I can tell you exactly when I first heard of it – August 30, 2009 at the memorial service for our own danceswithcats. I had never heard of it before that day, but I’ve done it at every grave visit ever since.
the closest I ever saw my GrandFather (Irish Catholic) come to throwing a punch, was when we went to visit his Father’s grave, and the stones had been removed from his headstone. We’re talking full-on rage here. The goundskeeper had removed them, he said, becuas ethey might fall off and dull the trimmers as they went round trimming the grass. My GrandFather felt rather strongly that they were being paid enough to pick up a stone and return it to the headstone when necessary.
I don’t know if this is across Ireland, or just our family, because we have a tradition of stone masonry. In fact some of the ones on my GGrandFather’s headstone had been specially carved ones made by the men on the teams he led over the years. But usually we would just pick one up in the days before the visit, and carry it around until the day. One from the person’s own yard or hometown would be especially appreciated. I still have three from my last home visit which haven’t made it up to NH to their destinations yet.
A naked stone would be a shame to the family in our home cemetary, a sign that the younger generation had no sense of obligation.
Out of curiosity, why not?
What about plastic flowers? 
If you do you’ll never get a second date with him.
It’s certainly not the case in any of the Jewish cemeteries I have been to in Toronto. I know my mother pays for my grandparents graves to be kept up with flowers planted every spring.
My parents are buried in a Jewish cemetery, and each grave has a little urn to place flowers in. Every time I go, there are lots of flowers all over the place. And rocks (well, actually pebbles).
Sorry to hear about Oscar Wilde’s grave. When I was there it was totally covered with lipstick kisses. They didn’t look like they were caused by actual kisses from people’s lips; they were way too uniform for that. But they definitely added a whole new dimension to the grave . . . beyond the carved sphinxes.
Honestly while I am sure my parents explained it to me at some point, I don’t remember. I seem to recall that it had something to do with the fact that flowers are temporary but I could be misremembering that.
You aren’t supposed to have flowers during the funeral either.
ETA: There are definitely a lot of flowers left at my parents cemetery too. It’s a tradition that not everyone follows.
Cairns used to be common as grave/deathspot markers in Spain, you still see new ones occasionally when someone died in a hard-to-reach place. The old ones are most likely to be found in unpopulated areas travelled by roaming herds. No putting stones on carved tombs, tho.
While it’s possible the practice got copied from other groups, I figure most people here would prefer to adscribe it to parallel evolution in different nomadic groups. Stones are a lot easier to come by than carved slabs of rock, after all.
As long as we are mentioning other countries’ traditions - in Germany people will plant flowers in spring/summer, put evergreen “blankets” on graves in the winter and bring other flowers year-round. You can even pay people to come and do this for you if you live far away. However, some cemeteries also only “rent” grave-sites for a specific period of time - after 20 years or so, they figure few people are alive to continue to take care of the grave, so someone else might be buried there. If people are still alive to take care of the gravel, they simply buy a new “lease” of sorts for an extended period of time.
Pamplona’s “new” cementery turned 100 years old in the last decade. The major thought they could just treat all those family plots that were originally sold as having been long-term rentals; anybody who was still using the old grave could pay again. After all, the niches are all 10y rentals…
The outraged yells got the notion derailed before it had even been put to the vote. Turned out that even in cases in which the original bloodline had become extinct, the tomb had been turned over to some collateral cousin or another, or been given to a beloved friend, or… pretty much not a single one was “abandoned”.
The then-major is from out of the area and is now our regional President; if that’s her worst misstep ever she will have had a great political career, but it’s often mentioned as the kind of mistake only an import could make.
I saw grave blankets growing up in Michigan and Pennsylvania as well; Mom told me it was so you could find the grave to visit it in winter when it’s snowed over.