The Rio Red grapefruit has become one of the most popular citrus fruits in the market and the home garden alike. It is a staple of Texas agriculture, where the Texas Rio Red has been trademarked as Rio Star. It has a very high brix ratio, which is the proportion of sugar to acid. Rio Red are more cold-hardy than the Star Ruby, which makes it more desirable for both market growers and the home gardener.
Rio Red is one of the more red-fleshed grapefruits available, with a red blush on its yellow skin. It keeps its red color throughout the growing season. It has a paler red halo in the flesh when cut horizontally. Its flesh is twice as red as the Henderson variety, and Food Network says that it is seven to 10 times redder than its parent, Ruby Red. It has so few seeds that it is considered a seedless variety.
According to Citrus Trees Online, the Rio Red grapefruit was discovered by Dr. Richard Hensz of the Texas A&I Citrus Center, now the Texas A&M Citrus Center. He had been working for several years treating grapefruit trees with ionizing radiation. Rio Red was discovered as a limb sport of Ruby Red in 1976 and released to growers in 1984.
Rio Red is hardy in climate zones 9 and 10. It is more cold-resistant than most other grapefruit varieties. This makes it a widely cultivated commercial variety. It also makes it desirable for the home gardener, who may have more variable conditions than a commercial grower. It does well as a container plant, if kept pruned, which enables a home gardener to keep the tree outside during warm weather then take it inside for shelter when cold weather comes. It is not as hardy as an orange tree, so protection from cold should be prepared for. Water your Rio Red thoroughly every week or every other week, unless you have very sandy soil, which drains quickly. For very sandy soil, monitor your moisture levels and water when the soil is dry one foot below the surface. Feed your tree one-third cup of ammonium sulfate per year of tree age in midwinter, late spring and early fall. This means feeding a five-year-old tree 1 2/3 cups of ammonium sulfate three times each year.
According to the United Nations Conference on Trade and Development, most citrus fruit that is marketed as whole fruit in the United States is grown in California, Arizona and Texas. Texas produces enough Rio Red and other red grapefruit that, in 1993, it named red grapefruit the official state fruit. Texas concentrates its production of grapefruit on red varieties because they have proved to be the most vigorous and climate-hardy. Food Network gives production figures of up to 1,000 fruit per tree per year.
Rio Red grapefruit grown from seed will grow true to type, but they will be more susceptible to diseases like foot rot and root rot. It is for this reason that most grapefruit trees are grafted onto sour orange seedling rootstock. That bud union must be preserved, so the tree should be planted slightly higher in the planting location than it was in its growing container. The soil around the tree should be kept at the same level as the surrounding soil, or even higher. Texas Agricultural Extension says that digging a watering trench or shallow bowl around the tree is a major cause of foot rot and should be avoided. Good drainage is important for the health of the Rio Red tree. The Rio Red is a very disease-resistant and vigorous variety. The major problem is that the fruit are rounder than most other grapefruit and they tend to become "sheepnosed," or elongated at the stem end.