New York Times: American colonists once watched for the spring bloom of the Nantucket shadbush, a sign that it was warm enough to bury the winter’s dead. Today, that shadbush and dozens of other flora native to the New York region face extinction, a result of urban development and the encroachment of invasive plants from foreign lands, scientists from the Brooklyn Botanic Garden report. Hoping to revive the plants, the scientists recently completed a 20-year project mapping species in every …