Now in its 11th year, the fundraiser originated with a staff member who had helped with a Boy Scouts tree sale in Cambridge and brought the idea to Somerville. In its first few years, Willey said, the sale grew, but now it has leveled out. This year, Willey said that 400 trees were ordered, along with a large selection of wreaths that arrived from Nova Scotia via Woburn’s Spence Farms the week after Thanksgiving.