Lighting the way in your greenhouse
“If you are a greenhouse grower, light can affect your crops in terms of flowering, fruiting, branching, and overall size. The challenge is that light is measured many different ways. The typical visible wavelength range of this light is between 390 to 700 nanometers. By contrast, photosynthetically active radiation, or PAR, is the light plants can use to photosynthesize. PAR is in the range of 400 to 700 nanometres. So, if your light levels are below 400 nanometres, your plants could literally be in the dark even though you still have enough light to see them.”
For this reason, Morton encourages growers to avoid traditional measures of light like lux and foot-candles. “Those are not useful measures when working with light in a greenhouse. They aren’t adequate to measure a plant’s needs. Measures like moles and daily light integral are much more effective.”
At its simplest form, a mole is just a way of converting and simplifying large numbers. “To illustrate, let’s say we make a massive bucket that is a metre by a metre (or 1 m2), to measure the light in our greenhouse. Assume that the photons of PAR (the light plants can see) are marbles. Every time your bucket fills, you have basically received one mole of light per metre square (mole·m-2).
“If you were to do this for an entire day, and then add up the total amount, you would have moles per metre square per day (mole·m-2·day-1). Moles per metre square per day is referred to as Daily Light Integral (DLI), a measure used to calculate light levels in greenhouses that is growing in popularity.”
Morton prefers DLI because it is a measure of light throughout the entire day as opposed to a snapshot in time. “Light levels fluctuate throughout the greenhouse from one moment to another. DLI is a cumulative value; as such, rather than just knowing how much light your crop got at one point during the day, you can assess how much light there has been throughout the whole day and whether additional lighting is required.”
But how much light is enough? According to Morton, the amount of light required varies from crop to crop and, to some extent, species to species. “The typical rule of thumb is an average of 10-12 mole·m-2·day-1. In Alberta, outdoor DLI can vary from under 5 to upwards of 45 depending on the time of year and location. As this is a measure of DLI outside, this will decrease substantially in a greenhouse given a number of variables such as spacing, glazing material, energy curtains, etc. In other words, in order to produce a quality crop during the winter months, supplemental lighting may be necessary to ensure your crop is filling its light needs.”
Whether supplemental lighting is used or not, Morton says it’s important to think like a plant.
“Buy yourself a light meter, track your DLI and respond to your plants needs accordingly. Staying on top of this measure will ensure a healthier, more robust crop. If you do start to look at supplemental lighting, as you will have a more accurate reading on your specific needs. Your crop and your business will reap the benefits.”
For more information:
Government of Alberta
Dustin Morton
780-679-1314
dustin.morton@gov.ab.ca
www1.agric.gov.ab.ca