The following info extracted from Wikepedia, contains very useful insight into the practice
The remembrance poppy (a Papaver rhoeas) has been used since 1920 to commemorate soldiers who have died in war. Inspired by the World War I poem "In Flanders Fields", they were first used by the American Legion to commemorate American soldiers who died in that war (1914–1918). They were then adopted by military veterans' groups in parts of the former British Empire: the United Kingdom, Canada, Australia and New Zealand. Today, they are mainly used in the UK and Canada to commemorate their servicemen and women who have been killed in all conflicts since 1914. There, small artificial poppies are often worn on clothing for a few weeks until Remembrance Day/Armistice Day (11 November). Poppy wreaths are also often laid at war memorials.
The remembrance poppy is especially prominent in the UK. In the weeks leading up to Remembrance Sunday, they are distributed by The Royal British Legion in return for donations to their "Poppy Appeal", which supports all current and former British military personnel. During this time, it is an unwritten rule that all public figures and people appearing on television wear them; some have berated this as "poppy fascism" and argued that the Appeal is being used to glorify current wars. It is especially controversial in Northern Ireland; most Irish nationalists and Irish Catholics refuse to wear one, mainly due to actions of the British Army during the Troubles, while Ulster Protestants and Unionists usually wear them.
The use of the poppy was inspired by the World War I poem "In Flanders Fields". Its opening lines refer to the many poppies that were the first flowers to grow in the churned-up earth of soldiers' graves in Flanders, a region of Europe that overlies parts of Belgium and France. The poem was written by Canadian physician and Lieutenant Colonel John McCrae on 3 May 1915 after witnessing the death of his friend, a fellow soldier, the day before. The poem was first published on 8 December 1915 in the London-based magazine Punch.
In 1918, American YWCA worker Moina Michael, inspired by the poem, published a poem of her own called "We Shall Keep the Faith". In tribute to McCrae's poem, she vowed to always wear a red poppy as a symbol of remembrance for those who served in the war. At a November 1918 YWCA Overseas War Secretaries' conference, she appeared with a silk poppy pinned to her coat and distributed 25 more to those attending. She then campaigned to have the poppy adopted as a national symbol of remembrance. At a conference in 1920, the National American Legion adopted it as their official symbol of remembrance. At this conference, Frenchwoman Anna E. Guérin was inspired to introduce the artificial poppies commonly used today. In 1921 she sent her poppy sellers to London, where they were adopted by Field Marshal Douglas Haig, a founder of the Royal British Legion. It was also adopted by veterans' groups in Canada, Australia and New Zealand.
Commonwealth of Nations
In Australia, the poppy is worn by many on Remembrance Day and is sold in the weeks beforehand by Legacy Australia. At Remembrance Day ceremonies the poppy is worn by the Governor General, State Governors, politicians, military and members of the public. https://au.news.yahoo.com/thewest/a/24611935/10-000-poppies-for-wwi-sacrifice/
The poppy is not traditionally worn on Anzac Day, although the practice is becoming more common, particularly at overseas commemorations such as in Britain. The traditional symbol of remembrance on Anzac Day is a sprig of rosemary.
In Canada, the poppy is the official symbol of remembrance worn during the two weeks before 11 November, having been adopted in 1921. The Royal Canadian Legion, which has trademarked the image, suggests that poppies be worn on the left lapel, or as near the heart as possible.
Until 1996, poppies were made by disabled veterans in Canada, but they have since been made by a private contractor. The Canadian poppies consist of two pieces of moulded plastic covered with flocking with a pin to fasten them to clothing. At first the poppies were made with a black centre. From 1980 to 2002, the centres were changed to green. Current designs are black only; this change caused confusion and controversy to those unfamiliar with the original design.In 2007, sticker versions of the poppy were made for children, the elderly, and healthcare and food industry workers.[ Canada also issues a cast metal "Canada Remembers" pin featuring a gold maple leaf and two poppies, one representing the fallen and the other representing those who remained on the home front
Following the installation of the Tomb of the Unknown Soldier at the National War Memorial in Ottawa in 2000, where the national Remembrance service is held, a new tradition formed spontaneously as attendees laid their poppies on the tomb at the end of the service. This tradition, while not part of the official program, has become widely practised elsewhere in the country, with others leaving cut flowers, photographs, or letters to the deceased.
Royal British Legion poppy
A volunteer makes poppies at the Royal British Legion Poppy Factory in London, where over 30 million poppies are made by a small team each year
A poppy on a bus in Southampton, England (November 2008)
The poppy is also worn on Memorial Day, celebrated on July 1 each year in Newfoundland and Labrador.
In the United Kingdom, remembrance poppies made of paper and plastic are sold by The Royal British Legion (RBL) and Haig Fund. These are charities providing financial, social, political and emotional support to those who have served or who are currently serving in the British Armed Forces, and their dependants. They are sold on the streets by volunteers in the weeks before Remembrance Day.
In England, Wales, and Northern Ireland, the poppies have two red petals, a green paper leaf and are mounted on a green plastic stem. According to the RBL, "The red poppy is our registered mark and its only lawful use is to raise funds for the Poppy Appeal". In Scotland, the poppies are curled and have four petals with no leaf and are sold by Earl Haig Fund Scotland. The yearly selling of poppies is a major source of income for the RBL in the UK. The poppy has no fixed price; it is sold for a donation or the price may be suggested by the seller. The black plastic center of the poppy was marked "Haig Fund" until 1994 but is now marked "Poppy Appeal". A team of about 50 people—most of them disabled former British military personnel—work all year round to make millions of poppies at the Poppy Factory in Richmond.
In the early years after World War I, poppies were worn only on Remembrance Day itself. However, today the RBL's "Poppy Appeal" has a higher profile than any other charity appeal in the UK. The poppies are widespread from late October until mid-November every year and are worn by the general public, politicians, the Royal Family, and others in public life. It has also become common to see poppies on cars, lorries and other forms of public transport such as aeroplanes, buses, and trams. Many magazines and newspapers also show a poppy on their cover page, and some social network users add poppies to their avatars. In 2011, a WWII plane dropped 6,000 poppies over the town of Yeovil in Somerset.
Some have criticised the level of compulsion associated with the custom, something Channel 4 newsreader Jon Snow has called "poppy fascism". Columnist Dan O'Neill wrote that "presenters and politicians seem to compete in a race to be first – poppies start sprouting in mid-October while the absence of a poppy is interpreted as absence of concern for the war dead, almost as an unpatriotic act of treachery". Likewise, Jonathan Bartley of the religious think-tank Ekklesia said "public figures in Britain are urged, indeed in many cases, required, to wear ... the red poppy, almost as an article of faith. There is a political correctness about the red poppy". Journalist Robert Fisk complained that the poppy has become a seasonal "fashion accessory" and that people were "ostentatiously wearing a poppy for social or work-related reasons, to look patriotic when it suited them". Kleshna, one of two businesses with an exclusive tie-in with the RBL, sells expensive crystal-clad poppy jewellery that has been worn by celebrities.
The Royal British Legion also holds a yearly poppy appeal in Northern Ireland and in 2009 raised more than £1 million. However, the wearing of poppies in Northern Ireland is controversial. It is seen by many as a political symbol and a symbol of Britishness, representing support for the British Army. The poppy has long been the preserve of the unionist/loyalist community. Loyalist paramilitaries (such as the UVF and UDA) have also used poppies to commemorate their own members who were killed in The Troubles.
Most Irish nationalists/republicans, and members of the Irish Catholic community, choose not to wear poppies; they regard the Poppy Appeal as supporting soldiers who killed Irish civilians (for example on Bloody Sunday) and who colluded with illegal loyalist paramilitaries (for example the Glenanne gang) during The Troubles. In 2008, the director of Relatives for Justice condemned the wearing of poppies by police officers in Irish nationalist areas, calling it "repugnant and offensive to the vast majority of people within our community, given the role of the British Army". In 2009, Sinn Féin's Glenn Campbell berated the policy that all BBC TV presenters must wear poppies in the run-up to Remembrance Day and urged the BBC to drop the policy, as it is a publicly funded body. In the Irish Independent, it was claimed that "substantial amounts" of money raised from selling poppies are used "to build monuments to insane or inane generals or build old boys' clubs for the war elite". However, on Remembrance Day 2010 the SDLP’s Margaret Ritchie was the first leader of a nationalist party to wear one.
Republic of Ireland
During World War I, all of Ireland was part of the UK and about 200,000 Irishmen fought in the British Army (see Ireland and World War I). Although the British Army is banned from actively recruiting in the Republic of Ireland, some of its citizens still enlist. The RBL thus has a branch in the Republic and holds a yearly Poppy Appeal there.
Each July, the Republic has its own National Day of Commemoration for all Irish people who died in war. However, the wearing of poppies is much less common than in the UK and they are not part of the main commemorations. This is partly due to the British Army's role in fighting against Irish independence, its activities during the War of Independence (for example the Burning of Cork) and the British Army's role in Northern Ireland during the Troubles. Nevertheless, the RBL holds its own wreath-laying ceremony at St Patrick's Cathedral, Dublin, which the President of Ireland has attended.
In the United States, the Veterans of Foreign Wars conducted the first nationwide distribution of remembrance poppies before Memorial Day in 1922. Today, the American Legion Auxiliary distributes crepe-paper poppies in exchange for donations around Memorial Day and Veterans Day.
In Hong Kong—which was formerly part of the British Empire—the poppy is worn by some participants on Remembrance Sunday each year. It is not generally worn by the public, although The Royal British Legion's Hong Kong and China Branch sells poppies to the public in a few places in the territory.
During Victory Day 2014—which marks Nazi Germany's surrender to the Soviet Union—some Ukrainians wore remembrance poppies instead of the usual Ribbon of Saint George, as the ribbon had become associated with pro-Russian separatists. A poppy logo was designed by Sergiy Mishakin, containing the text: "1939-1945 Never Again".