Why is ajuga invasive?

I'm not trying to defend the plant -- I'm just trying to understand. I've had a patch of the 'chocolate chip' variety in my yard for 3 years and has never outgrown the perennials and annuals that are next to it. Did I just get lucky?

Yep!

I have tried several times to grow ajuga and it has yet to live a season for us.

Ajuga, like most ground covers, can be aggressive in the home garden. Invasiveness really refers to its naturalizing tendencies beyond the home garden. Some states list ajuga as a problem invasive as well. I know my native plant society does. That being said, I've never noticed it in natural areas the way I have some other escapee ground covers.

And, of course, every gardener has something they *should* be able to grow, but just can't! Cleome is supposedly to be a ridiculously easy annual to grow -- if anything it's supposed to be hard to keep ahead of the self-sowing in the garden. I couldn't grow cleome if my life depended on it.

'Chocolate Chip' ('Valfredda' is it's current name) is a known slow growing cultivar and somewhat pickier than it's sibling cultivars.

Ajuga may at times move into the lawn, and as a groundcover it may do what it is supposed to do in your garden beds, but like MaryMD7, I have never seen it in any 'natural' areas.

Grow it happily!

Perhaps, it has not yet made it into any natural areas because gardeners such as myself, recognizing a huge mistake as it swallowed it's bed mates and stormed across the lawn, have employed draconian measures to eradicate it.

Thanks for everyone's input. I think I have a better understanding about the difference between aggressiveness and invasiveness, at least with respect to the ajuga and in my particular climate. My sympathies for anyone that is battling it in their climate.

How does the lighter "Silver Queen" compare to the darker variety like "Black Scallop"? Is one more invasive than the other? I'm thinking of planting this in Redwood City, CA. I wouldn't mind to see it spreading because it's very pretty.

I have grown both and have had a patch of the darker variety for years. It wanders into the lawn occasionally, but only a few easily pulled plants on runners. I have not found it growing anywhere other than where it was originally planted. The ferns, hellebores, tiger lilies and epimediums in the same bed have spread much more enthusiastically.