BOSTON — The solemn display of tens of thousands of U.S. flags that first appeared on Boston Common for Memorial Day a decade ago, honoring service members who have died defending the nation, is slowly becoming a national movement. The flag gardens, as they are known, can be seen this weekend in Texas, Louisiana, Ohio and New York, all started by local residents inspired by the Massachusetts Military Heroes Fund tribute established in 2010. “We are extraordinarily proud that what we intended to do for the Commonwealth of Massachusetts has been brought to a scale that we never fully anticipated,” said Tom Crohan, president of the nonprofit’s board of directors. The concept has even spread to Canada, where thousands of Canadian flags are planted in Toronto on Remembrance Day in November, when Canada honors its military members who have died in the line of duty. The Massachusetts Military Heroes Fund was founded in 2009 to support to the families of active duty service members killed since 9/11. The first sea of fluttering stars and stripes included 20,000 flags, one for every Massachusetts resident who gave their life in war since World War I.