PATRICK Harvie has waited a long time for this election. It’s been 13 years since the Scottish Greens hit their high-water mark of seven MSPs in the "rainbow parliament" election of 2003. Harvie, then as now a list member for Glasgow, was among the slightly stunned new intake.

But at the following election, most of his colleagues were ousted by an SNP surge, and he was one of just two Greens to survive at Holyrood. He became co-convener amid the wreckage in 2008. The party kept its head above water in 2011, but only just, with Harvie holding on in the west and Alison Johnstone making it through in the Lothians.

Then the referendum arrived and Harvie came to the attention of a wider audience as one of the most eloquent and thoughtful advocates for Yes. Afterwards, the Greens’ membership increased five-fold, a bigger multiple than even the SNP enjoyed. People, money, energy and optimism flooded in and the dynamics of the Holyrood vote changed with them.

The party used the General Election to talent spot potential candidates and drill new recruits in basic electioneering. But the real target was always this May. With the polls suggesting solid support across Scotland, the Greens now have a realistic chance of achieving their best result yet and the sweet satisfaction of overtaking their old enemies, the LibDems.

“I don’t think there’s any region in Scotland where we’re not capable of returning one MSP,” says Harvie. “In Glasgow and Lothian it’s possible we could get a second. That would take us into double figures. We’ve always had a big, strong message and confidence that a lot more people are open to voting Green, but we’ve not had the activist numbers and capacity to connect the two and get the message to the people.”

His doorstep pitch, a sideways criticism of the SNP, is that Greens would help deliver “a bolder parliament and a better Scotland”. He cites rent control, fuel poverty and land reform as issues where the party has already helped push the government in new directions.

“Most people recognise the SNP are going to be the biggest party again. But the SNP have generally been at their best when under pressure, forced to answer tough questions, forced to raise their game. We’re going to get a better result for Scotland if there’s a strong, diverse parliament with some Green voices in there.”

He says the SNP’s minimal tax tweaks have added “new urgency” to this argument.

“How long did we argue for these powers? And for what? To do nothing with them? That’s not what I was making the case for. I wanted Scotland to have the ability to make these decisions so we could do something different from what the UK has been offering us for far too long. It’s a huge missed opportunity if that’s the tax proposition we see implemented.”

Besides replacing council tax with a fairer, updated property tax and ultimately a land value tax, the Greens would also use Holyrood’s new powers to shake up income tax.

Their plan pivots around the average earnings point of £26,500. Those earning less would pay less tax, those earning more would pay more tax. There would be an extra band to make the system more progressive, so instead of 20p, 40p and 45p, income tax would be levied at 18p, 22p, 43p and 60p in the pound. Not surprisingly, that top rate of 60p has drawn a lot of attention.

Nicola Sturgeon says even a 50p rate would be “daft” and “reckless” as high-earners might quit Scotland take their loot with them. Harvie admits a 60p top rate might have a “behavioural effect”, and so the Greens haven’t assumed any income from the 17,000 taxpayers affected, but ultimately he couldn’t give a monkey’s.

“Frankly, if a relatively small number of people chose to leave, if we lost the greediest people in Scotland, I would wave them goodbye. If someone is so motivated by being super-rich that they’re willing to uproot their family, their home, move somewhere else purely in order to be that much richer than their neighbour, I find that a deeply ugly and unpleasant sentiment.”

He denies the 60p rate is symbolic but struggles to explain why it’s set where it is. If you want more money for public services, why set a 60p rate that might raise nothing?

“It seems reasonable to us.” But if it might not raise anything why not set it at 55p or 65p? It doesn’t seem rooted in arithmetic, a calculation that a 60p rate would raise X pounds revenue. So why have you settled on 60p? “You’ve got to put it somewhere, haven’t you?” he shrugs.

Oh, come on. That’s lame. “I’m genuinely open to a discussion whether it should be 60, 55, 65,” he splutters. But if it might raise nothing, why not leave the top rate as it is? “Because what we want to achieve is the redistribution of wealth in our society. It would be entirely wrong to achieve that in relation to people who were on £40,000, £50,000, and £70,000 but then ignore the wealth which is being hoarded by the super-rich.”

He’s not too hot on the numbers for one of the Greens’ other big ideas either. That the party wants to ban fracking is well-known. Harvie says it’s wrong that the SNP are asking for votes in areas that might be affected without saying if they would licence shale gas drilling after the election. But the other side of the Green plan – taking the massive Grangemouth refinery belonging to would-be frackers Ineos into public ownership – is less familiar to the electorate. And apparently to Harvie as well.

How much is Grangemouth worth? His eyes widen and flicker. “I’d like to re-tool that facility to have a critical role in the development of new industries like non-hydrocarbon chemical feedstocks,” he recites.

It’s Scottish Green policy to have it in public hands. How much is it going to cost? “I’ve no idea. I don’t know,” he admits. “I would love that to happen. I don’t know what it would cost.”

The latest Ineos Grangemouth Plc accounts value the property, plant and equipment at around £300m and the employee pension scheme at £400m. “That would be an immense challenge.”

He’s better on why fracking should be banned, however.

“The big question is where is our economic future lies, both in terms of jobs and our energy system. It’s clearly in demand reduction, renewable energy generation and new sustainable industries. Fracking would not only be a setback. It would be a huge distraction from what we should be doing. No-one who’s paying attention thinks fracking is a long-lasting industry. Wells last a few years and then they’re kaput. If anybody in the SNP sees this as a huge economic boon for Scotland, they’re kidding themselves.”

Will there be much about independence in next week’s manifesto?

“We still believe Scotland has the right to ask this question of itself at some point,” he says. “But I don’t want to have another referendum until we have genuinely convinced enough people, not just to get to 50 per cent plus one, but a clear vote. I’m ready, but a lot of others are not. Forcing the issue before we’ve won the deeper argument might be a mistake.”

Harvie is one of three Green constituency candidates – his “baldy head” logo is currently plastered across Glasgow Kelvin – however the party will rely on the lists for its MSPs.

Are you worried Rise or Ukip might take enough list votes to scupper a Green revival?

He says he’d be “surprised” if either party gets an MSP, and insists voters know Greens have a track record in getting elected and delivering.

And how do you counter the wall-to-wall BothVotesSNP message?

“I’ve spoken to people on the doorstep who find that slogan off-putting," he says. "The SNP would do well to remember how deeply that kind of mindset rooted itself in the Labour party when they were the dominant party in Scotland, and how much it ended up damaging them.”

The 43-year-old counts as a grizzled veteran in this campaign. He is the only Holyrood leader who was also leader in May 2011.

You’ve done this job eight years. Can you see yourself doing it for another parliament as well? “If the party wants me to."

If you fail to make progress, would you step down as co-convener? “That will be a decision for the whole party. I don’t have it in mind. If we come back with a very small group again it’s going to be depressing. If we approach or exceed double figures, I will be overjoyed. What I have in mind is to run the best campaign that we can for the next month, try to get the best result, and then work hard to represent the Green agenda, our voters and the wider interests of a sustainable Scotland.”