The award, presented by guild president Professor Simon Owens, is made once a year to international figures from the horticultural world.
Guild vice-president Bob Ivison said: "It's an annual award looking out for those who contribute to the world of horticulture nationally and internationally, which meets the aims and objectives of the Kew Guild and deals with education, botanics - a range of things."
Barber said: "I guess Kew must agree with me that public parks are worth fighting for. London benefits enormously from having so many within the metropolitan area.
"If you want all the benefits that public parks can provide, you must protect their quality. Kew Gardens is a shining example of how well-cultivated plant species enhance a park landscape."
Barber's latest campaigning includes drawing attention to the "butchering of plane trees" in a local town centre where he lives.
Myerscough College also recognised Barber's achievements this week, with an Honorary Fellowship. These accolades take their place alongside many more including the OBE he received from the Queen last year.