How can we remedy the situation of low light levels here in Holland? We want to grow marijuana in a greenhouse.
You can supplement the light with artificial lighting or innovative designs. During days in which light is low, you can use metal halide or high pressure sodium lamps at the brightest part of the day to supplement sunlight. No more than 5 hours of supplemental light per day will be needed in most cases. You can use light sensors to regulate the lighting. The growth and yield of your marijuana plants will increase substantially when using an automated system.
If you want to increase the natural light that makes it into the greenhouse, cover the north wall with a reflective material. A reflector above the north wall can be used to deflect light right back into the greenhouse.
Greenhouses with low light frequently have soil that is too cool and slows marijuana plant growth. Using soil heaters to keep the soil temperature at around the low 70’s F or a container/hydroponic system where the marijuana plants are kept above ground, the growth rate will increase.
A CO2 system designed to deliver the gas directly to the plant canopy would increase growth rate, as well. You could have it automated to turn on when the light reaches a high intensity.