"London is the focal point of biological invasion because of the heat island effect," he told the London Natural History Society on Saturday. "By understanding what goes on here, we can predict and manage risk elsewhere."
Foremost among emerging problems is the tree of heaven (Ailanthus), he said. "In some parts of the world it is known as 'tree of hell'. It's incredibly destructive. It undermines buildings and virtually nothing lives on it."
Spencer added that Catalpa and Paulownia were "a couple of decades behind" on the road to becoming a problem. "They're also significant invasives elsewhere in the world," he said. "They should be substituted with plants that have low or no seed dispersal."
He described Buddleja as more of a threat to urban biology than Japanese knotweed. "It's thought to be good for wildlife, but there's no evidence of any clear benefit from it, and it may be bad for some populations," he said.
Even apparently innocuous garden plants such as the commercially available Allium triquetrum (three-cornered leek) Spencer described as "a fiend - it appears early and smothers other flora".
Horticulture has also been responsible for unwittingly introducing Fumaria capreolata (white ramping fumitory) and Urtica membranacea (membranous nettle), Spencer said.
He added that work was needed to record the spread of such plants. "We have a lack of demonstrable evidence in urban areas," he said. "Non-natives have not been considered proper botany."