I think Diana Walstad lists plants that can use bicarbonate as a carbon source? In that case they should all do well in harder water, if they can't they are more likely to struggle.
Diana Walstad (1) is no scientist and does not follow rules of scientific ethics, (2) her book is rather outdated and she never cared to study later literature because, in fact, she moved to a quite different field of interest.
So, as for her list: Even at the time she has been writing her book there was growing consensus that submerged plants are not divided into two groups - can and can not - but rather there's a continuous scale of varying affinity of species to bicarbonates.
V. spiralis is a plant on which Diana Walstad carried out an experiment. This plant was shown to distinctly dislike acidic soft water!
Yep. This is a perfect example of her low honesty. She performed the experiment at two values of pH: 8 and 4 (four!). Only very few plants can live in such strongly acidic water. Yet that does not mean that
V. spiralis cannot thrive at pH 6. She didn't try, but felt free to conclude that not only
V. spiralis, but
majority of all water plants prefer alkaline water. She arranged this "scientific" experiment in such a way to 'prove' her thesis.
In another place she concludes that water hardness is THE decisive factor. Mind you, water hardness is mostly an industrially important number. Calcium and magnesium are important for all waterworks because their compounds tend to precipitate in plumbings. But from plants' point of view, there's no reason to see Ca & Mg as more important than, say, potassium or sulfur.
Of variables which generally and profoundly affect living conditions for plants, two (interconnected) stand clearly atop: pH and bicarbonate content (rather than alkalinity).
the plants are showing clear signs of iron induced chlorosis, and I would take that as a pretty good indication that Rotala rotundifolia is much happier in softer water.
Again, it's not soft water (low GH) but pH and bicarbonates which strongly affect iron availability.
some plants may do better in hard water because they can synthesise carbon from bicarbonates
All higher plants prefer CO2 over HCO3-, without exemptions. But those with higher affinity for bicarbonates are in a
competitive advantage at higher pH.
I usually ignore other folks/experts opinions and try it anyway. Half the time they're just plain wrong or have confused cause and effect. I've lost count of the times plants have thrived or died when and where they "shouldn't" have.
Unfortunately, you're right. Most guides are fairly useless. I like
Flowgrow database. But it also features many disputable suggestions, and even outright nonsenses. I mean, they often combine recommendations for pH and KH in such a way that if you tried to follow it to the letter, you'd have to inject
hundreds of mg CO2 per litre. By the way, just a few days ago I asked them about it and they were pretty rude to me.
Where can I get a list of aquarium plants that includes recommended water hardness?
As far as I know, nowhere.
I think it's due to prevailing habit of CO2 injection and using strong artificial chelates. When you read the guides (par example Flowgrow), it's basically the same for any species. "A LOT" approach - a lot of light, a lot of CO2, a lot of macronutrients, a lot of micronutrients, and if the plant happens to possess rich roots, they usually recomment nutrient-rich substrate (which is a clearly mistaken reasoning).
With CO2 injection and strong artificial chelates you can overcome many difficulties stemming from natural preferences of a given species. So, recommendations made by people who run hi-tech tanks are mostly worthless - they don't refer to standard (natural) conditions.
That's why I've been performing my experiments, strictly without CO2 and artificial chelates, and in this way I've been collecting data on what our plants truly like & hate. An endless source of fun and enlightenment. I've published some articles on my findings, unfortunately, in my native language only. It's too difficult for me to translate them to English. Besides, I think the original publisher has some rights to them, perhaps I can't publish them elsewhere without consent. Never asked, anyway...
