Well. I had a chat with a friend, and he had an awesomely good idea for sorting out the remaining caltrop involving a stepladder strapped to a bike and a tall friend.
After my previous efforts, this is what I could see from the bike path.
Oh, and I could also see more three corner jacks that had landed on the path since I gathered them all just a few days ago. I extended my stepladder. I had a lovely chat with a cyclist who stopped to find out where I had found my bike trailer. Sadly for him but happily for me, I bought it from the maker in the 1980s–it’s a great low-fi trailer but no longer available so far as I know.
So there’s good news. No more caltrop overhanging the bike path. I say again, bike path. Bike and caltrop should never be mentioned in a single sentence!
And there’s bad news. That blob in the left hand front corner is my secateurs sitting on top of the retaining wall, and here is what I could see standing on my tiptoes. Caltrop extending up the slope for another metre or more, covered in three corner jacks at all stages of ripeness. I pulled at every stem I could reach, but clearly the taproot(s) are further back and my feeling that I could pull this thing out if only I was taller or higher up–was a fantasy I was entertaining when the ugly truth was out of my sight. On the up side–thousands of potential punctures eliminated by my efforts to date. On the down side, plenty more where they came from!