You will be the first Green MP in the House of Commons. Did you expect to win the Brighton Pavilion seat?
We knew we had a really good chance - at the European elections, we came ahead of all the other parties. But until it's happened, you can't quite believe it.
How did you celebrate winning?
The result didn't come through until six in the morning, so we were all quite tired. There was champagne at the count, and then we went for some breakfast.
What lessons have you learned from fighting this election?
As a small party, targeting our resources was crucial. But we need to repeat the Brighton result countrywide, with no state funding for political parties and the electoral system stacked against us in every respect. It's designed to keep out the smaller parties.
What's next for the Greens?
There are lots of local elections coming up. But, for now, the issue is electoral reform.
Who are your allies at Westminster?
We are a party of the left: as well as environmental policies, the issues we're standing on are fairness and social justice. That said, we will support issues on a case-by-case basis.
Could the Greens be part of a coalition?
I think we would rule out a formal coalition, but we're very interested in talking about ways we might co-operate.
How big a political shift does your election represent?
It's only one seat - but it's the first seat. It was only 24 years between the first Labour MP and the first Labour government. We're not counting on that, but this is the beginning of a new political force at Westminster.
In many European countries, having Greens in government is the norm.
Absolutely. But to do that under a first-past-the-post system is difficult.
Can we still prevent climate change?
If we act now, very quickly, we can avoid the worst, or at least have a very good shot at it. But politicians think you tackle either the economic crisis or the environmental one.
All the parties make environmental pledges. Do you believe in them?
No. They dress up their policies with green tinges. I have sat in the European Parliament with Tory MEPs who water down legislation and delay timetables. Credit to Labour for introducing the Climate Change Act, but the targets are not ambitious enough, and they don't have the policies to achieve them.
Do green tinges undermine your message?
They confuse people. One day, climate change is the greatest threat we face; the next, it's nowhere on the agenda and we have to make huge spending cuts. Evidence shows a green economy would be far more labour-intensive than our fossil-fuels economy.
You're the first leader the Greens have had. How has it changed the party?
It was controversial, but having a leader has not diluted our message, and it gives us an extra bit of leverage with the media.
Is your recent success partly down to that change in the party's leadership structure?
Yes, but not my leadership - everyone's collective leadership and endeavour.
What difficulties did the change cause?
I honestly think all of that is behind us now. People think of political leadership as something done to them, rather than something they play a part in. But the Greens can bring collective leadership to the political debate.
What inspired you to work in politics?
I was very involved in CND. Then, in 1986, I read Seeing Green by Jonathon Porritt and realised there was a coherent political approach to all the issues I was interested in. I joined the party on the day I finished the book.
Looking forward, is there a plan?
To do everything I can to further green political approaches.
What would you like to forget?
The six hours waiting for the election result.
Are we all doomed?
Absolutely not. What we need to do is urgent and important, but incredibly positive.
Interview by Alyssa McDonald