What zone are you in? Seems this shouldn't be happening in Z5 (which of course may not be Z5 any longer)

Why do you think it could not happen in a zone 5? There are midwestern gardeners in zone 4 and 5 that report heavy self seeding of rose of Sharon.

I've grown ROS for decades and have never seen a self-sown weed.. This neighbor and her ROS has been here 10 years and this is the first time she's run into it.

I just did a bit of googling and I see that there are many cultivars that do not self seed. That would explain my personal experience. The names were familiar. For my neighbor, I guess I would have to conclude warm winter or warm at the wrong time or something along those lines. And wrong cultivar.

Thanks for your inputs.

I think for self seeding the summer temperature is relevant. They need heat to ripen. I’ve only had a single seedling in many years. But as our climate changes it’s getting more likely. Did you have a hot summer last year?

I've had lots of ROS and they all self-seeded! I consider them a nuisance plant...and unfortunately, our new (old) house came with a few! Not only do they self-seed, they die off for no reason, then pop up nearby in another color, grow like weeds, and some have a habit of spreading branches rather than upright. She's my least favorite flowering shrub, even though I love her flowers!

@floral_uk that's interesting. They bloom late here -- not until August generally. And her's blooms later than other ROS in the neighborhood, so I don't think it's last summer's weather. But, this spring (probably during germination time), we did have several unusually hot days unusually early. And that happened a couple/few times. Maybe that woke them up before they had a chance to rot in the ground!

Theyre late here too. I think the heat is important for seed ripening rather than just germination. Did you have a warm fall?

Cannot remember what I had for lunch yesterday!!! 😀

I'm glad WendyB looked up information on Hibiscus syriacus selections. The USNA went about hybridizing polyploid plants for just this reason: to have hardy flowering shrubs with superior ornamental characteristics that would not be additionally pestiferous with excess seeding. That led to the Goddesses: 'Aphrodite', 'Diana', 'Minerva' and non-goddess 'Helene'.

These plants were the product of the diligent efforts of the late Dr. Don Egolf in the '60s, '70s, and '80s (when he was taking a break from producing outstanding Viburnum selections). Except for 'Aphrodite', these clones produce little to no seed; this will be obvious by the absence of the dehiscent capsules that populate the fecund members of the Rose-of-Sharon tribe. Here are some USNA fact sheets on these plants.

https://www.usna.usda.gov/assets/images/as_pdf_image/Hibiscus_syriacus_Diana.pdf

https://www.usna.usda.gov/assets/images/as_standard_image/Hibiscus_syriacus_Aphrodite,_Helene,_and_Minerva.pdf

I grew three plants of 'Aphrodite' that I received from Don Shadow. They eventually succumbed to the vagaries of a few really cold winters, regular deep droughts, and more shade from Celtis occidentalis than an ROS should have to tolerate.