Guest Author - Susan Taylor
Among long-time orchid growers, the most common advice given to new orchid growers who are unable to bloom their plants is "Give your orchids more light." This is the one best piece of advice for those who have kept their plants alive, but can't get them to rebloom.
Many times the advice will be that you can tell if a plant is getting enough light by the color of its leaves. For new growers it is hard to understand when experienced growers say "yellow/green leaf color" and not "dark green." Unless you know exactly what they are referring to, this is not really very helpful. More helpful is the advice that a reddish leaf color in Cattleyas indicates that they are at the top end of their light needs. This reddish color is really obvious and if you have it on your Catts, you're giving them plenty of light. Unless they're in full sun all day long, it's hard to give most Catts or Dendrobiums or Vandas too much light. You will see large yellow areas on the top of the leaves. Move those into a more shaded area.
Let's go back to the yellow/green color. If the orchid plant looks anemic in comparison to a fern, or palm, then you're probably giving it enough light. If it's a nice dark green like the same fern or palm, then it's not getting enough light to flower. It will continue to grow for you, but it is not likely to flower.
This is not true for Phals, which require much lower light than the Cattleya Alliance plants. With their larger leaf surface it is easier for them to absorb light thus they do not need as much light hitting their leaves. Direct sun will burn the leaves. They need to exhibit a much darker green color than a Cattleya, Dendrobium or Vanda.
In the house you need to provide different lighting during the summer and winter months, or augment your natural light with fluorescent lights or grow lights. Remember that naturally orchids come from the tropics where there is little variation in hours of light throughout the year. In the northern or southern hemisphere we have wide variations in number of daylight hours -- especially during the winter months. At least ten hours of light is required for most orchids.
A location in your home with southeast or southwest exposure will provide much better light than one only receiving from one direction and will work much better in the long run. Group your plants with those needing the most sun where it is strongest and closest to the light source. Put Phals further from the light source and slightly shaded by the high light varieties. Then, if necessary or possible, add a fluorescent light. You should be able to re-flower most of your orchids.
And, if you have a location outside where you can safely put your orchids for the summer with partial shading, this is ideal. They will thrive outside and the natural light will go a long way to make them flower.