How do I kill a butterfly bush so it stays dead?

Question

I have four Butterfly Bushes (Buddleja davidii) in my yard from a previous owner. I believe two are the Black Knight cultivar, but I'm not sure what the other two are.

I spend a lot of time every year keeping them (and their dozens of offspring) under control and I've finally had it with them — I want them gone.

This is the current state of the largest of the bushes: a 6"/15cm high stump, about 16"/40cm by 12"/30cm with four large trunks. The other bushes have smaller trunks, but they'll be in a similar state as soon as I can get the time to cut them back.

However, in the time I've been in this house, ice storms have knocked them back to the ground twice; they regrow just fine once the weather warms up, so I know they're tenacious.

A complication is that my sewer and gas lines both run along that side of the house, so I'm limited in the digging or picking I can do there.

What do I have to do to get rid of them for once and for all?

Answer 1

I would paint brush-b-gon (or a similar small tree weedkiller) on the stumps after cutting them back. Using an old paint brush means the treatment is highly localized and does not affect neighboring plants, etc. This is the approach used by many ecological groups when removing invasive bushes.

Incidentally, davidii amounts to an invasive in the UK, where it freely grows in abandoned industrial areas, along railway cuttings, etc. I guess the purple brings in some color and butterflies (and I remember having one as a kid that would be covered with small tortoise shells and peacock butterflies), but it is very much a weed when it escapes.

Answer 2

You don't mention where you are located, but here in Central Florida, I've been able to take out incredibly invasive, persistent, and prolific sprout-from-the-trunks plants by covering it and sealing it down in thick, black plastic.

I use 3 mil contractor clean-up bags cut open into one large sheet. On even mildly-warm days, I've measured it at over 140°F (60°C) under there. There's no light, so even if something manages to sprout in that heat, it isn't going to last very long. Bonus that, once the plant dies from either "heat exhaustion" or light/energy deprivation, it starts rotting very quickly. By fall, it is basically a pile of loamy mulch.

Answer 3

You could burn it out or use a pick axe to split the base or paint it with something like root stump killer (overhere i would suggest TBK stump killer in the UK) expensive stuff and must be used neat not dilute as the instructions say so(never works if you do!), one must damage the stump first and mash up the exposed bark on the outside to allow the chemicals to be absorbed- seal it with a shower cap and leave it to die- stump killers are usually based on white spirit or helicopter fuel type chemicals, nasty stuff! and of course keep pets away and children- paint on using an old paint brush and try not to get too much on the soil or you'll have to leave it for several months before planting- oh don't smoke around it too as its very flammable! and wash it off your hands within fives minutes or so as it can be absorbed through the skin- usual precautions apply with PA1 and PA6a pesticides and spraying certificate holders- nothing special just common sense type stuff.