Looking at the polling graphs it becomes clear when the 'Green surge' started. It was with the European elections. The Green party not only got an expected boost from those elections but has clearly been able to carry momentum forward since then. This leads me to a number of thoughts.
Firstly how did the Greens maintain momentum after the Euro election results when their vote share actually when down slightly not up. Well two aspects of the results were good for the Greens. Firstly the fact they beat the Lib Dems and secondly the fact that their number of seats went up rather than down. This latter fact helped give the impression of forward momentum. If the number of seats had gone down instead the impression might have been given of going into reverse which could have had the opposite effect.
Now why did their number of seats go up to three rather than down to one when their vote share fell? It was because they got the final seats in South West and London regions. Why did those win those seats? Because UKIP failed to beat then to the final seats in those regions. Why did UKIP fail to win those seats? Because of the votes they lost to Mike Natrass' 'An Independence from Europe' spoiler party. Why was that party on the ballot paper? Because Mike Nattrass had been deselected by UKIP having fallen out badly with Farage and the party leadership and feeling rather bitter about it he decided to set up his own party and put up the deposit money to run in all English regions.
So my (highly tenuous and speculative) theory is that if the Green Party make gains at next year's local elections, possibly gain one or two seats in the General election and make a significant impact in many other seats it will be because Nigel Farage and Mike Nattrass couldn't get along with each other and that this is a case of 'the Butterfly effect' in action!