It's good to be Green.

After years of parrying barbs about tie-dyes and tofu, the party is getting the last laugh on those who dismissed them as a political punchline.

Undeodorised tree-huggers and yurt-dwelling vegans no more; the Greens are a growing force in Scottish politics. They head into the Holyrood election in better shape than ever before and with brighter prospects than rivals who once dwarfed them.

Of course, they're still Greens and so they have no leader as such. Patrick Harvie is Scottish Green "co-convener" and he sat down with me to talk about offering a progressive alternative and keeping the SNP on their toes.

Once upon a time, the Greens were associated purely with environmental and ecological issues. They were the party you voted for if your motive passion was the ozone layer or climate change or pollution. While those causes are still hardwired into the soul of the Scottish Greens, they have greatly broadened their appeal and now offer distinctive policies on the economy, taxation, justice, health and education. No longer the nagging social conscience of Concerned of Kelvinside, the Scottish Greens are a fully formed alternative to the centrist parties.

And if the polls are anything to go by - they've tended to be more reliable north than south of the border - that alternative could find itself replacing the Liberal Democrats as the fourth party of Scottish politics.

"It's entirely conceivable that we can get at least one seat in every region," Harvie tells me. "Certainly in regions like Glasgow and Lothian we are confident we can take a second seat as well. That would take us into double figures, which would be marvellous - the best we've ever done."

The Scottish Greens had a good referendum and saw their membership leap to 9000 in the months following the vote against independence. Activists are buoyed by the most ambitious campaign the party has ever put together. A budget of £350,000 has paid for staff in every region of Scotland, including eight regional coordinators, three full-time national campaign staff, three press officers, and a policy wonk. In financial terms, the campaign is three times bigger than any previous effort.

Although Harvie rejects the term "radical" - not out of political timidity; he just reckons asking the rich to pay a bit more tax isn't revolutionary - the stall he is setting out for next month's election is bolder than any of the other left-leaning parties.

Four thousand extra teachers, 200,000 green jobs, four new tax bands (with a 60p rate on income over £150,000), and a levy on supermarkets to deter the promotion of junk food. The Offensive Behaviour Act would go, as would custodial sentences shorter than 12 months, and sex work would be decriminalised. Rounding off their manifesto are promises to ban fracking, introduce a written constitution, and give voters the power to decide when a second referendum on independence should be held.

At the centre of it all is a holistic view of the economy, where the common good is prioritised over GDP growth.

Harvie tells me: "It is central to why we exist as a party to challenge the notion that everlasting economic growth on a planet of finite resources is either possible or desirable. You look at the history of this country, over my lifetime, you see periods of growth - sustained, long periods of growth - where inequality keeps getting worse. You see periods of stagnation or recession where inequality keeps getting worse. So there is something deeper going on here than just whether the economy is growing by 2%, 3%, or 0% in any one year.

"If we want an economy that works for people's interests then we have to recognise that we own it, collectively. It's our economy; it doesn't belong to a bunch of traders in the City; it doesn't belong to international investors. It is this country's economy and we have a right to ensure that it works in a way that benefits the common good."

If you are predisposed to the Greens, this is the gospel you have longed to hear from the mainstream parties. If you're not a fan, this will only confirm your conviction that Patrick Harvie is the high priest of druidical communism and his doctrine represents the shackling of economic dynamism and the dragging of human society back to the cave.

Harvie is familiar with the jibes that often fly in his party's direction. He argues: "The kind of economy we've got at the moment deserves to be thought of as an unrealistic, bizarre fantasyland. Some people stereotype Green ideas as unrealistic. They're the more realistic ideas on the planet. The idea that we can continue with the current model is the fantasy, and it's not a pleasant fantasy."

One of the most far-fetched aspects of that fantasy, as Harvie sees it, is a property market driven by speculative investment rather than meeting housing needs. Housing is one of the key pillars of the Scottish Greens' pitch to the electorate and the party wants to see non-domestic rates levied on vacant and derelict land.

Harvie insists: "Our land should not be about speculative investment for short-term gain. It should be about using the land in the way that creates the most social benefit and economic benefit that is in the interest of the common good rather than in the interest of a tiny number.

"This country is weird. This country is unusual in European terms. Not every other country has this problem, where the bulk of our housing is provided as speculative investment. The bulk of housing in many other European countries is about providing for social need... A great deal of the wealth of our country is shorn up in fixed assets that are not being properly taxed. That wealth should be invested in the real economy."

That's all very well but, absent Nicola Sturgeon's secret life as a kitten-strangler coming to light, the SNP is on course to walk the election. Even if the Greens end up with seats in double figures, what influence are they going to have?

Harvie believes there is scope to needle the SNP from the left, arguing: "People know that we're not the kind of party that will say the party of government at any one time is either perfect or a disaster. People know that we take a more balanced approach to these things and that we've got a track record of getting the SNP to move on some issues, even when they didn't need to because of the numbers. Things like rent controls, things like the fracking moratorium."

His approach to the Nationalists is respectful but not deferential. "If you think the SNP are going to be the next government, who do you want in there putting them under pressure? Do you want people who just say, 'Oh, SNP bad!', or people like their own backbenchers who'll just nod through everything? One backbench rebellion in nine years? That's not holding the government to account."

He adds: "A good solid group of Green MSPs holding the government to account will be constructive. It'll be bringing pressure to bear where it actually makes a difference. Where it gets results. That in many ways is what the SNP need if they're to do a better job."

If another small party tried to assert its influence over a majority government, it would be met with gales of laughter. But the Scottish Greens have a secret weapon: Their instincts are broadly those of the great mass of SNP members and activists. And it is this crossover that gives them a measure of leverage with Nicola Sturgeon. Nationalist backbenchers may tug their forelocks in The Presence but the Scottish Greens can nudge the current and future First Minister off the fence and onto progressive territory.

Meet the Scottish Greens, the new power brokers of Holyrood.

Stephen Daisley is STV's digital politics and comment editor. You can contact him at stephen.daisley@stv.tv.