I have neighbors who are asking questions about ornamental grasses and their risk of reseeding. They are specifically interested in Pennisetum ‘Hameln’ and Cortaderia ‘Richardii.’ I have known Stipa (Nassella) to reseed but I have never noticed any reseeding from either of these two groups in the Seattle area. I know they can reseed freely in other parts of the country, but I suspect the seeds rot in our winters. How can I find out?
I consulted online lists of invasive species and found a few references to Pennisetum species (but not the cultivar you mention) which are problematic in Oregon and California.
According to the University of Florida Extension, the variety Pennisetum alopecuroides ‘Hameln,’ is not known to be invasive.
Although not classified as invasive, the species P. alopecuroides is still a spreader by self-sowing. Rick Darke, author of The Encyclopedia of Grasses for Livable Landscapes (Timber Press, 2007) and a Pennsylvanian, says it “self-sows, usually at a manageable level; however, some of the fall-blooming varieties such as ‘Moudry’ and ‘National Arboretum’ are particularly fertile and can be very weedy if conditions are suitably moist. They often become established in irrigated cool-season turf.”
The Royal Horticultural Society plant profile for ‘Hameln’ says it is early-flowering (which might mean that seeds have time to establish themselves, though a droughty summer such as we sometimes have in the Seattle area might prevent this).
One sterile cultivar is Pennisetum advena ‘Rubrum’.
As with Pennisetum, there are species of Cortaderia (C. selloana, C. jubata) which are listed as invasive in various parts of the world. Here is an excerpt from Los Angeles & San Gabriel Watershed Invasive Plant-Monitoring and Outreach Program’s WeedWatch information on Cortaderia:
“It is unknown whether the cultivated and ‘sterile’ varieties of Cortaderia are able to cross with the wild species of Cortaderia and produce viable offspring. Until this scientific research is conducted, and considering the rampant ecological damage already caused by both C. selloana and C. jubata, it is not recommended to plant any members of the species, including cultivars, varieties and supposed ‘sterile’ varieties.”
Author Rick Darke says that all species of Cortaderia may be grown from seed (therefore, species are not sterile).
A Pacific Northwest palms and subtropical plant online discussion group, Cloud Forest, (no longer available online) includes some dialog on Cortaderia richardii:
“Another terrific grass is Cortaderia richardii, which is a NZ form of the humongous pampas. It is short (to 5 ft.) with elegant flowering pattern (circular and reaching instead of vertical). It’s evergreen and a great specimen. Needs a 6 x 6 space minimum. ”
“Cortaderias scare me a little, ever since my neighbor’s specimen seeded itself all over my poor rock garden (heck it is all over the neighborhood). Obviously sellowiana; I don’t think C. richardii is as aggressive. The other one is a devil to try to remove once it gets wedged in! The leaves cut like razor blades and the crown is extremely persistent. I actually think they look kinda grand, and if I lived on the coast I’d be willing to use them as windbreaks (which is how they got all over the place). I don’t think a hurricane would injure them.”
“Cortaderia richardii is the magnificent cousin of the monstrous pampas grass. Petite and elegant in comparison. Mine have never self-seeded. My friend’s seeded a bit. It may be hotter by his house.”