I have one small kanna plant that I started from seed back in June (I think). I planted 10 seeds, and six germinated. They were extremely slow to germinate. I think it was close to 3 weeks before I started seeing anything pop up. I read somewhere they they like something with a little sand in it like a cactus mix... so I mixed a little sand in some regular potting soil. I don't have exact ratios that I used. Definitely not more than 50/50. Probably more like 25% sand and 75% potting soil.
I didn't really do anything special other than keep the soil moist by covering it with plastic wrap & patiently waiting. The reason I only have one now is that my cats knocked my newly sprouted seedlings off the window ledge that I had them on when they were still freshly sprouted. Some got lost in the mess, others broke and died. One hardy little guy made it though and is now about 4" tall. I really don't do anything special for it. I just make sure the soil doesn't dry out. Currently I water it about once every 5 days or whenever the top layer of the soil starts to look or feel dry. It's now in a window that gets very little direct sunlight.. probably not more than 2 or 3 hours of direct sunlight in the early morning.
It doesn't seem to be very picky about temperature or humidity as far as I can tell.
However, it's very slow growing! I think that's normal but maybe partly because it needs something that I'm not giving it. I'll probably move it to warmer room into a window that gets more light soon since the room it's in get's pretty cold at night and less light now that we're getting into winter.
I initially started growing this because I was interested in using it to ward off the winter blues. I used to take St. John's Wort through the winter months for that and I've read kanna can be used similarly. However, I've also read that it's an SSRI so you don't want to mix it with MAOI.. Since I've been enjoying changa lately and intend to work with Ayahuasca, I don't know if I'll actually ever harvest from my kanna plant, but who knows, maybe. It's still a happy looking plant and I'm very proud of it=)