"Is this species native".
This is an interesting and often contentious area. According to the authorities that decide such matters, a plant species is native if:
"it was already present before the formation of the English Channel." (Around 8-10,000 years ago).
Here is a 'check-list' of all plant species considered to be native in Britain.
The Postcode Plants Database - Natural History Museum
(There is a lot more info on this site which will answer many other related questions).
However it is not as simple as just producing such a list. The paper below lays-out some of the problems with the determination of a species as being 'native' or not.
http://watsonia.org.uk/Wats15p231.pdf
Obviously there was no-one around 8,000 years ago listing these plant species. So such a list has to be based on many criteria. Seeds and pollen of many have been found in datable ancient deposits, particularly in ancient peat . But much is still based on a well informed guess.
Since the Englished Channel was formed many new species have constantly found their way to our shores, and continue to do so by purely natural means.
Orchid seeds for example, particularly that of the
Ophrys, (Bee Orchid) group, are incredibally small and can easily be blown here from plants in France, as with many species whose seeds are adapted for distribution by wind. Such seed may have established plants since the Channel was formed, but long before there was anyone able to record such an event. And this would have continued for many thousands of years, with new species finding a niche as the climate gradually warmed.
It is virtually impossible to know if such species were here before, or not until the years after the English Channel's formation. Does it really matter?
Some species we know only too well are not native, such as Oxford Ragwort, when it is known almost to the very day when the seed escaped into the wild.
It is also not just a case of, "Does the species look native". If you didn't know otherwise,
Rhododendron ponticum looks totally at home here in the Welsh mountains.
Field Botany is not an ancient science, and although some species can be recognised from earlier writings, it's not until the likes of Gerard and his Herball of 1596 did our flora start to be recorded with any diligence.
There will probably never be a definitive list of 'native plants' acceptable to all; opinion will constantly be challenged, and rightly so.
We can often say with some clear evidence that a species is
not native, but proving the reverse is not always so easy.
Dorts.
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