I have 3 Peganum plants in pots that (by some miracle) I've managed to not kill off for the last 10 years, but it's too cold in Auckland (Southern Hemisphere) for them to set seed, so I've never managed to get any seed from them to increase the numbers. As a noxious weed, it's not possible to buy or import viable seed.

Has anyone managed to successfully carry out root division as a means of increasing plant numbers? I'm guessing that Winter is the preferred time, as there is no aerial growth and I figure that it will be best to have them all ready to go once they recommence growth in the Spring. I'd prefer not to leave it until they start regrowing in the Spring because they usually take me by surprise, and by the time I've noticed them growing they've put put 1cm + shoots, which are east to knock off..

If I don't get any responses then I'll just divide one of them in August and leave the others until Spring - and let you know the outcome.

I've heard their roots are quite sensitive to digging up and transplanting, so root division may be tricky. However, one source says to divide in late spring.

Are you sure Auckland is too cold for them to set seed, or could it be other environmental factors (soil etc.)? If they don't set seed in the warmest part of NZ, why are they classed as an invasive weed nation-wide?
They set seed in Southern Europe and grow in cooler parts of the US (including Washington state - although I'm not sure if they set seed there), so I would be surprised if that wasn't possible at your location at 36.8 degrees of latitude. But if that's the case, perhaps you can give them a warmer microclimate / sun trap with the use of equator-facing dark stones or bricks, or even erect a mini greenhouse? If you don't get them to set seed, it may defeat the point of growing them.

They do set seed in washington state, although I've only heard of them being grown on the dry side where its 42°C right now.
But looking at the climate stats for Auckland, it should be warm enough.
Peganums, younger ones at least, can successfully be transplanted if they are given sufficient water after transplant. I would only attempt to divide one in spring, preferably when the new growth is budding out.
Are the commercial esphand seed irradiated in your country? Even in the US where we irradiate nearly everything (including travelers) the commercial esphand is generally viable, I guess because its incense.

Everyone seems to think washington is all temperate rainforest. This is what much of the state looks like:

Thank you both for your responses.
Some very valid points are made.
Yes, it is very likely that factors aside from climate are at play - the most pertinent, I suspect, being the cultivator!
<<The paranoid conspiratorial rant starts here>>
It would not surprise me in the least if these plants have been branded as noxious for no reason other than that they spread like wildfire in the US - especially combined with the (more important) fact that they are psychoactive. This is enough to convince the "scientists" who advise government of the need to restrict them.
<<End of paranoid conspiratorial rant>>

Thanks for your input Jagube.

I have no doubt that, if these plants were to be released to the wild in sand dunes, that they could/would set seed and completely replace native dune-stablising grasses and become a serious problem. That is why I grow them in pots on my deck, rather than releasing them into dune environments on private land (to which I have very ready access).
Why I have been unable to produce seeds, I do not yet know.

Aside from that, the harvesting of dried seeds (without them dropping to become weeds) is a somewhat cumbersome process, which I learnt several years ago when I had an obsession with (growing) Morning Glory.

But I digress.

To answer some of both of your questions:

All seeds coming into NZ are now heat treated, at the importer's expense. This is tried and tested. I am kicking myself that I let the sparrows demolish 90% of the seeds that I successfully germinated before such measures were implemented. But C'est la vie.

So, why grow them?
I'm going to harvest the roots every couple of years and do a standard Manske extraction on them. I did this last year with a small amount (26g) of roots that I had collected over a few years, with a reasonable yield (~5.5% harmine HCl, yet to be analysed).
Due to an anticipated increase in demand, I need to increase production.

I think I'll split one of them soon, and the other once growth commences in Spring. As long as I have buds, they should grow OK. I did try to plant a section of root that didn't have any buds, and sadly it didn't produce any buds at all. Will keep you posted.

Update on this: I divided the plants, and all of them survived. Therefore, root division is a viable means of propagating Peganum harmala. The Harmine HCl came back as 50% pure, with the remaining 50% assumed to be water of crystallisation - no other compounds detected. So ~2.75% Harmine from roots as the HCl salt, no other alkaloids after 3x Manske runs.