I thought I would contribute to this forum and hopefully this information will be helpful to someone. I've made a 4000 long dirt block bridge at about 768 feet in the sky. I'm on an Expertmode world, though I don't think it makes a difference. I put my world on multiplayer so it would constantly run in the background and I would hopefully have a better chance of them spawning, even when sleeping irl. I've awoken the past two days disappointed that only blink-root had grown on my super long bridge. But I remember something that happened yesterday. I saved and exited my multiplayer world and I re-loaded it, doing this I had 4 strange plants appear on my bridge. This happened the day before also in which I re-loaded and had 1 appear on my bridge, that one was half as long as this bridge though.
I would also like to add that I've tried clearing as many dye plants as I can on the surface world and I've started the process of placing wood on the surface to cover most areas that allow growth. I'm only about 1/8th done however, so a long way to go and unsure if this is even helpful lol.
Not sure how I could get my world to constantly run in the background in single-player, if so it might make a difference in whether a re-load is needed for them to appear. But so far, in my personal experience on multiplayer I've had to re-load my multiplayer world and they've appeared immediately.
Edit: Restarted again after about an hour and clearing dye plants again and had 2 more strange plants spawn.
Edit 2: Another re-load after about 30 minutes and more clearing and I found 2 more. I've made sure to find all sky islands and clear them of dirt and dye also.
Edit 3: I turned in all my plants. I had about 15. More then half the dyes I got was reflective gold and reflective silver...The rest were Purple Ooze dye, reflective copper, and acid dye.