One of the limiting factors for winter greenhouse production is low light levels, this is especially important for growers at Northern latitudes who can experience both limited light intensity and photoperiod. Those who are growing crops at northern latitudes need to take special consideration for how they will plan year-round growing. Since we work with many growers who face the challenges of growing in a northerly climate, we have compiled some considerations when deciding on their lighting options.
Let’s Talk DLI (Daily light integral)
What you are growing will help to decide the amount of recommended light intensity and light duration you will need for your crop to get a healthy and productive harvest. DLI or Daily light integral is the number of photosynthetically active photons (in the PAR range of 400-700 nanometers) accumulated in a square meter over the course of a day – in a broader sense it is the amount of photosynthetic light delivered to plants each day. It is usually expressed as moles of light per square meter per day and is a function of light intensity and duration. DLI affects plant quality, photosynthesis rate and plant growth. DLI will change throughout the year and will be higher in the summer and lower in the winter, variations will change depending on your geographic location. The maximum DLI we can receive in the outdoors is about 60 mol/m2/d1 on a cloudless summer day and on a dark winter day, in a northern latitude like Alberta, the outdoor DLI can be less than 5 mol/m2/d1. It’s important to recognize that even though greenhouse structures appear to be “clear” or “transparent” light transmission is usually only between 40-65 percent therefore daily light integrals inside the greenhouse are usually much lower than what is recorded outside. This is due to building materials, glazing, shade structures and other materials and environmental factors.
In general, plants are categorized into several groups: low light plants such as ferns and orchids, medium-light plants such as shade-tolerant bedding plants and leafy greens, and high light plants like full sun bedding plants, cut flowers and fruiting vegetables. Plants such as tomato, pepper, cannabis and roses are recommended to have very high daily light integrals for quality production and yield. For example, if you have less than 5mol/m2/d1 you would need to supplement up to 30mol/m2/d1 for cannabis as it can require up to 35mol/m2/d1.
While suggested DLI varies depending on crop – this guide from Purdue University outlines minimum and recommended DLI for many popular greenhouse crops. Aside from low light plants such as ferns or orchids, many medium and high-light crops will see quality and yield increase with increasing DLI. Plants receiving low daily light integrals tend to be of lower quality and will stretch in search for light.
HPS vs LED for Northern Growers
Lighting is one of the most important factors for growers in a Northern climate where winter light levels are low. Choosing the right lighting system that works with your growing space is imperative to ensuring a successful crop.
LEDs have been shown to save money in the long run by cutting down on electrical costs as well as having a longer life when it comes to burning hours and replacement. However, in many cases we find growers who are located at Northern latitudes continue to use HPS (high pressure sodium) lights in their crops. A reason for this is that LEDs do not emit as much heat as HPS lights, and many plants such as peppers, cannabis and tomatoes tend to benefit from the radiant heat emitted by the HPS lamps in the winter. The heat emitted from the lamps is not an efficient way to heat a greenhouse, but the radiant heat provides a great benefit to the plants. Please take note that you cannot rely on HPS lamps to heat a greenhouse, but they can help cut down on costs.
On the other hand, LEDs while energy efficient, do not emit radiant heat, therefore more heating is required during the winter months to compensate and the heating costs can outweigh the energy savings of the lumi