' with even deeper red-colored fruit flesh. The Citrus Pages mentions that very few orchards in Texas consist of 'Star Ruby' and those that are planted with 'Rio Star' likely are just 'Rio Red', making up about 75 percent of the Texas orchard acreage.