The Guardian piece, “High Lines and Parks: why more green isn’t always greener for cities” is a great question.

But the enquiry is confused. The writer, Owen Hatherley, has taken a wide scope of movements and ideas and blurred their failures on something undefined; what is green?

I think a better way to break down the notion, "why green isn’t always greener for the cities" is from the article "The Surprising Role of CO2 in Changes of the African Savanna”.

The following changes are the result of a CO2 fertilisation effect;

We associate more vegetation under the vague, ‘green is good’ banner but this fertilisation effect is having negative impacts on wildlife. Cheetahs, preferring to hunt in open spaces where they can maximise their speed, become inhibited and starve. Furthermore;
 
"Although some might view an increase in desert plant growth as positive, an expansion of woody vegetation in savannas and grasslands could have serious negative effects, Midgley cautioned. It could threaten and wildlife populations and water supplies, as trees and shrubs use more water than grasses. It could even amplify global warming, since trees, being generally darker than grasses, can absorb more solar radiation.”

The above photos, all 3 of which are ‘green’ (and non-anthropocentric, though anthropogenic) reveal different ecologies with different outcomes for a variety of species. The good and bad depends on which perspective you take (pick a species), yet all is classified as "Green". To apply this back to the city, and garner some insight, you have to ask yourself "which green" is being used.

However, this lumping of green as a singular ecology perhaps raises a fundamental question that isn't addressed, the fetishisation of ‘Green’ and the paradox of how we easily idealise it yet simultaneously, we don't acknowledge it - we don’t accept there’s a variety of ‘Green’, or perhaps filter out the ones that do not fit the ideal. "The algae’s ruining the view" (yet provides 71% of our oxygen). 

Green comes in variety of flavours. Green is not always greener, but with thoughtful application, I think it is.