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DU420
Yep the mums could get going but you could try smaller post and weaker lights to hold them back a bit.
The potash will stay awhile provided you don't do any heavy flushing.
I use it as a general purpose soil mix for indoor and out.
I'm not a fan of expensive additives. I use this mix on my greenhouse tomatoes in pots. Works a treat with them. The potash improves the taste of the tomatoes.
Well the Mums are only in 500ml pots under 2 48w cfls so should be ok. Do you add any liquid ferts to that or just water?