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Optimizing Marijuana Flowering

Flowering cannabis is the most exciting phase for most gardeners. There is nothing more exciting than watching months of work come together in such a beautiful way - not to mention we are that much closer to harvest!

The first step of transitioning a plant from vegetative growth to flowering is to determine the length of flowering cycle for each strain - now is the time to date our labeled plants. Uncle Pete prefers to put the date of harvest on each plant immediately when it goes into the flowering room.

Optimize Vegetative Growth First

The most common mistakes which compromise flowering are made before our plants ever get here. It is quite common for new gardeners to spend all of their time, energy, and money on flowering while neglecting the vegetative cycle. Once our flowering rooms are set up and the environment is properly controlled, the yield of the harvest is determined by the plants we put in. Don't make the mistake of skimping on vegetative growth and then expect a miracle in the flowering room. Every day of the plants' life leading up to flowering will play a role in the outcome. The more experience the gardener has with a specific variety, the easier it is to manipulate plants into the size and structure we want in the flowering plant. Generally speaking, the gardener wants to grow bushes with multiple tops. The more bud sites and tighter the node spacing the better. Additional detail may be found in the basic topic Optimizing Vegetative Growth.

Setting Up The Flowering Room

Light and Space

Assuming the gardener has invested time wisely during the vegetative growth cycle, there are some nice bushes heading into the flowering room. We grow our plants in either 10 or 20 gallon containers as discussed in the basic topic Optimizing Vegetative Growth. In a 10 gallon container, we expect to yield 14-16 ounces of dry flower depending on variety. In a 20 gallon container, we expect to yield 20-30 ounces of dry flower. These yields are reached under 1200 watt double-ended HID lights with 10 foot ceilings. Uncle Pete’s flowering room is 12x24 and holds an average of 20 plants, covered with 10 lights. The entire grow including vegetative growth room and work area is housed inside a 24x24 garage.

During the initial period of flowering - before the plant begins to bud - cannabis plants will "stretch" for 2-3 weeks. This means that not only do they continue to grow new branches, but also lengthen their stems between branches. Some varieties can double or triple their height during this period. Make sure you have enough vertical space under the lights for the varieties you are growing. Experience is the best teacher for making the most of the space you have in your garden without exceeding it.


As always, environment will be the crucial element of success for the flowering cycle. Plants will not thrive with more than a 20 degree temperature swing. Between 60 and 80 degrees Fahrenheit is the target range. Use oscillating fans to ensure good air movement around and through the plants.

A highly productive cannabis garden uses HID lighting for 12 hours per day. HID lamps create a lot of heat which must be controlled. Ventilation of specially-designed sealed lighting fixtures will ensure that the plant's environment doesn't get too hot. Ensure that the temperature is maintained at all times by using a centrifugal fan to exhaust the heat captured by the fixture. With multiple lights or with unsealed fixtures, an air conditioner may be required. In winter when the lights are off, a heater may be required. Be very careful when determining electrical requirements for your garden - don't plug too many things into an outlet or you will create a risk of fire.

Manipulating Marijuana Photoperiod To Induce Flowering

Marijuana plants begin to flower when they are introduced to a 12-hour "night" period. This is accomplished by creating a light-tight flowering room where the lights alternate on and off every 12 hours. In keeping with our consistent advice to maintain environmental control, day and night photoperiod must be controlled by an automatic timer.

When the lights are off, there must be complete darkness, with no light leakage. Even a pinhole of light can cause light stress to a cannabis plant, causing the plant to create male flowers (and therefore seeds), vacillate between growth and flowering, or even not flower at all. Turn off all the lights in your flower room and give your eyes a few minutes to adjust before carefully examining the grow room, paying special attention to areas around doors, windows, vents, etc. Some grow room equipment even has lights that can disturb photoperiod - check these carefully before purchase.

Effective Plant Support and Maintenance

To optimize flowering, the gardener must ensure that all of the plant's branches are supported well. As the plants begin to build flowers, the branches will naturally bend, collapse, or even break from the additional weight. This is the plant's natural tendency to want to fill with seed and fall to the ground. By supporting the branches, we trick our plants into putting more and more weight on to make that effort to fall to the ground. This is a layman's explanation of complicated, but predictable hormonal responses in the cannabis plant. We know that cannabis thrives and survives on every continent because of its ability to adapt and overcome. Using this knowledge is how we coerce our plants into the desired behavior.

Almost all marijuana varieties require additional support of their branches to avoid this breakage or collapse during flowering. The simplest way to support cannabis branches is by tying them up to bamboo stakes inserted into the Pro-Mix. There are many, many techniques that are used, however, including plant cages and trellises. Advanced techniques for training, pruning, and plant support are covered in the intermediate topic Pruning and Training to Maximize Yield.

For very large plants, use 4 foot cattle fence to put around containers during late vegetative growth as shown below:

Buds like these don't just happen.

These cages give plants the support they need to avoid a total collapse during flowering and provide an excellent support to tie many branches to. Use bamboo stakes in addition to the cages to support and spread your plants where necessary. The plants should be confined to their assigned space under the light, while allowing them to maximize that space - this is a balancing act. The goal is to fill the canopy in the room so that little light is hitting the floor while maintaining enough space for air and light to get through the plants. You should be able to see light thru all of your plants.

Pick yellowing leaves constantly. Maintain good plant health by keeping up on this endless chore. As the cannabis plant matures, we cut the nutrient supply, so it is normal for the plants to use the nutrients stored in the lower leaves. This is the natural order of things. Keeping up on this task separates the serious gardener from the novice.

Uncle Pete's Organic Flower Formula

Flowering Feeding Solution (4 gallons)

Adjust pH to 5.5, feed on Monday and Wednesday only. Do not save excess solution.

  1. 50 ml BioThrive Bloom

  2. 20 ml CaMg+

  3. 15 ml BioBud

  4. 20 ml/gal BioMarine

  5. 40 ml/gal Terpinator

Watering Solution (4 gallons)

Adjust pH to 6.0, water on Friday only. Do not save excess solution.

  1. 20 ml Black Strap Molasses

  2. 1/4 tsp Myco Madness

Target feed rates for Monday/Wednesday:

  1. 1 gallon pot - 1/3 gallon

  2. 2 gallon pot - 1/2 gallon

  3. 5+ gallon pots - 2 gallons per 5 gallons of container


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