"Horticulture has been incredibly lucky - it's not been constrained or penalised like agriculture and pharmaceutacals," he said. "Things like roses and Delphiniums have been around for a long time and are highly bred. But plantspeople are bringing in species that may become invasive like the grass Anemanthele, which is spreading in London."
The horticulture industry should take a more proactive approach to breeding and select plants that are not likely to cause problems, like using hybrid trees with low fertility as street trees rather than Ailanthus (tree of heaven), which he said "we will come to hate".
As plant recorder for the London Natural History Society, Spencer has urged naturalists to log sightings of the tree as part of the Defra-backed Recording Invasive Species Counts project. But he cautioned: "The mappings we have grossly underestimate of the distribution."
Grass-roots action has helped to stop the spread of invasives such as Ludwigia (water primrose), he added. "At most sites, it's eradicated or under control. But without knowledgeable people looking out for it, we could not have taken the steps we have to control it."
He warned against "basing our priorities (on which species to control) on aesthetic criteria", saying of Buddleia: "I still haven't found evidence for its benefits (to wildlife). It's undoubtedly an invasive species, it out-competes other plants and may have a negative impact on their pollination."
He added that a beetle, Cleopus japonicus, is already in use as a biological control against Buddleia in New Zealand, but said: "It would need trialling and studying before we could consider it here."
- See interview, p15.