All is still quiet in the world of the window boxes and, in a 90cm x 12cm place where nothing much happens, it's tempting to start thinking big. Peering at my plants through the bathroom window, I've started wondering what it's all about. Is a decision to grow things on the ledge of a building I will never call my own, in the chaos that is Holloway, about more than decoration? Is urban growing a kind of activism?

Let's focus on food. An edible window box isn't going to change the world but it could be part of a wider movement that just might. The minute you start growing your own, no matter how small the scale, you become aware of others who are doing the same. Some urban growers are protesting with their produce – a positive kind of protest that explores alternative ways of living and working.

Grow Heathrow has returned a derelict market garden back to its former productive glory, while challenging stereotypes about squatting and highlighting environmental issues. I ask William Ronan from the project if he sees urban growing as activism. "The way in which we meet our basic need to feed ourselves is deeply political, and political movements have a rich history in making food a focus of their organising."

"Instead of lobbying power-holders through methods like petitions, marching and media stunts, urban food growing puts political power in the hands of the community. We remove our reliance on food systems that destroy eco-systems, manipulate workers and enrich the bank balances of corporation shareholders. We don't have to eat vegetables tainted with chemicals, air miles and poverty wages."

Growing Communities in Hackney Photograph: Growing Communities

In Hackney, Growing Communities actively challenges agribusiness and supermarket dominance by growing tonnes of inner city salad and promoting community led trade. Kerry Rankine from the social enterprise says urban growing "concentrates people's minds on how much effort goes into producing the food we all take for granted. For many, it's a way to start thinking about alternatives".

While Kerry thinks small-scale growing can be a form of activism, she believes as a means of creating change it's not a great lever by itself. Urban growing is part of wider changes that need to be made, including supporting small farmers around cities and mentoring new growers.

So what would an activist's window box look like, if William and Kerry were pushed to indulge me? It would be sown with rare varieties not sold by mainstream companies, and with locally sourced seed. An activist grower would reject the often sterile F1 Hybrid seed that's wiping out old varieties of veg.

Kerry and I even think about window box potatoes, specifically the rare 1918 'Arran Victory'. It's an ambitious plan requiring a deep container and a very sturdy ledge, but one that emphasises that experimenting with heritage edibles is a key way to transform a passive box into an active one. Sounds like an excuse for a Seedy Sunday.

Read more of Helen Babbs' Diary of a window box garden here. Helen is the author of the book My Garden, the City and Me: Rooftop Adventures in the Wilds of London.