I am considering planting Nepeta 'Walkers Low' behind KnockOut roses and Cityline hydrangea. Is this plant too invasive for this setting?
In my experience it is not invasive. The clump slowly gets larger but is easy to control.
This variety is not invasive in my experience growing it in zones 5-6. The clumps gradually enlarge but there are no invasive running roots or reseeding of note. Commonly grown Nepetas are not typically considered aggressive perennial border dwellers.
It does sound like your planned usage of "Walker's Low" might hide it behind taller shrubs (it would probably work in between them or in the foreground).
Depending on the variety you will get some volunteers near the mother plant but Im not sure that i would say that they are invasive. There are some sterile varities that will not seed. I would google them to find out which varieties are sterile and go with that unless you want volunteers.
I have planted Walkers Low nepeta with several roses, including Knockout. I did plant the nepeta in front of the rose as I anticipate the knockout roses getting larger than the nepeta. They did not spread for me but the oldest one I have - 4 yrs old - will make a mound approx 2ft across and prob 18 inches high. It blooms pretty much continually here in my 7b climate - central New Mexico. I love it and keep adding more to my garden, especially with roses.
It doesn't fit the accepted definition of invasive - a non-native (exotic) plant that poses an economic or ecological threat. But it is a member of the mint family (Lamiaceae) and does spread although very manageably so. I would not even consider it an overly aggressive plant in this regard. And 'Walker's Low' is not a reseeding variety anyway. It was also named the Perennial Plant Association plant of the year for 2007, which means it is a very garden-worthy plant and with no overwhelmingly bad habits :-)
The "low" part of the name is a bit misleading, as this is not a low growing plant - it can reach 24-36" (the cultivar name is for the garden in Ireland - Walker's Low - where the plant was first discovered).
One thought that comes to mind is that nepeta is considered a xeric or low water use plant, so I'm not sure how appropriate it may be in a grouping with roses and hydrangeas, both of which appreciate frequent watering.
No. It's not invasive at all. It's probably the best plant to put with roses. The color complements roses in many hues. The Nepeta is easily propagated by simply cutting a sprig and planting directly in the garden. I am planting tons of Nepeta. I put it and Russian Sage around Easy Does It rose and got so many compliments last year. I think you will love it!
If I remember correctly I had "walkers low" - and as Gardengal mentioned the name is misleading as they are rather tall - I had another variety as well though I don't recall the name offhand - they were nice, but not as "tidy" as I wanted them to be.
I eventually replaced with Geraniums, Bloody Cranesbill as an underplanting to my rose bush.