Light dep -- tarp can be pulled many ways
Light dep -- tarp can be pulled many ways

Days became longer than nights after the spring vernal equinox on March 21. From then until July or August is a good time to start covering your light dep setups.

Method 1: Old School Light Dep

The old school method is to close in the evening at 7 p.m. and open in the morning at 7 a.m. This method works great if you have enough ventilation during the whole grow, especially the bud cycle. Ventilation is an important component to all light deps.

Method 2: So You Can Sleep In

Another method is to close at 5 p.m., opening after dark and letting the plants wake up naturally with the sun the next morning. This method works best if you have the sun shining on your plants as soon as it comes up. Knowing the direction your land faces the sun is critical for a successful light dep.

Method 3: Faster Results

Another method is one that shortens the duration 3 to 4 weeks without any loss of size, weight or quality of product.  Close at 6 p.m., open at 7 a.m. to give the plants 13 hours of darkness. A lot of ventilation is needed in this one.

How to Ventilate Light Dep Grows

When closing at night, deps can be opened up after dark and closed up at 4 a.m. before sunrise; this is for ventilation. Fans are necessary to keep the air moving to prevent mold. There are other time methods but these are tried and true with great results!


  1. I see most people around here using the tarps on at 4am off at 10am schedule. Its nice and cool and in the mornings so no ventilation is needed. One of the most interesting schedules Ive heard of is oldschool growers putting tarps on at 5pm then off at 9am, and then skipping a day and covering again at 5pm and off at 9am, a schedule that ultimately yields appox 12 hours of darkness a day and you have to cover your greenie half as much. Your method 3 for faster results sounds a little dubious to me, anyone have any experience with this one? Does it really work?

    • I have done the 4am to 10am for the past few years, takes some commitment to get up that early, but being a full time farmer I find it quite useful to wake up before the sun is out and start the daily chores, due to the lack of intense heat and only 45 min to an hour of direct sunlight on the tarp I see no mold! No knowledge or experiences with the 13hr darkness process.

      • From what we understand, the 13 hours is more important when flowering some sativa or sativa dominant strains that originate near the equator. Last year for example we ran 2 phenos of a Vietnamese Black x Bodhi ‘ s BSHB. The BSHB dom pheno started flowering just around day/night of 11.5/12.5 and went till Dec 18th. However it wasn’t for another 3 weeks, closer to the 11/13 photoperiod that the Vietnamese Black pheno started flowering. Whew, she ran until Jan 22nd. She proabaly could have gone another week. So, at a 12/12 photoperiod, the Vietnamese Black pheno would have taken much, much longer to induce flower setting.
        It’s called Photoperiodism or Critical Photoperiod. Cannabis plants are classified as short day plants, and genetic coding, as well as phenotypic expression, and acclimation determine the “Critical Photoperiod”. We read that it had to do with the light cycles being consistently close to 12/12 around the equator.
        Lastly, DJ Short says you will see phenotypic expressions in plants under a 11/13 light schedule that won’t show when under a 12/12. Makes sense, but we have not ran controlled experiments on this yet.
        Super props to everyone. It would be nice to get some solid info on this.

  2. I too would like to hear a firsthand account of method 3. Cutting flower time in half while sacrificing nothing seems fishy. I have pulled tarps at 7 and 7, and 5:45 on 10:00 off both with equal success. I prefer 5:45/10:00 because the plants get better ventilation with the tarp off all night. It also suits the spot my greenies are better and an off grid situation. If i had full sun until 7 and grid power i would pull 7 and 7 to maximize light.

  3. 8 p.m. to 8 a.m.
    It’s not rocket science. It’s a weed!
    You guys sound like grape or wine snobs
    Just grow how and whatcha like!

  4. I’ve been pulling tarp from 7pm to 10pm for the last 5 weeks (morning light hits greenhouse at 7am)…all my seasonal plants have started going into flower also..being that the dep is this far into flower and its almost Aug 20th…shouldn’t i be able to stop pulling tarp and just let it grow?

  5. I only do 11/12. I have a few friends I supply starters to so we’re all doing the same strains. 12/12 is simple so they stick with that. Mine always comes out with a lot more color and heavier tricromes yet I don’t notice a shorter duration. In fact I wouldn’t mind going an extra week or 2.

  6. There is no such thing as cutting flowering time in half. Doing 13 hours of darkness shaves off about a week of a normal 2 month flowering cycle. It all depends on your situation, specifically your westward horizon and when your pulling tarp (sunrise and sunset) For me, I have redwoods to my west and my greenhouse is shaded starting at 5:30pm in August. Sunrise being at 6:30am I lose nothing by pulling tarp at 5:30 and taking it off once it’s completely dark outside. I get 13+ hours of darkness and I have bad ventilation for a minimal amount of time.Keeping tarp on all night is not a good idea, your bound to get some mold. Having tarp on with direct sunlight is not ideal either. Timeing your grow with sunrise and sunset can be very helpful. Also timing your grow with having maximum sunlight during the flowering stage can be helpful. The sun shifts up to 15 degrees over the summer, for me being around redwoods my hours of direct sunlight changes drastically over the summer. By hours even.

    -to Michael… for me, leaving a tarp on my green house from 8pm to 8am could cost me tens of thousands of dollars in mold damage, dispensaries wouldnt buy it. It may not be rocket science but smarter growers make a lot more money than lazy less intelligent growers. I don’t do this for fun…

    • Glad to hear good about pulling tarps at night to let plants breath. I work at a recreational farm and I’m taking away my boss next year outdoor grow. He’s telling me my idea is bad because I won’t get quality light dep with pulling tarps at night…… his reason is moon light and star light effects the quality of pot….. B’S B’S…

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