Seems like the only hybrids I get come from the blue flower lemongrass, or whatever it's called. I have about 5 of them in my garden and I switch around the plants that surround them to see what will happen. It also seems like I only get hybrids in a certain area of my garden, in the upper right and left corners.

Other things that seem to be happening:
Placing the blue flower lemongrass next to a level 3 plant gives me a level 1 plant.

Placing it next to a level 2 plant gives me a level 3 plant.

Placing it next to a level 1 plant gives me a level 2 plant.

This isn't solid, irrefutable research or anything. Just casual observations.