Article 2-5 A Guide to Pre-Flowering

by Eric Biksa

Many annual flowering crops grow through a transitional period before initiating intense floral production. This often over looked phase provides the grower with an opportunity to increase overall yield and floral production. By providing the plant with ideal transitional conditions, you are paving the way for the final few weeks of flowering to reach optimum production, as the plant has developed the required reserves of nutrients, growth regulators, and physical framework in order to successfully complete it’s life cycle.

The following outline is based on an eight-week flowering cycle, although some varieties are able to yield impressive results in six weeks. So, please adjust accordingly, as this is not written in stone and is offered as a suggestive guide.

WEEK 1 (Begin when the plants have been introduced to critical dark cycles, for example, short day plants require10-14 hours of uninterrupted darkness.

  • In the soil or soilless medium, attempt to keep moisture levels at the roots equal in light and dark cycles, but be careful not to over water. This helps to encourage a final spurt of growth, so a little less time can be spent in the vegetative cycle.
  • Try to keep light and dark cycle temperatures similar, as this too, will help with a vigorous growth spurt.
  • An oscillating fan gently sweeping the plant canopy will help to keep osmotic pressure in the plant high, so that it may take advantage of the last leg of the vegetative growth.
  • Now is a good time to remove any “sucker growth” at the lower portion of the plant canopy. This would be the lower third of a plant. This material can be used as cuttings for your next crop. You can easily root the lower prunings from growth during the first few weeks of flowering under 24 hours of gentle lighting.
  • Apply a high quality kelp-based foliar spray high in cytokinins, such as growth plus. Gently mist the plants and raise the lights high off the canopy, or just keep a household lamp burning during normal light cycles. This should be done in order not to scorch the foliage. DO NOT interrupt the light cycle and tend your plants during the dark cycle. However green light during dark cycles is not detrimental.
  • Metal halide lighting is ideal during this stage of pre-flowering.
  • Ease off the nitrogen, slightly increase phosphorous, and make sure there is plenty of potassium available to the crop.
  • A light foliar spray with a product containing plant available calcium and magnesium (i.e. Calcium Nitrate or Calcium Chloride and Epsom Salts (magnesium sulfate) or a liquid blend such as “Cal-Mag”), this step is highly recommended.


  • Slowly begin to cut back moisture levels at the roots in the dark cycle, do not begin to cut back root moisture during the light cycle just yet. You can get a feel for this by picking up the containers, or by a simple and inexpensive moisture meter for soil/soilless crops.
  • Provide a light/dark temperature differential of about 5° – 10°F. This will help to further notify the plant to produce, as it’s life cycle is begging to go into the reproductive stage.
  • As above, a weekly plant available calcium and magnesium based foliar spray should be applied. These help to fuel cell and chlorophyll production, which seem to peak during the pre-flower cycle.
  • Continue to gently remove any lower branches on the plant which receive little light, and draw energy away from more actively producing flowering sights.
  • It is desirable to apply certain growth regulators as directed at this time. Blossom Booster or Blossom Blood are both excellent for helping to build framework for a bountiful harvest.
  • A phosphor coated or clear metal halide is recommended at this stage. It will help to keep internodal distance closer, allowing for larger, more compact and dense blooms.
  • Further reduce nitrogen, and increase phosphorous and potassium at the roots.


  • Continue to increase light/dark temperature differentials. If possible by the end of the third week you should have a difference of 15°F. Make sure that humidity during dark cycles is not greater than 65%, as fungal problems may occur.
  • Spray the last weekly foliar feeding of calcium and magnesium as outlined above. 150ppm Calcium and 50ppm magnesium are optimal.
  • Moisture at the roots at the end of the third week, should be significantly reduced by the end of the third week during the dark cycle. Take care not to stress your plants from lack of water, as this can lead to a crop of hermaphrodites, which can occur with drastic swings in soil or hydroponic media moisture content.
  • If you have height restrictions, now is a good time to tie down taller branches, and carefully remove flowering sights not exposed to optimal light intensities. Overall plant height will increase slightly, but the majority of growth will have occurred in these first few weeks. When considering the right shape for a flowering plant, keep in mind the apple tree. You will notice it is wider than it is tall, and has an open center for air circulation, which allows light to penetrate evenly to all active growth/flowering sights.
  • At the end of the third week you should be burning your H.P.S or H.P.S. conversion lamps, as their red spectrum will encourage profuse flowering, as most varieties have slowed vertical growth to a crawl by this stage.
  • Further reduce nitrogen, increase phosphorous, and maintain adequate potassium levels.
  • In week four, apply growth regulator such as blossom booster as directed.
  • Follow your usual regimen for flowering from here on in. As mentioned previously, the above is not meant to establish hardfast rules for flowering. It is offered as a suggestion that the pre-flowering stage should be recognized as a unique stage with it’s own requirements in the annual life cycle of flowering plants. Taking the time to accurately learn your plant rhythms will allow you to give what your plants want, when they need it. For the grower, this means consistently increased yields, improved quality and reduced pest and disease problems.