According to daysoftheyear.com, "National Garden Month itself is a fairly recent observance. It started with National Garden Week when, in 1986, US President Ronald Reagan signed a declaration to make the observance official the following year in 1987.
Realizing that only seven days simply could never be long enough, National Garden Month was born in 2003– and people have been celebrating this important occasion during each springtime ever since. Now it’s time to get started with the celebrations for National Garden Month this year!"