This is a recurring question on the various forums .... here's an old response I posted a while ago...
This is a question that comes up frequently and typically the poster would like a yes / no response (wouldn't we all?).
The answer I see frequently from '
experts' is 50ppm. Above 50 is questionable & below is 'safe'. I've also seen posts from people saying they grow plants well with 90ppm (D'Amato notes that 100 ppm is generally considered safe - but lower is better (Savage Garden pg 7)). From what I've seen, I don't think there is an easy answer (except for the cop-out: "
It depends...").
What does the answer depend on?
- your growing conditions.
--- are the plants growing on a sunny windowsill in a low-humidity house? Here you will be watering the plants frequently, the water will evaporate and the residual minerals will be left behind to concentrate in your media - quite quickly. This will produce an unhealthy media environment for the plants.
--- are the plants growing outside in coastal Oregon or England? Here, the frequent rains will both reduce the need for manual watering and also flush the pots with rainwater to keep any minerals from being concentrated.
--- how often do you replace your media (repot)? One nursery owner told me that his well water is full of minerals but the plants he doesn't sell get repotted with fresh media each winter and therefore continue to grow well for him.
- species of plants.
--- some species (like Neps) have somewhat of a reputation for being able to grow well with a wide variety of water purity. Some species of Drosera & Neps come from areas with limestone/dolomite and actually prefer hard water over soft. Other species are picky about their conditions and need everything just so ... I struggled to grow D. villosa (and did eventually give up). I changed many parameters but during this process, at least I knew the issue wasn't mineral-laden water....
- you
--- more specifically - your level of experience, your knowledge of the plants and ability to detect small changes early in the potential death spiral. Frequently new growers will post pictures of their plants infested with pests. More experienced growers will take a quick glance at the pics and see the pests while the newer grower never noticed and didn't even know what to look for. The changes that happen from mineral-laden water often produce a slow decline in plant health and can be especially difficult to notice. If you're aware of what to look for when tiny changes start to happen to your plants and you can easily rule out many other possibilities (like pests), you are much more likely to be successful with hard water than others who cannot detect small differences....
- the actual minerals that make up the ppm
--- I have heard that the well water in some areas of Texas & California have minerals that are not compatible with plant growth and may actually poison the plants. If this is true, I suspect that these conditions are rare and most would be aware of this condition before using the water for their plants.
My apologies for the long-winded response - hope this helps ...