This is all part of the preparation for the catholic festival of Toussaints, or All Saints, which is on the 1st November - and has been since the ninth century. It is one of the main religious observations.
The flowers are for dressing the tombs of deceased relatives. Toussaints, or All Hallows (hallows is the old English for saints), is the day of the dead, the time to remember those who have died. On the day, all the cemeteries are crowded with people coming and going, placing these enormous flowers, or other arrangements, on the graves of their relatives.
Unlike the UK, almost all French people are buried rather than cremated, and have marble memorials, not just headstones, erected over them. It seems that every town, and many villages, have their what used to be called in England memorial masons. There are few such enterprises left in Britain, but they seem to thrive in France. Our part of Normandy is rich in granite, so there are even more firms than usual, cutting and polishing granite tombs, headstones and other memorials. In our village over the last fifty years all of the retail activity has dwindled, so that now there remains a small grocers plus everything else, a butcher two mornings a week, a depot de pain (not even our own boulanger any more). The one thriving business is the funeral director, which has its own granite tomb making arm.
It would be harder to have the same sort of festival in the UK, not only because the catholic influence ended 600 years ago, but because, simply, there are no tombs to visit and leave flowers.
For some reason, chrysanthemums, particularly white ones, are associated with the dead, not a major component of most bouquets of decorative flowers as in Britain; if you are invited to dinner at a French home, do not take chrysanthemums as a gift.
The night before All Saints day was of course 'All Hallows evening' or hallowe'en for short. Whilst there are many who dispute the non christian origin of a celebration at that time of year, there is some evidence of an Irish festival, and certainly most early societies celebrated the end of the harvests, and the preparation for winter around the equinox. That detailed written evidence does not survive is of course because the christian churches suppressed and/or absorbed all the old festivities.
What we can see is that there are remnants of common primitive practices. People dressing up in various ways, the presence of or driving out of spirits, the use of lights and candles, rituals not used at other times, are all pretty standard. As is the use of severed heads in one form or another. Real heads, freshly cut off, are I believe no longer that common, but the use of substitutes such as hollowed turnips or pumpkins seems to be increasing.
Halloween used to be a minor regional activity in the UK. In Somerset in the 1940s and 50s it was never mentioned (we had wassailing, much more important, and effective). It was more of an event in the North of England – probably the Norse connection – and Scotland, the Celtic/Gaelic history. Those groups seem to have taken it to North America, where it was expanded and commercialised out all sense.
Twenty years ago, there was no mention of it here in Normandy, but the growing influence of the supermarkets, and their desire to sell more and more of more and more to the public has resulted in halloween becoming a big marketing event. Sadly. The same with Christmas, which was a minor celebration, with a big family meal on Christmas Eve, and not much else. Some churches and civic buildings such as mairies might have been outlined with a string of white bulbs, but now every town tries to outdo its neighbours with the ostentation and in my view vulgarity of its municipal Christmas lights.