Invasive Campanulas

My experience with these is that C. saxifraga is completely well-behaved with no spreading from the roots, and that C collina does spread but reasonably slowly (here, at least; I wouldn't apply the term "invasive"). 
I only had C. fenestrellata for one season, which suggests that it is on the more sensitive end of the campanula scale, most of which are happy in ordinary garden conditions here.  (If I manage to get seeds for it, I will certainly put it in the rock garden next time.)  I have no experience with C. chamissonis.

I had C chamissonis for some years and it was certainly not invasive here. Unfortunately it died to soon. However, my climate is very different from yours!

Years ago I grew C. saxifraga, it was very choice and short-lived. :'(

I've not grown C. collina, but it sure looks nice from Lori's photo in the NARGS Photo Gallery at:
http://www.nargs.org/nargswiki/tiki-browse_image.php?imageId=1029

And, I've not grown C. fenestrellata, but it looks amazing and seems to be a common enough rock gardening offering.  In the link below, scroll down to photo #6; but watch out, your eyes might get stuck on Campanula species #4 & 5. :D
http://www.srgc.org.uk/discus/messages/141/34439.html

I had posted a photo on this forum of C. chamissonis growing in Peter George's garden, and it looks like a very choice slow growing plant.
http://nargs.org/smf/index.php?topic=373.msg3593#msg3593
http://nargs.org/smf/index.php?action=dlattach;topic=373.0;attach=6779;i...

When I saw this topic title (a very good one by the way), I thought you might be looking to find out which Campanula species are invasive.  Well, I have two that are incredibly invasive, beautiful yet monstrously rampant thugs in my yard; C. punctata and takesimana. These were discussed here:  http://nargs.org/smf/index.php?topic=193.msg1626#msg1626

Hi Barbara.  Campanulas as a rule don't like it here because it's so dry, with the exception of Campanula portenschlagiana.  Campanula chamissonis seems to be permanent here but I would never call it invasive.  It increases very slowly, perhaps due to the lack of water, and it's quite lovely.  The others I've never tried.  There may be a couple of varieties of C. chamissonis, but they seem to behave identically in the garden.

Thank you for the input, everyone! I've never had the nerve to try C. punctata or C. takesimana even though I've seen them in catalogs of reputable nurseries, and everyone knows about C. rapunculoides!

The only campanula that I've tried that might be considered invasive is C. poscharskyana, but it's beautiful, continued flowering through hot and dry periods when I forgot to water, and is easy to control. I'd still recommend it to someone who wanted an easy plant That would spread through cracks in a wall. It no longer fits my style, at least for now. I still have a garden that I want to build someday where I can imagine its robustness working perfectly in the design.

What's the average annual precipitation there, Anne?

I thought it was curious to see in Graham Nicholls' Dwarf Campanulas and Associated Genera that he considers Campanula punctata and C. takesimana to be synonyms.  Actually, it makes things a lot easier for me, because I can never remember what was supposed to differentiate them!

I see my C. saxifraga have been hanging in there since 2008, so I hope they continue on.  So far, they don't appear to be fussy, and seem to do fine in regular soil.  These couple of plants in regular soil (below) are larger, though (not surprisingly), than the ones in the rock garden.

Barbara,
For recommendations, the C. betulifolia group is nice, large white flowers on non-spreading rosettes. And C. waldsteiniana bloomed for me the first year from seed, though it remains to be seen what it will do the second year. Small lilac flowers on wiry stems. Fenestrellata is a tidier version of garganica with more upright stems and did well for me in a wall in zone 5-6. Rotundifolia should do well almost everywhere, though I'm not sure about your aridity. They don't put on much of a show, though, unless massed.

Jeremy wrote:

Barbara,
Rotundifolia should do well almost everywhere, though I'm not sure about your aridity. They don't put on much of a show, though, unless massed.

C. rotundifolia is native to the local mountains (circumpolar, I think), and it grows very well with a little supplemental water, blooming off and on all season. I've used it in perennial gardens where people want "pretty" and "cottagey" but also want to use native and xeric plants. Sounds a little over-constrained, but it works!

I'm currently growing all 4 of the species that you mention, and all are relatively easy. In general, those Campanulas that are rhizomatous have the potential to be invasive in the rock garden, and C. collina is one of those that can be a problem. I grow it in the open garden with other larger perennials, but not in the rock garden. C. chamissonis is rhizomatous, but spreads very slowly and it is wonderful in a sunny crevice in the rock garden. C. fenestrellata is commonly known as C. garganica subsp. fenestrellata, and it is a very well behaved plant. It spreads to about 16 inches or so, but it is a clumper, and can easily be divided when it gets a bit bigger than you'd like in one spot. C. saxifraga is a tap rooted species from the Caucausus, and if it's given a tight, sunny crevice it will grow beautifully for years. Here in New England I have to keep slug bait around it but I don't imagine that slugs are a problem for you.

I love Campanulas, and grow a lot of them. With a few exceptions like C. zoysii and C. dzaaku, they are generally not too hard to keep, as long as you pay attention to their soil and siting preferences. Some really do require tight crevices, and some are quite happy in the open, but most will do fine in a partly sunny setting with some moisture and good drainage. Given your humidity situation, you should be able to grow most of them quite easily. If you have Graham Nicholls' book, follow his general instructions, always factoring in your low relative humidity. I've found that the easiest to grow are from Turkey and the Caucausus. Particularly nice is C. choruhensis, either the white or the rose colored form. I have both and they are hardy, stay small and bloom like crazy. I've got them tucked away in a rocky area that is shaded in the afternoon, but gets plenty of sun from daybreak until around 2 PM. It's easy from seed too!

I would avoid C. cochlearifolia as in my area it is VERY invasive.  C. rotundifolia spreads a bit much for my liking as well...especially by seed.  C. chamissonis spreadts pretty good for me but it is such a beauty, I don't mind and it gives me a chance to spread it among friends.  I have a hard time wityh the Turkish types as they need more heat and drier winters than I can provide.  C. collina is well behaved for me...so far.  C. saxifraga is slug fodder here...in fact many campanula are...however, we have a potted C. saxifraga in our alpine house at the BG and it has been in the same pot for 20 years!  C. poscharskyana grows very well here but I would noty consider it invasive...just a bully as it is robust.  Still, they provide a spectacular display. C. zoysii is difficult here...we kept it for about 5 years in ther alpine house then it upped and died.

Several of the garden border Campanula are pests here...punctata, takesimana (a synonym as Lori pointed out), rapunculoides and persicifolia in particular.

Campanula chamissonis (pilosa) in my garden.

I've since changed my mind about C. collina!  I bought 3 plants in 2005, so it did take some years for their spreading nature to come to the fore, but I did pull them out this summer... a shame as they were quite attractive. 

'Tis a beautiful plant, but not for the rock garden. Try it in a border and you'll love it.

That's where I just got rid of it from...  ;D

If anyone would like to learn more about dwarf Campanulas then Graham Nicholls' book "Dwarf campanulas and Associated Genera is a must. Published by Timber Press 2006.