I can't remember when Decoration Day started to be referred to as Memorial Day - but that was the term when I was young. In Illinois, where I grew up, peonies were the most popular flower to be placed on the graves in memory of loved ones. The peonies almost always were in sync with the holiday.
Here is a part of an article on the start of Decaration Day.........
"Memorial Day was officially proclaimed on 5 May 1868 by General John Logan, national commander of the Grand Army of the Republic, in his General Order No. 11, and was first observed on 30 May 1868, when flowers were placed on the graves of Union and Confederate soldiers at Arlington National Cemetery. The first state to officially recognize the holiday was New York in 1873. By 1890 it was recognized by all of the northern states. The South refused to acknowledge the day, honoring their dead on separate days until after World War I (when the holiday changed from honoring just those who died fighting in the Civil War to honoring Americans who died fighting in any war). It is now celebrated in almost every State on the last Monday in May (passed by Congress with the National Holiday Act of 1971 (P.L. 90 - 363) to ensure a three day weekend for Federal holidays), though several southern states have an additional separate day for honoring the Confederate war dead: January 19 in Texas, April 26 in Alabama, Florida, Georgia, and Mississippi; May 10 in South Carolina; and June 3 (Jefferson Davis' birthday) in Louisiana and Tennessee."
How the red poppy became a part of the day is explained here......
"In 1915, inspired by the poem "In Flanders Fields," Moina Michael replied with her own poem:
We cherish too, the Poppy red
That grows on fields where valor led,
It seems to signal to the skies
That blood of heroes never dies.
She then conceived of an idea to wear red poppies on Memorial day in honor of those who died serving the nation during war. She was the first to wear one, and sold poppies to her friends and co-workers with the money going to benefit servicemen in need. Later a Madam Guerin from France was visiting the United States and learned of this new custom started by Ms.Michael and when she returned to France, made artificial red poppies to raise money for war orphaned children and widowed women. This tradition spread to other countries. In 1921, the Franco-American Children's League sold poppies nationally to benefit war orphans of France and Belgium. The League disbanded a year later and Madam Guerin approached the VFW for help. Shortly before Memorial Day in 1922 the VFW became the first veterans' organization to nationally sell poppies. Two years later their "Buddy" Poppy program was selling artificial poppies made by disabled veterans. In 1948 the US Post Office honored Ms Michael for her role in founding the National Poppy movement by issuing a red 3 cent postage stamp with her likeness on it."