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Take a cardboard box, line it with aluminum foil or something equally reflective, put a lamp or a few lamps over the box to cover the plants and you're set. The more plants you have, the bigger the box you need, obviously.

ideally, you need no light leaks as this can stress the plants into becoming male. we only harvest sensimilia, which means 'without seeds'. male plants produce pollen and pollenate females. females then produce seeds. you need a good air exchange in a grow box. temps of around 25-27 are good for MOST strains of canna. the plants need air moving over them as they have no muscles to push spent gasses away from their leaves. a fan in the box will provide this. a good idea is some sort of extraction fan. this will help you keep the heat down, and provide FRESH air into your box. i have an industrial, purpose built fan, but a small bathroom fan could be applied here if necessary. bear in mind that plants can stretch up to 4x their original height when you change the photoperiod to 12/12. plants only flower when they receive less than 14 hours of light. check the link in my signature