State officials are urging residents to keep it local when they buy their Christmas tree this season. Agriculture commissioner Gregory Watson said buying a tree grown in Massachusetts is good for the local economy. Watson attended an annual Christmas tree-cutting ceremony on Friday at Vandervalk Farm in Mendon. Christmas trees are considered a crop and nearly 98 percent of all trees sold are grown on farms. Recent figures from the US Department of Agriculture show there are 284 farms in Massachusetts that produce Christmas trees.