Trifoliate Orange Hybrids
Viciously thorny trees from China. Hardy to -15 F; used by Burbank in his (unsuccessful) experiments to develop cold-hardy citrus. Fuzzy orange fruits have about the same size, hardness and juiciness as golf balls. Intense perfume when ripe.
Often used as understock for dwarf citrus orange, these thorny little hybrids are all that remain of Burbank's experiments with citrus. They are hardy, hybrids of ancient Chinese oranges (Poncirus trifoliata) and can survive to 15 degrees below zero. Burbank's objective was to make raising citrus outdoors in, say, Minnesota feasible by working this Chinese plant into lemons, oranges and grapefruit. But even in Sebastopol the climate was too severe to continue development year after year in order to obtain a successful hybrid.
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