Some alien weeds aren't too bad. More like visitor wild flowers really. In the south-west the roadsides in May and early June are adorned with beaked hawksbeard, which is quite jolly. There's Oxford ragwort, a native of Mount Etna, that cheers up the railways in the London area. I went up Etna looking for it a couple of years ago and couldn't find a single specimen! We have three-cornered garlic, impressive enough en masse but which is too aggressive and which ousts native hedgerow plants in Cornwall and Scilly.   But Japanese knotweed and giant hogweed? No thanks. They are unalloyed menaces. Exterminate!