Citrus US-1516 (USDA Hardy Rootstock)

Every grafted citrus tree is two plants pretending to be one: a familiar fruiting top, and a rootstock below the graft union doing the unglamorous work of roots, vigor, and disease resistance. US-1516 is one of the latter, and a good story all the same. The cross was made by the USDA in 1975, a pairing of opposites: African pummelo, the giant of the genus, crossed with Flying Dragon, the contorted, fiercely thorned, cold-hardy form of trifoliate orange (Poncirus trifoliata) that lends so many hardy citrus their backbone. The seedlings went into the ground at the Whitmore farm in Groveland, Florida in 1976 and then, in the patient way of tree breeding, were watched for forty years. Kim Bowman's program at the USDA lab in Fort Pierce released them at last in 2015, into the worst of the huanglongbing epidemic, the bacterial greening disease that has hollowed out Florida's groves. On infected ground they keep their grafted tops healthier and more productive than the old standbys. We offer them ungrafted, which is an unusual thing to sell and an honest one: this is a tree for the cold-hardy citrus tinkerer, the person who wants to practice budding, raise their own understock, or simply grow the trifoliate-blooded foundation and see how far north the plant will go. They come nearly true from seed, vigorous and uniform, and they ask only that you have plans for them. Graft them bold, or just let them teach you the lower half of the tree.