In my last house I had a huge garden bordering on fields. I actually had one giant hogweed growing down by the ditch. I knew what it was, I've seen them quite a few times round here. It was magnificent, and did no harm, as I didn't go up and touch it or wipe it on my eyes. Loads of plants are dodgy, eg aconite, nightshade, laburnum, yew etc, but they aren't triffids, they don't run after us and attack. I agree that children should be warned not to touch, but here nobody fusses about giant hogweed, it's just there, and no-one suggests 'eradicating' it. Mine grew to about nine feet (3 metres). I liked it!!