Garden designer Andrew Fisher Tomlin and others took to Twitter to complain about the new roof terrace.
Fisher Tomlin described it as "really sad" and "crazy waste of an opportunity to create a green space in central London".
Tracy McQue of McQue Gardens said the space was "pretty depressing".
Ian Price of Ian Price Design said: "Plants don't produce useful things like wifi. Only silly things like oxygen."
A college representative said the college had plenty of green spaces and three beehives, an orchard and a community garden at its Hackney campus.
But in this instance "we wanted a social space that we could use as a classroom. Sometimes we might have 30 people who need to sit down there so a garden would not be appropriate. The design does have plants and a landscape designer was consulted to check they were suitable."