First, butterfly bush, Buddleia davidii, is a listed invasive species in CA although a non-regulated one. So you might want to consider removal and replacement with one of the newer sterile cultivars on the market. If you opt not do this strongly encouraged suggestion, then you can just cut them back to the desired height. The shrub responds well to this kind of treatment and it occurs naturally with winter dieback in colder climates. You can do this at any time from now until around early-mid April.

If you don't choose removal, at the very least be sure to deadhead all the spent flowers ASAP to prevent the seed being dispersed and contributing to the invasive problem.

Here in Texas they aren't considered evasive.

The OP is in CA, though.

I did some research. - Buddleia davidii isn't listed in the California Invasive Plant Inventory. - http://cal-ipc.org/paf/ - I also can't find any concrete evidence that it is considered invasive in Northern CA. I'm surprised to hear it was but I see Oregon had a problem at one point. I'm not sure if it's much trouble here given California is so harsh on that type of thing and I still see it for sale in most greenhouses. .. I may consider removing them if I can get a solid answer.. Do you have any reference links I can glance at?

I don't think they're considered evasive anywhere. Rather often the opposite is the case, in fact. I've even seen them growing out of the front of a building.

It's very possible that the plants here are sterile. There don't seem to be any others in the area. It doesn't appear these ones are trying to spread either. It would be a shame to rip them out without properly identifying them.

It is listed on theUCDavis invasive species list but as a limited threat. And it is a listed invasive in both WA and OR. In fact, Oregon banned sales of this plant and it was due to this concern that breeders became intent on developing sterile or seedless cultivars.

Embo, I do believe that comment sailed right over the top ;-))

Amanda, one of the biggest issues of invasive plants is the ease with which they spread. You may not see them pop up in your garden or even necessarily very close by but the seeds do spread an amazing distance, carried by the wind, water or animal life. The fact that this plant has naturalized throughout parts of California is an obvious indication of this.

When doing research on invasive plant species, I read that each flower head of a butterfly bush can produce up to 40,000 seeds and with a very high 80% rate of germination potential. That means a single blossom on a singe plant could be responsible for 32,000 more plants!!! And BB will grow anywhere other than in soggy soil - railroad tracks, along the side of the road, cracks in abandoned parking lots, the foundations of old buildings, cracks in rock walls. Even apparently out of the front of neglected buildings :-)

If your plant is older than around five years or so, it is not sterile. These are relatively new to the market within that time period.

I appreciate the information. These were planted in the last six years and it sounds like they may fall into the "Or So" category. I'd hate to rip them out until their identified. I'd still like to see about getting them healthy and thriving in the mean time..

Amanda, I know three things about Buddleias. First, their roots get eaten by nematodes and they will kill the plant in a few years. This is a major problem here in Florida. I have repeatedly tried to grow Buddleias; they grow well the first year or two and then die out rather quickly. I have no idea if they are a problem in California. I suspect not.

Second, they have to be deadheaded if you want them to bloom well. So, regardless if this plant is sterile or not make sure you cut off the dead flowers right after blooming.

Third, they hate wet feet. If their roots are in water for even a day or so they will tend to die. Again, I don't imagine this is a problem where you live since the plants are in the ground.

I believe they respond well to pruning, so you can probably cut them back quite a bit, give them a bit of fertilizer (organic if possible) and make sure they get enough water and light and they should do well.

Don't know what kind you have, but they produce large blooms that are very fragrant and attractive to many different pollinators, including, of course, butterflies and also hummingbirds.

I'm not sure I would agree with Tom on all points :-) Nematodes are certainly not a problem here in the PNW and they can tolerate wet soil just fine but soggy, poorly draining soil is another matter! And while I will endorse the deadheading as soon as possible, it doesn't seem to affect blooming even slightly here - they will continue to push flowers all summer in my area, often blooming well into fall.

And pruning IS recommended. Old wood tends to be less productive than newer growth and it is suggested they be cut back to a low level at the end of each winter or in early spring - to 1-2'. In colder parts of their range, they are treated more as a perennial rather than a shrub, as they die back to the ground each year, accomplishing the same thing.