The magnesium thing is worth looking in to. I think there are some varieties that require that much sun to bloom, but to tend to turn purple/ red. I have several, IE gerberara, laelias and Onc. spledidium ( or what ever they call it now) that do require lots of light, but do turn purple. That includes some of the Laelia Xs such as the one I posted yesterday. Some don't seem to change color, even though they share the same space. So far, the only ones I've 'burned' are some Phals that I was experimenting with. I worry alot more about yellow leaves~!
