I keep mothers in hydro, large mothers, right now. Here they are under the plasma light. I use soilless for a substrate for my hydro mothers. You can use pretty your same nutrients and simply drain to waste. I lower the light on my mothers (I want them to stretch internodal length), and I usually feed them regular veg food and alternate with tap water (plain tap water no pHing etc...), to keep them growing slowly. As they get to tall if I don't need more clones I simply give them a haircut shortening them and throw away the extras.
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I don't usually keep mothers but right now with moving through strains so quickly I do not want to lose my Kosher Kush girl who is just phenomenal. So I'm careful. Anyway I usually take clones off my vegging plants 2 weeks after I've flipped them to flower. That is the time I trim off 18 to 24 inches from the bottom where light doesn't penetrate. So again lower nitrogen (rooting is sped up).
hth,
Annie