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.

US-942 began life as a rootstock, bred by the USDA and released in 2010 after years of Florida field trials, a careful cross of the Sunki mandarin and the curious Flying Dragon form of trifoliate orange, Poncirus trifoliata. As a rootstock the record is stellar: compact, productive trees, strong resistance to Phytophthora and tristeza virus, and better tolerance of citrus greening than most, which is why growers across the citrus belt have come to trust the number.

What surprised everyone is the fruit on the rootstock itself. Where most trifoliate hybrids carry the bitter trifoliate aftertaste as a family curse, US-942 mostly sheds it. The little fruits ripen bright orange inside and out, not much larger than a ping-pong ball, thick and soft in the skin like a small mandarin and easy to peel, with juice that runs somewhere between the sweet citrus of a calamondin and the tart brightness of a kumquat, and few seeds. Herschel of Madison Citrus, who knows the tribe as well as anyone, calls US-942 "one of the best-tasting trifoliate hybrids there is," and we would not argue.

In the garden, grow US-942 in full sun and sharp, well-drained soil, in the ground through the warmer zones or in a large container to overwinter farther north. Hardy, disease-resistant, compact, and productive, the tree suits the collector of unusual citrus and the grower who wants a tough, good-tasting fruit off a plant that asks little. A rootstock that turned out to be worth growing on its own merits.