I have had some issues getting the milkweed growing in natural light to grow big, so today I attached a light fixture to the post across from them to give them a bit more light, but not enough to mess with the variables of the experiments. I tracked the amount of light, as you can see from the photos. The plants under the grow light are receiving about 2,171lux of light and, originally, the plants growing in natural light were only receiving 45lux. With the addition of this fixture they are now growing under a light source of about 120lux. Even with this setback, the milkweed is still growing strong. You can see that the ones under grow lights have thicker stalks, which means the sap is more abundant and has a lower viscosity than its counterpart growing in more natural light.
