Guerrilla gardening is political gardening, a form of nonviolent direct action, primarily practiced by environmentalists. It is related to land rights, land reform, and permaculture. Activists take over ("squat") an abandoned piece of land which they do not own to grow crops or plants. Guerrilla gardeners believe in re-considering land ownership in order to reclaim land from perceived neglect or misuse and assign a new purpose to it.

Some guerrilla gardeners carry out their actions at night, in relative secrecy, to sow and tend a new vegetable patch or flower garden. Others work more openly, seeking to engage with members of the local community, as illustrated in the examples that follow. It has grown into a form of proactive activism or pro-activism.

The earliest record of the term Guerrilla gardening being used was by Liz Christy and her Green Guerrilla group in 1973 in the Bowery Houston area of New York. They transformed a derelict private lot into a garden. Thirty five years on the space is beautiful, is still cared for by volunteers but now enjoys the protection of the city's parks department. Guerrilla gardening of this form - that is gardening on someone else's land without permission - has been around for centuries, there is even a reference to it in the bible [cite?]. Two celebrated guerrilla gardeners, active prior to the coining of the term, were Gerald Winstanley and The Diggers in Surrey England (1649) and John "Appleseed" Chapman in Ohio USA (1801).

Guerrilla gardening is happening all over the world. In Northern Utah apple trees commonly grow along the banks of canals. Asparagus grows along the smaller ditch banks. Many of these plants were seeded 150 years ago by the workers who dug the canals, by burying their lunch apple core in the freshly dug soil, or by surreptitiously spreading seeds along a new ditchbank. Guerrilla gardening continues today as individuals secretly plant fruit trees, edible perennials, and flowers in parks, along bike trails, etc. Some guerrilla gardeners do so for the purpose of providing food. For example the Tacamiche banana plantation workers in Honduras illegally grew vegetables on the abandoned plantation land rather than leave with the plant's closure in 1995.

The term guerrilla gardening is applied by some quite loosely to describe different forms of 'radical' gardening. This includes gardening as an entirely political gesture rather than one with genuine horticultural ambition - the most famous of which is probably the London May Day protest in 2000 when no long term garden was expected to take root. Guerrilla gardening has also been used by a number of writers to give a radical edge to their gardening books. One of these is a book titled "Guerrilla Gardening" was published in 1983 by John F. Adams aimed at encouraging amateur gardeners to grow heirloom varieties that are not the result of corporate hybridization. Another was a book by Barbara Pallenberg called Guerrilla Gardening which instructed how to make a garden on a small budget.