Terraria – farm Chlorophyte above ground level

I don't think what you're trying to do is possible, but the easiest solution in this case is to just dig your tunnel under the desert a little bit deeper. Eventually the sand gives way to stone if you keep going down, and it shouldn't even take that long to get there. Make a stairway down until you have an ample buffer of stone above you and then dig your tunnel across.

I'm assuming you mean the vines, as you speak of dirt blocks, and the vines grow from those. They do look like liane.

The dirt block needs to have grass on it. And grass spreads from other grassy dirt blocks. Or, you can use grass seeds, or the Staff of Regrowth.

The only requirement I can find is just a grassy dirt block. Sunlight, underground/aboveground. It all doesn't matter.