I have used this method for many, many years. Works well.

how long should brugmansia seed be soaked?

How old seeds and what type would benefit from this? Does anyone know the science behind this? Does it need to be warm hydroperox? How long a soak?

Well, i don't know if it helps seedlings break through the seed coat, but I do know that soaking seeds in a 3% solution of hydrogen peroxide for 10-30 minutes, followed by at least 10 rinses in clean water will kill any pathogen growing on the outside of the seed. Germination appears increase after this sterilization because some seeds get attacked by pests as soon as germination begins and the embryo is destroyed before it even emerges from the soil. This is routinely done in plant research laboratories to prevent powdery mildew, mold etc. It also prevents damping off. You can also use a 3% solution of bleach, as long as the hypochlorite level is low: javex is an excellent brand. The cheaper bleaches are not as pure and won't have the same effect. As for the hydrogen peroxide, I'd recommend using a food grade solution.

As for a 'biological' explaination, hydrogen peroxide is a plant signalling molecule (like ethylene) used by the plant to mediate pathogen and environmental stress responses. It also *MAY* help in root development. I've heard of some people actually watering or spraying plants with low doses of hydrogen peroxide to help plants overcome pathogen attack, or make them hardier if a pathogen make its way into the garden. Personally, I don't like spraying my plants with too many chemicals, and if you do have a diseased plant, it is better to sacrifice it before it infects everything else. Plus, not much is known about hydrogen peroxide plant signalling yet, the hydrogen peroxide may stop pathogen attack, but what else is it telling your confused plant to do?

I took some honey locust Gleditsia (?) and scratched them with a file (My experience has been they may sometimes soak for a year without germination unless they go through some freeze/thaw cycles.) and I put some in a household peroxide soak and some in a bottled water soak. Both samples started to swell in about 12 hours. Those in the bottled water produced roots about 48-72 hours before those in the peroxide.

The unscrached controlls in water and in peroxide show no indication of doing anything as was expected.