Dunstan Citrumelo: Hardy Grapefruit that 'Approaches Edibility'

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The citrumelo is an old hybrid that crossed the Trifoliate orange with grapefruit in an early attempt to produce cold-hardy, edible citrus after disastrous freezes in Florida in the late 1890s.  The first citrumelos were produced by Dr. Walter T. Swingle of the United States Department of Agriculture.  Swingle was successful in producing several citrumelo cultivars.  He found that like most 50% hybrids of mainstream citrus (already complex hybrids) with Trifoliate orange (probably a pure species), the Trifoliate characteristics dominated the offspring.  This tendency worked to the advantage of retaining cold-hardiness, but was a definite disadvantage when trying to produce a fruit that could be used for anything more than a furniture polish.

The citrumelo has proven itself over time to be a very reliable rootstock for grafting, and many citrus today are still grafted on citrumelo, which provides a robust root system and good cold tolerance to the scion.  Most of these cultivars by themselves make excellent ornamental plants, but they tend to be very sour, with bitter Trifoliate overtones and aftertaste.

The standout exception is the Dunstan citrumelo, probably the best 50% hybrid involving Trifoliate orange. It produces a large, grapefruit like tree that produces an abundance of fruit that taste like an ordinary grapefruit that is slightly underripe. It can be eaten fresh but is probably better as a diluted juice for those who like grapefruit juice. 

The origin of the Dunstan citrumelo is a little unclear.  It was apparently developed after the original citrumelo cultivars produced by Swingle, and it is also fairly certain that Swingle did not develop the Dunstan himself. (Swingle tended to name cultivars either after prominent U.S. government officials or after the location where the cultivar first fruited.) A clue is found in the cultivar name - Dunstan. There was a Dr. Robert Dunstan of North Carolina who was a Romance Languages professor by day but an avid amateur plant breeder in his spare time.  Among Dunstan's successful experiments were producing blight-resistant American chestnut hybrids, and producing the first successful hybrids involving European grapes and American muscadines.  Given Dr. Dunstan's interest in producing hardy, disease resistant plants, it's likely that the Dunstan citrumelo was one of his more obscure crosses.

Another clue lies in the origin of the Dunstan citrumelo.  Again, something of a mystery.  However, the oldest plants I have been able to locate all are in North Carolina.  Guess where Robert Dunstan was from and did most of his plant research - North Carolina.

As pictured here, Dunstan citrumelo eventually grows into a large tree for a citrus, particularly if grown on its own roots.  It has large Trifoliate leaves that are mostly evergreen. Hard freezes below 15F or so will usually defoliate the tree, though this does not harm the tree in any way. On its own roots, Dunstan will eventually grow to about 20 feet tall and broad, functioning much like Muscogee Crape myrtle in the landscape. Like all citrumelos, it is very hardy - hardy at least to Atlanta, GA, and probably well north of that. Fruit are about the size of a Navel orange, perhaps a little larger, and ripen in early November.  Like many hybrids involving Trifoliate orange, fruit will drop when completely ripe, and do not hang on the tree indefinitely (as grapefruit will do). 

I have been growing Dunstan citrumelo for about 20 years and have found it to be a worthy addition to my collection. After the Christmas 2022 freeze here, where temperatures dropped to 11F and stayed below freezing for 55 consecutive hours, Dunstan citrumelo was not only undamaged, but bloomed and fruited heavily the following spring.  

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