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As cannabis cultivation has become more mainstream, an increasing number of innovative products and practices have become commonplace. While outdoor cultivation is usually done in the warmer regions of the United States, the majority of growers in states coast to coast are taking advantage of the controlled-environment benefits that indoor and greenhouse growing provide.

With indoor cultivation, a cultivator can harvest a few crops per year. While the grow area may be more restricted than the sometimes-vast square footage of an outdoor grow, the ability to closely control temperature, humidity, and photoperiod provide a number of options for indoor growers. This level of control puts them in control of the key environmental factors necessary for growing high-quality cannabis.

Using Blackout Tarps for Growing Cannabis Can Manipulate Photoperiod

Manipulating photoperiod indoor—and particularly in greenhouses—can be accomplished in a few ways. Supplemental lighting in greenhouses is one widely used practice for extending day length and allowing the shortening days of fall to remain at summer day length to prolong vegetative growth and delay flowering.

When it comes to shortening photoperiod, blackout tarps serve a very valuable purpose. When the sun is shining brightly outdoors, a greenhouse interior can be completely dark when using a good blackout tarp. Blackout tarps put growers in the driver’s seat when it comes to limiting daylight. In a greenhouse, there’s no reason to wait for shorter fall days for plants to flower. By using a blackout tarp, plants can be in flowering mode in a week.

Indoors, blackout curtains and tarps—as well as blackout grow tents—give indoor home grow-room cultivators the same level of control. One decent-sized grow room can quickly transform into two smaller grow rooms so simultaneous grow projects that require different photoperiods can occur.

Using Blackout Tarps for Growing Cannabis Indoors

For novice growers who grow from seed and grow only one crop at a time, the value and purpose of a good blackout tarp or tent may not be immediately apparent. Let’s identify some of the useful purposes of these items for simultaneous cultivation projects.

Mother Plants

If you have a special mother plant from which you cut clones, you want to keep her under 18 hours of daylight at all times. A simple way to accomplish this is through the use of a blackout grow tent. This tent can fit into the corner of a grow room while a full crop of clones or seedlings are growing in the main grow area. With the grow tent lights set at 18 hours and the grow room set at 12 hours, all light is kept separate, which provides the means for dramatically increased production. The mother plant will remain healthy and happy in her permanent grow-tent home.


As well as being a great permanent location for mother plants, blackout grow tents also provide a handy temporary home for young clones. Once they are cut from mother plants, young clones can occupy a grow tent that’s set to a constant 18 hours, just like the mother plant tent. These tents can occupy a grow room that’s set to a permanent 12 hours of daylight and darkness.

When the plants growing under 12 hours of light are ready for harvest, clones at their full vegetative growth size can come out of the tent and occupy the 12-hour growing area to flower. Meanwhile, new cuttings can go into the grow tent.

This way, a continuous rotation of plants can occur without ever needing to change the photoperiod on a light’s timer. This approach will dramatically increase overall yield as opposed to growing one crop at a time.

Single Plants

Some people like to grow just one plant or a few plants at a time, Blackout grow tents are perfect for this. These types of tents can be set up in the corner of nearly any room, and all light at any photoperiod will remain sealed within the tent, regardless of other household activities. They make perfect sense for people who are growing a small number of plants for personal use.

Grow Room Partitions

Blackout tarps also provide an easy way to partition a grow room to essentially make two rooms out of one. The tarp will thoroughly divide the room from ceiling to floor. By doing this, you can accomplish two separate cultivation projects simultaneously, in a similar manner as the grow tent(s) in the grow room but on a larger scale.

Using Blackout Tarps for Growing Cannabis in Greenhouses

Cannabis cultivation in greenhouses captures some of the best of both outdoor and indoor cultivation, which is why it’s become increasingly popular. Capturing the benefits of the sun’s full-spectrum natural light eliminates or reduces electrical costs dramatically, and the protected, sealed environment is relatively easy to control. In the greenhouse grow space, it’s possible to extend daylight with the use of supplemental grow lights, as well as reduce the length of day by using blackout tarps.

Greenhouse blackout tarps are large and heavy, typically made with layers of durable material that’s hemmed and constructed for long-term use. The weight and size of these tarps depends on the model and size of the greenhouse it serves. While some are manageable for one person to lower and raise, typically two people are necessary, with fully automated systems on timers now being the norm.

For large-scale cannabis cultivation, multiple greenhouses equipped with blackout tarps opens up opportunities for a wide range of cultivation practices. Production and yield for grow operations like this can be very large, with multiple grows in both vegetative and flowering phase possible simultaneously.

Light Leakage When Using Blackout Tarps for Growing Cannabis

Whether used for greenhouse or grow-room purposes, all blackout tarps must be fitted and secured so that no light leakage occurs. Even small amounts of light entering the blackout space can be extremely detrimental to the plants within and seriously limit the effectiveness of these tarps. Nearly all of the blackout tarps sold are guaranteed to be impenetrable to light, so making sure they fit the exact room or greenhouse they serve is imperative.

If light leaks into a grow room that’s supposed to be dark, it results in stress to the plants within. Photochrome (light receptors in plants) imbalances will occur, and since photochrome imbalances profoundly impact flowering in adult plants, they are extremely undesirable. Overall yield will suffer, as will flower size and structure, not to mention potency. All cannabis cultivators should strive for photoperiods that result in several hours of complete light and several hours of complete darkness. Light leakage jeopardizes states of absolute darkness—which is always the best recipe for success when growing cannabis.

Under extreme cases of light leakage, a thriving crop of flowering females can actually revert back to the vegetative mode and stop flowering altogether. Another danger of light leakage is the occurrence of hermaphrodite plants (plants that exhibit both male and female sex organs). Hermaphrodites are a common occurrence in the cannabis world, and are never a good thing—except for specialized purposes like the creation of feminized seed.

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Whenever using blackout tarps, carefully evaluate the growing area for any light leakage that may be occurring. Then, address that danger immediately. Light leakage equates to an uncontrolled environment, and you want your growing area as controlled as possible. It never hurts to periodically revisit the grow area to ensure that no light leakage is occurring due to unintentional shifting or movement of the tarps. Heed this warning and prevent light leakage so you can maximize the yield and potency of your crop.

Final Thoughts

Blackout lights create wonderful opportunities for both indoor and greenhouse growers. Used correctly, they can provide a means to dramatically increase production and engage in new and interesting cultivation projects. If you are becoming increasingly ambitious with your cannabis growing and have never given blackout tarps and tents a try, you will probably find that they are well worth the money.

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