I won’t even wade into the definition of what a native plant is. That alone can be fairly controversial. And people who love native plants have different ideas about them.
What do I mean? I might say that I am growing native plants and I might be referring to my echinacea. A very strict constructionist would say that coneflowers are not native to Connecticut and therefore I can’t consider them native.
To me, that’s silly–but I do know people who will only plant regionally appropriate native plants. Bless them.
Other folks might be growing the double form of coneflowers–these photos are of a neighbor’s plants–and still consider them native.
Technically these double forms are considered “nativars.” That’s a cute form of native and cultivar, combined.
But here are things to consider when planting these types of plants.
First, what is your goal? Are you just planting ornamentally? If so, plant what you like and what will be hardy for you.
If you are planting for wildlife, consider how closely the nativar mimics the native plant. In the case of the coneflowers, the “cone” is replaced by petals. So there is no place for insects or butterflies to nectar. That’s not a good “mimic.”
On Friday I will show a different nativar that maintains the attributes of its parent.
Gardendaze 
You would not believe how many to not grasp this! Cultivars of native plants are quite trendy, and I suppose that the majority of them are relatively harmless. California is a big place, so there are those who consider Joshua tree and desert fan palm to be native just because they are in the same state. There are more regions just in our small county than most states have altogether. There are more native species in California than there are in many states combined. The entire Midwest likely has less natives than California does. The butterfly gardening fad complicates things even more, because people want to attract butterflies and other insects to their garden, with no regard to the native flora that is being deprived of their pollinating services.
I DO believe how many people haven’t a clue because if one more person tells me that they planted a butterfly bush for their butterflies, I will just cry.
Yes, most cultivars are harmless, particularly if all you are after is ornamental gardening. In all our talk of gardening for this and that, we have lost sight of the fact that gardening can be ornamental.
I expect what purists object to–particularly with something like the coneflowers that I had photos of–is that we have taken natives and made them less useful to wildlife. But that’s really one of the few examples I know of. I have another nativar today (Friday) that actually is probably more useful for home gardening and still great for wildlife.
Karla