Nicola Twilley's blog on the geography, infrastructure, and history of food, from rice mutations to strategic food reserves.
HN commenters link it as a detailed source on atomic gardening, apple rootstock archaeology, and strategic food reserves like maple syrup stockpiles.
Cigarette butts seem to be the most common by count, but what about weight and volume?
On the streets of NYC, I don't really notice the cigarettes, but I do notice the enormous amount of plastic bags (much of it stuck in trees), plastic or paper cups, straws, takeout containers. And gum. If you ever look at a sidewalk and see dark splotches [1], that's discarded chewing gum. It's absolutely everywhere. Not as invasive, of course, just odd.
I don't know what SF is like, but NYC has a fascinatingly ugly system [2] where you're supposed to put trash and recycling out on the sidewalk for it to be collected, where it's effectively temporary litter. NYC's sanitation workers are notoriously careless about handling the trash, and my pet theory is that a sizable portion of street litter actually originates in the sanitation workers spilling trash on pickup day.
NYC's trash problem is also exacerbated by the fact that landlords can get away with not doing their part in keeping the outside of the building litter-free.
[1] https://www.ediblegeography.com/wp-content/uploads/2011/07/Splotchy-pavement.jpg
[2] https://i.redd.it/x0iye2nog6m31.jpg
https://en.wikipedia.org/wiki/Atomic_gardening
EDIT: you may be thinking of this article specifically http://www.ediblegeography.com/strange-and-beautiful-seeds-from-the-atom/
> For example, a mutagenic Ruby Red grapefruit grown without the use of pesticides can be labeled as organic and thus be sold for a premium, despite the fact that organic foods by definition cannot be genetically modified.
https://www.ediblegeography.com/rootstock-archaeology/
There's some detail here about how British researchers developed apple tree rootstocks, including photos of whole trees that have been excavated and reassembled.
Couple it with this documentary:
https://docuwiki.net/index.php?title=Apples:_British_to_the_Core
According to this article,[1] cultivars produced with this method include Calrose 76 rice and Todd's Mitcham peppermint, which respecively account for roughly half of California's rice crop and a majority of the world's peppermint
[1] http://www.ediblegeography.com/strange-and-beautiful-seeds-from-the-atom/
Check out the global strategic maple syrup reserve (https://www.ediblegeography.com/syrup-stockpiles-wine-lakes-butter-mountains-and-other-strategic-food-reserves/)
https://en.wikipedia.org/wiki/Atomic_gardening
and
http://www.ediblegeography.com/strange-and-beautiful-seeds-from-the-atom/