Radical environmentalists – especially those belonging to the Green Party – are often described as ‘watermelons’ – because they’re green on the outside, but supposedly red on the inside.

It’s a juicy metaphor, but as Matthew Holehouse of the Telegraph argues, a somewhat inaccurate one:

“Karl Marx and his pupils championed economic growth and personal consumption: five year plans, tractor factories and fridges for all. The row, for them, was whether the planned economy was a stronger engine than the free market.

“The Greens want something very different.

“Caroline Lucas and colleagues regard economic growth as incompatible with protecting the planet and a fulfilling personal life… The party’s manifesto argues for zero, or even negative growth and falling levels of personal consumption…”

Being pro- or anti-growth is a pretty fundamental divide. So fundamental, in fact, that it puts the Greens on one side and just about every other political philosophy on the other.

Conor Pope – a Labour supporter – uses a piece for the New Statesman to make many of the same points. He has little sympathy for those in his party who would pander to the Greens:

“The problem for Labour is not just that we are faced with the threat of a ‘Ukip of the left’ – we already have Ukip for that – but that we are losing votes to a party so utterly useless as the Greens. Let’s not be coy about that. We can wait until after 7 May to try and build coalitions if we need them. Now is not the time to retreat to an ‘I agree with Nick’ type narrative. Less lovebombing, more bombing.”

In respect to the fundamentals of economic policy, he’s right. There is very little commonality between the Greens and Labour. But on other issues, the inconvenient truth is that many of the Green Party’s “crackpot ideas” would be Labour Party policy if the latter thought they could get away with it.

For instance, are we to suppose that Ed Miliband, in his heart of hearts, is a monarchist? Or does he secretly share the Green Party’s republican aims? Looking at Matthew Holehouse’s survey of Green Party policies, there are a number of other points on which large parts of the Labour Party – including the upper echelons – must be in private agreement.

For instance, I’d hate to think how few Labour MPs could contemplate the following without drooling:

“Under Green plans, inheritance tax – ‘to prevent the accumulation of wealth and power by a privileged class’ – will no longer just tax the dead.

“Under radical reforms, it will cover gifts made while the giver is still alive – raising the prospect of levies on cars, jewellery or furniture given by parents to their children. There will be exemptions for some large gifts, ‘such as those received on marriage’.”

Or this:

“Independent schools will lose their charitable status and pay corporation tax, while church schools will be stripped of taxpayer funding. Religious instruction will be banned in school hours.”

Then there’s a whole raft of Green social policies that would find widespread support in the Labour ranks (not to mention the Lib Dems and certain sections of UKIP and the Conservative Party):

“The trade and cultivation of cannabis will be decriminalised under Green policy, along with possession of Class A and B drugs for personal use. Anti-rave laws would be scrapped…

“All elements of the sex industry will be decriminalised, and prostitutes could no longer be discriminated against in child custody cases…

“Assisted dying will be legalised, and the law on abortion liberalised to allow nurses to carry it out…”

The Green Party is best thought of as a free-range environment in which middle-class liberal lefties can say what they really think. Stuck inside the battery cage of conventional politics, there are many Labourites who wish they could do the same.

Perhaps we need a fruit-based metaphor for the opposite of a watermelon – i.e. for someone who is Labour red on the outside, but harbours Green sympathies within.