Ask Ed: Questions and Answers – International Dark Periods

Question: Dear Ed,

I looked up the daylight hours from May through September in my part of California and discovered that the most daylight hours on June 22, the longest day, is 14:50. This triggers the question – Why do plants in indoor gardens need a minimum of 18 hours of light in the vegetative stage when Mother Nature only gives you 15 to begin with? Is 18 hours of light really necessary? Can I start clones under 15 hours of light/9 hours of darkness and then force them to flower by increasing the dark period to 12 hours?

-Claudia J., California

Ed’s Rosenthal: Claudia,

The amount of uninterrupted darkness that is required to induce flowering varies by variety. Strains that flower late in the season require a longer dark period than those that flower early.

To find the length of the dark period required to trigger flowering, place the plants in a space with 17 hours of light and 7 hours of darkness. Then each day increase the dark period by 5 minutes. The earliest plants, if they are in your library, will indicate within the first six weeks. Within two months, when the dark period has increased to more than 11 hours, virtually all the plants will indicate. Since it takes the plant some time to grow flowers once it has been triggered, counting back 10 days from that date, will give you the day flowering was triggered and show you how much uninterrupted darkness the plant needed to be forced to flower

In the northern hemisphere the longest light period and the shortest dark period occurs on the first day of summer, 22 June. Starting the first day of summer, summer solstice, the length of the light period decreases each day. On 22 Sept., the first day of autumn, light and dark are equal at all latitudes. The light period continues to decline until its nadir, 22 Dec., winter solstice, when the light period starts to increase again.

Suppose you were growing in Humboldt County and your plants were ripe 10 October. Figuring that the period from start of forcing to ripening was 60 days, a little more than eight weeks, the plants initiated the flowering process on August 11. On that date in Arcata, latitude 40.530 N a city in the middle of Humboldt County, there were 14 hours of daylight and an hour of twilight (dawn and dusk) for a total of 15 hours of light. This indicates that the plant required nine hours of darkness to initiate flowering.

If the plant had been grown in San Diego, near the Mexican border, latitude 32.420 N, the dark period of nine hours would have occurred, 24 days earlier on 18 July, and the plants would have ripened Sept 15.

Generally speaking, plants that require more time to ripen also require a longer dark period to initiate flowering. Short-time ripening plants usually need less darkness to flower.

Indoors, you can keep most plants growing vegetatively under eight hours of darkness, 16 hours of light. The reason garden books recommend 6 hours of darkness, 18 hours of light is that growers are assured that the plants have less than the minimum amount of darkness that they require to initiate flowering no matter what variety they are growing. Indoor plants can be grown using continuous light to produce the fastest growth.

Plants growing indoors for transplanting outdoors later in the season will face less stress if the light regime was six or eight hours daily rather than in continuous light. The reason is that the change in light regimes is sometimes enough to set the plants to flower or shock them so they stop their growth for 5-10 days while they adjust to the new light regime.

Below is a graph of the darkness in your region, you can find more here.

Send your grow questions to Ed Rosenthal at [email protected].