In article ,
Jeff Layman wrote:

That would have been true 50 years ago, but things like Dutch Elm
Disease and now the threat to ash from Ash Dieback and possible threat
of the Emerald Ash Borer are major ecological problems. The threat is
not only from plants, but what may be brought in on those plants. It's
true that our "problems" with Himalayan Balsam and Japanese Knotweed
pale into insignificance when compared to Water Hyacinth in tropical
countries, Hakea in South Africa, and the now non-existent Opuntia in
Australia, but that doesn't mean there isn't a plant out there which
couldn't become a threat.

None of those (nor virtually any other such disease) is transferred
via seeds. There are two myths he

The only terrestrial plant that is a serious ECOLOGICAL problem is
Japanese knotweed, though there are some aquatic ones. Himalayan
balsam is highly invasive, but doesn't eliminate whole ecologies;
in fact, I have looked carefully and have never seen it form a
monoculture. The people who make a flap about it (and most other
'alien' species) simply have no idea how dynamic the UK's ecology
has been for the past 10,000/1,000/100 years (pick any). We are
NOT New Zealand, Madagascar etc.

Dutch elm disease has probably been native almost since elms
arrived (see Rackham), and the other diseases aren't all that
different. Also, we can't stop the ones that are carried by
insects from crossing the Channel, because of the occasional
easterly gale - all we can do is delay. Our problems with such
things are almost entirely by the way that we have stressed the
ecologies by preventing them regenerating naturally and by things
like air pollution (*). Without attending to those, no defence
will work in the long term; with attending to them, no other defence
is essential.

(*) https://www.theguardian.com/environm...air-plan-delay


Regards,
Nick Maclaren.