And why not? Things have never been better for his party, who are punching above their weight at Holyrood with a raft of proposals from bans on smacking and fracking to the imposition of the 20mph speed limit coming to fruition.
First Minister Nicola Sturgeon is also pushing on with plans to explore a Citizens’ Basic Income, another Green policy, despite evidence it will cost billions.
All this with only six pro-independence supporting MSPs. No wonder Mr Harvie is claiming “Everything’s Gone Green”.
This is perhaps how coalition politics is supposed to work. Smaller parties can extract concessions and increased influence as a result of favourable electoral maths. At Westminster, the DUP is certainly deriving maximum benefit by propping up a hirpling Tory party under Prime Minister Theresa May.
However, while coalitions can prove short-term successes, history tells us that the long-term effects are more mixed.
The Liberal Democrats under Nick Clegg have yet to be forgiven by voters for their coalition with David Cameron, which seemed all too chummy from the very start. And the chumminess at Holyrood is also upsetting many.
For some, the Greens have long been regarded as a party of principle, a place where floating voters who want to push the environmental agenda can safely lay their vote.
But the perception is growing that the Greens are prepared to lie down to the SNP when it suits.
The decision to fight only three Westminster seats out of 59 at the last UK general election led to strong criticism and accusations that the party was stepping aside so as not to harm the SNP by splitting the Scottish Yes vote.
This decision denied many genuine Green voters the opportunity to cast their vote up and down Scotland. The reason given – that the party could not afford to field more candidates – was laughable.
Some may point to the recent policy successes and conclude: Good decision; smart politics. But if Mr Harvie’s party becomes shorthand for “SNP lapdogs” rather than “principled upholders of a strong environmental agenda” he may find that his new-found influence is merely a short-term blip.