Time to Flower: How to Harness Photoperiodism for your Indoor Grow

Flowering is the most important aspect of a plant’s lifecycle, as it offers the opportunity to reproduce. In a natural environment, shortening daylight hours represents the onset of winter — some plants must reproduce then if they are to pass on genes for the following spring. Other plants respond to the lengthening hours of daylight leading into summer. Hours of light in a certain environment, also known as the photoperiod, plays an enormous role in when and how quickly a plant will flower. This evolutionary response is known as photoperiodism.

Photoperiodism was originally studied by Wightman W. Garner and Henry A. Allard at the U.S. Department of Agriculture in Maryland during the early 20th century. The two researchers wanted to see if they could breed a tobacco plant to bloom in the summer, rather than only in the winter. As they discovered, tobacco will only bloom if given certain hours of darkness. Their results were published in 1920 and altered our understanding of how plants respond to the changing seasons.

LDPs versus SDPs: What’s the Difference?
There are two types of photoperiodistic plants: long-day plants (LDPs) and short-day plants (SDPs).

  • Long-day plants are species that mature as daylight hours lengthen. Some examples include barley, spinach and lettuce.
  • Short-day plants flower when there are increasing nighttime hours. These include tobacco (as Garner and Allard determined in their research), soybeans and cannabis.

This response to light found across several flowering plant species — whether they respond to the lengthening day or night — is a key characteristic to understand when cultivating an indoor crop. Indoor cultivators are able to replicate this natural cycle: the seasonal shift from summer solstice to autumnal equinox, and eventually fading into the dark winter season. Therefore, cultivators also have control on when and how quickly their plants flower.

For instance, Garner and Allard were able to induce tobacco flowering in the midst of summer, by leaving their tobacco plants in darkened cabinets for long periods of time. As technology has progressed, it is easier now than ever to control the hours of simulated day and night a crop receives.

ProGrowTech Lights Offer a Simulated Seasonal Shift

Growers today can utilize their lighting systems to determine exactly when a plant flowers. These advancements in lighting technology make simulating the changing seasons accessible for anyone from microgreen growers to the seasoned cannabis cultivator. With ProGrowTech lighting systems, altering the hours of light and dark is just another tool to fine-tune your indoor growing facility and achieve exceptional results.

One of the most common mistakes growers make is going immediately from one extreme — the middle of winter, when there are more than 12 hours of darkness — to the height of summer when the days are lengthy and bright. This could never occur in a plant’s natural environment, and as a result, a plant may be shocked by this sudden change in light. Distressed plants may have brown wilting leaves, and buds fail to form. Shocked plants may fail to reach maturity or even die.

We can avoid shocking our crops by eliciting a more gradual season change that is much more representative of the outdoor environment. ProGrowTech lights offer the ability to elicit a slower simulated season change that feels gentle rather than abrupt.

With the right lighting technology, photoperiodism may be harnessed to the cultivator’s advantage.