No, if someone uses a water softener, their KH will be extremely high(the salt does not exchange HCO3, only Ca/Mg) and the GH will be very low.
In general though, the GH is is higher than KH in most cases.

GH high= fine okay for every plant I've grown(300 or so species at least).
KH is fine up to about 5 or so, for all plants, but over this general range, some do poorer, you can grow them, but they do better and are easier to grow at low KH's.

So the Ca is not so much an issue as is the KH.
I've found this plant growing in near freezing temps, in road side ditches all over Florida, in rock hard limestone rivers/spring(very common), in blackwater ditch pools, pH about 4.7 to 5.0 and no GH/KH.

So I'd say it is a very tough, wide ranging habitat easy to grow weed.
At least that's what the natural habitat would suggest


Regards,
Tom Barr