I sometimes refer to "native" and "naturalised" plants in this diary. Last week I posted a number of entries concerning various plants I saw within about a mile or so of each other in Hertfordshire.
 The first plant I noted (above) was Lords and Ladies aka Cuckoo Pint (Arum maculatum) which is native- a truly wild flower of woods, hedgerows and shady places.

 Common Gorse (Ulex europeus) is native. Grows wild, sometimes cultivated for various purposes e.g. for hedging and landscaping.

 Winter Heliotrope (Petasites fragrans). Non-native, a naturalised garden escape of North African/Mediterranean origins. Richard Mabey states in 'Flora Britanica' that it was introduced to the UK in 1806 though he doesn't mention his source for this date.

 The Common Snowdrop (Galanthus nivalis) might be native, might be an introduction. There is a plausible theory that it came to these shores in Norman times when planted in monastic grounds as a symbol of purity coinciding with Candlemas. Certainly various Galanthus species have been grown in gardens across the ages and will have spread.
 Winter Aconite (Eranthis hyemalis) is undoubtedly an introduction being of southern European climes but it has naturalised here and there in the UK.

 Stinking Hellebore (Helleborus foetidus). Native but also cultivated as a garden plant; so it can occur as both an indigenous wild thing and a naturalised escape.

 "Native" plants are generally considered to be those that existed here after the ice age ended (about 11,700 years ago) and before rising sea levels separated the land mass from the continent (around 8,500 years ago). It's a fascinating history but then again does it alter my appreciation of Snowdrops if they were here ten thousand years ago or only a thousand?
 It becomes contentious I suppose in the case of more recent arrivals. We no longer take the Victorian view that Japanese Knotweed is suitable for the shrubbery. We generally like Buddleia but sometimes regard its invasiveness as a problem. The jury is out on a plant like Himalayan Balsam which may or may not be a threat to native habitats, ecologists differ on that point.
 Anyway, even a short walk in the countryside makes it evident that the flora of Britain has been millennia in the making.