Can I cut off pieces and just stick them in water?
Would I have better luck in the ground with something plastic on top of them?
I want to root some now while the weather is still nice and warm.
You'd probably get a lot better results from traditional cutting propagation methods than from water. The subject has been covered numerous times in the Plant Propagation Forum, so instead of rehashing, I'll just provide the link from the search results below.
BTW, non-sterile types of Buddleja/Buddleia are an environmentally-damaging pest-plant in many areas, so check with your appropriate local agency to determine if this plant is an invasive in your area. If it is, there are sterile cultivars that you could safely use without harming your area's ecosystem. The sterile cultivars have been mentioned in this forum at least a few times and should be easily found with a search of this forum or from a quick Google search.
Here is a link that might be useful: Search of Plant Propagation Forum for Butterfly Bush
That link was awesome! It had great pics!
I'm propagating a sterile form.
Many of the sterile forms are patented plants.......and therefore restricted from propagation unless licensed. I'd look for the cultivar 'Asian Moon', a sterile form developed by the University of Arkansas and not patented.