@dhudson it depends on if you grow hydro or soil.
It isn’t harmful for hydro, but it can cause salt and precipitate build up in your res if used too frequently and possible pH issues if you don’t keep a clean res.
Now as far as organic soil it’s a God send. You can use it as often as you like really, but I alternate between using l-ascorbic acid and molasses so I can get sugars to my microbes too. It doesn’t just work as pH down either, but it also neutralizes chlorine and chloramines. On top of that studies have proven that vitamin C is essential for photosynthesis and overall plant health. It increases vigor and plant resistance to ozone issues and smog. Plants produce their own vitamin C, but they can also absorb it via foliar feed and through the roots which will increase the efficiency of photosynthesis and other systemic functions because the mitochondria don’t have to expend the energy to create their own vitamin C. It also increases the plants ability to create other antioxidants as well as helps it increase the efficiency of producing flavenoids, phenols, and terpenes. Vitamin C is also an essential cofactor for metablozing enzymes and in processing organic and inorganic metals needed as micro nutrients (calcium, magnesium, copper, iron, etc…) There’s a lot more to it as well, but I haven’t read that study for a while so I don’t remember 100% of the specifics, but it’s good for plants.
B vitamins (specifically B-1, B-2, B-6, B-7, B-9) and vitamin E, will also help the plants. Bs and E will increase vigor, increase biomass, increasr drought resistance, and cold resistance as well a load of other things. This goes for most vascular plants actually.
Foliar feed is 200-400 mg a liter for vitamin C if you’re interested in trying it. If you add b vitamins you can use about 200-400 mg a liter for them too. Vitamin E is not water soluble, but you can put it in the soil and it will be broken down and taken up. Vitamin E can be used safely at around 1000 mg a gallon in watering each week as far as I have found in studies. (Yes, I said it isn’t water soluble, but the water will bring it deep into the soil to be broken down instead of it sitting on top)