guerrilla gardening
n. The surreptitious or unauthorized planting of flowers, shrubs, vegetables, and other flora in a public space.
Other Forms
Sam Tylicki calls himself a "guerrilla gardener."

His weapons? Shovels, watering cans and vegetable plants. His target? A small section of a vacant city-owned lot at 5406 Fleet Ave. His problem? He didn't get permission from the city to use the property, so Tylicki is trespassing. That means the city can kick him out, along with his green peppers, collard greens and other vegetables. But getting permission is against the philosophy of guerrilla gardening, an international environmental movement whose followers "reclaim" public space by planting fruits, vegetables and flowers.
—Olivera Perkins, “'Guerrilla gardener' battles City Hall,” The Plain Dealer, September 22, 2001
1977 (earliest)
An architect was saying just last night at Hack Schoolman's house that he liked vines, and how tall did I think a kudzu vine might grow?

"Right over the face of a forest." I said, and he said good. He is thinking of planting some to grow over the Rayburn House Office Building. Well, the heart sank, of course, because here was the architect of that building. I presumed, and imagine being expected to be polite right through supper.

But no, he was not that architect, but instead feels, as all right-thinking people do, it is singularly ugly. For his part, he said, he is taking up guerrilla gardening, planting vines and trees without permission, to cover, conceal, camouflage the Rayburns of the world.
—Henry Mitchell, “Any Day,” The Washington Post, September 30, 1977
The original guerrilla gardeners were probably the gypsies, who would use out-of-the-way locations near the side of a road or path to plant potatoes and other vegetables. They would then continue on their nomadic way and return later to harvest the crop.

Today's guerrilla gardeners are an activist bunch who view their politicized plants as symbols for reconnecting with the land in the face of urban blight and as a way of green-thumbing their collective noses at The Man. The planting-as-protest began in the 1970s with a New York group called the Green Guerrillas. These urban horticulturalists started off crudely by lobbing seed grenades (Christmas tree ornaments filled with soil and wildflower seeds) into abandoned, debris-filled lots, but eventually converted hundreds of these lots into flower- and vegetable-filled community gardens. The movement has since spread around the world (one slogan: "Resistance Is Fertile") and now operates under the more general rubric of guerrilla gardening.

Note, too, that those who grow marijuana plants on public lands are also sometimes called guerrilla gardeners. (Although, back to the protest angle again, some activists somehow managed to plant a few marijuana seeds near the British parliament in the annual May Day protest of 2000. By July, the plants had sprouted and were apparently growing quite nicely. Police confiscated the crop immediately.)

I should also mention the possibility of a third sense of the phrase taking root: To remove plants from public lands for use in one's private garden. This sense is being cultivated by a book published in 2001 called Guerrilla Gardening: How to Create Gorgeous Gardens for Free.