How supplemental lighting can be used for outdoor cannabis cultivation

How supplemental lighting can be used for outdoor cannabis cultivation

The following article by Dr. Allison Justice appeared on MJBiz Daily  August 4, 2021. Learn about supplemental lighting and its application in outdoor cannabis cultivation. 

Traditionally when growing cannabis, the thought is to simply apply over 18 hours of light per day to keep plants vegetative and switch to 12 hours to induce flowering. 

This works, so why change it? 

As cultivation of outdoor cannabis spreads across the US, understanding how cannabis responds to light becomes very evident.

Some growers are finding out that supplemental lighting is necessary even with outdoor grows.


Let’s back up a minute and make sure we are on the same page. 

What is photoperiod?  It is a physiological change based off night length. 

So even though we do think and speak of photoperiod as daylight hours, that isn’t quite correct. 

For example, if you were to turn your grow lights on for only 5 minutes in the middle of the 12-hour dark cycle, your plants will stay vegetative. 

How cool is that?  Cannabis, poinsettias, garden mums are all “short-day” crops.  This means that under short days (aka. a long night), the plant will become reproductive.

Below you see a typical indoor schedule for growing cannabis (Figure 1). 

typical indoor cannabis crop schedule

Figure 1. Typical indoor cannabis crop schedule.

Vegetative propagation takes around two weeks, the vegetative cycle can be anything from 1-10 weeks (all depending how big you want your finished plant to be, typical is 3 weeks), and flowering can last 8-12 weeks, depending on cultivar. 

This works perfectly well and is easily achievable for multiple harvests per year.  Once you move outside, things get a little trickier. 

When growing outdoors, our natural day length never extends over 18 hours, and most areas only get under 12 hours per day in the winter months. 

For the states early to adopt cannabis cultivation (Washington, Colorado, Oregon), breeding began along with selections for those regions. 

2020 has been the first year (in modern history) that hemp has commercially been grown in regions below the 30th parallel which include places such as Southern Texas, Louisiana and Florida. 

These cultivars do not always translate well when grown more southern as the photoperiod is very different.

To visualize the daylength difference, refer to Figure 2.  Let’s consider two very different regions of the US- Lansing, MI and Baton Rouge, LA.

Figure 2. Daylength comparison to hemp variety which initiates flower at 14 hours and 10 minutes.

When growing indoors or in a greenhouse with photoperiodic control, the United States has much greater temperature, seasonal and daylength swings as it is further from the equator. 

In other words, it has longer days for a longer amount of time.  Looking at the graph below, we see that neither location ever hits below 12 hours of daylight. 

Does that mean cannabis will not flower? No. 

Fourteen hours and 10 minutes is indicated in yellow as this is a usual critical photoperiod for commercial hemp varieties.

For Lansing, this equates to approximately seven weeks with daylight hours long enough for the plant to stay vegetative. 

On the other hand, for Baton Rouge, the plants potentially could stay vegetative for 1-2 weeks.  Though the farmer is paying the same amount per seed or liner, the yield will be devastatingly different. 

If we know our varieties more intimately, we can be suited for success.

In The Hemp Mine variety trials we’ve found that “Southern Sunset” initiates flower at 13 hours and 24 minutes. 

As shown in Figure 3, planting an alternative variety with a shorter critical photoperiod can provide the farmer with a longer vegetative cycle. 

This will make a huge difference in the potential size of the plant equating to greater yield as well as profits.

Figure 3.  Daylength comparison to hemp variety which initiates flower at 13 hours and 24 minutes.

However, a farmer must beware that substituting this same plant in a region does significantly shift the flower initiation date later - meaning a later harvest. 

For northern regions, this could mean a frost before the flowers are mature. 

The Hemp Mine has developed a Hemp Planting Map (HPM) with conjunction with The University of Tennessee to give farmers a quick and easy guide to understand daylength and the dangers of frost for planting and harvest. 

Now we understand why certain varieties perform differently at varying latitudes, but how can we manipulate that in a cost-effective way?

It does not take much light to keep plants vegetative. 

For example, poinsettia and other short day crops require very little light. 

This light can be applied at the beginning or end of the day’s natural light or to achieve night interruption with a photon flux density of 1-3 μmol·m−2·s–1

Night interruption is the practice of providing low-intensity photoperiodic lighting to plants during the middle of the night.

By interrupting the dark period, the plant will think it’s a long day. This is typically done in the middle of the night (ex. from 10:00 p.m. to 2:00 a.m.).  This can be achieved easily with stadium/flood lights on multiple sides of the field.  It is important to remember lights will need to be applied from multiple sides as the side which does not receive direct light can still flower.  String lights are also a good option though more costly in a field setting due to the light count needed plus needing a structure constructed to hold the lights.  LEDs are a great option for electrical savings.

 hemp in field

Picture: (Miller Farms in Michigan growing Peach Haze).

Semi-Auto Flowers

Though horticulturally ”autoflower” is misleading, in the cannabis industry the terminology works.  These plants are not autoflowers (aka. day neutral) at all. 

Rather, they are photoperiodic with a flowering response time greater than 17 hours of light per day. 

When grown outdoors at any location in the US (except Alaska), it will flower as daylength will not go over 16 hours. 

At first this may seem like a variety which can only be grown indoors. 

In fact, by understanding how we can manipulate these plants with light it provides a whole new way of growing with the potential to have many crops per season. 

This year at The Hemp Mine’s annual field day, we will display this system of growing on a half-acre plot (see picture below). 

You’ll notice one 300-watt LED on the telephone poll lining the field. 

Whereas these varieties would normally flower immediately after transplant, because of the photoperiodic lighting, we are able to grow these plants to a size which will produce a profitable yield. 

What else does this mean? In warmer climates, we can run multiple cycles per year without the need of a greenhouse or blackout cloth. 

Additionally, as true “autoflower” varieties (which are only available via seed) are still being optimized through breeding, this is a great way to optimize clones and have a profitable harvest with consistency and size.

The more we know, the better we grow!

hemp in field

Picture: (The Hemp Mine in South Carolina).


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