Transition to Bloom Phase: When & How to Switch

By Andrew Taylor
Published: June 29, 2020 | Last updated: May 13, 2022 05:08:54
Key Takeaways

Switching your cannabis plants from veg to bloom isn’t as simple as flicking a light switch. Andy Taylor examines the key variables needed to successfully make the transition so your crop flourishes in bloom.

Switching from Veg to Bloom

Plants need at least 13 hours of light each day to stay in veg — a few long nights may be enough to trigger budding. Plants begin budding when they get at least 12 hours of uninterrupted darkness each night (12/12). This must continue until harvest. Even if darkness is interrupted briefly, flowering will be hindered. In fact, plants may revert back to veg unless 12-hour nights are maintained. Employ a timer to help ensure consistency. For outdoor grows, light deprivation techniques will need to be employed to trigger flowering when nights are too short.


When to Switch?

Knowing when to switch involves two main considerations:

Age of the plant: Cuttings can be switched as soon as desired, however, for optimum yields it is generally best to wait until the clone has a strong root system (typically two to three weeks old). For seedlings, if space (and time) permit, better yields can be obtained from a six to eight week veg period. Alternatively, they can be switched straight after germination, however, they are biologically incapable of budding until about three weeks of age.

Height of the plant: Depending on the strain, plants will generally double in size during flowering. So, if room height is limited, the switch to 12/12 will need to be done no later than when the plant has reached 50 per cent of the available height (remember to consider the minimum gap distance between lamp and foliage).

Topping and low-stress training (LST) are very useful techniques for keeping plants short and bushy and will help enable a long veg (six to eight weeks) if required.


Diagram showing topping and low-stress training are used to position more flowering sites in the “sweet zone”

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Bloom Phase (Powering the Flower)

As a rule of thumb, in the first two to three weeks of flowering the plant will continue to grow in height (termed “flowering-stretch”) and buds will begin to form. Following this, floral growth will dominate and veg growth will slow then stop. Some general principles:

  • Lights are switched off for 12 hours per day — uninterrupted! Use high pressure sodium (HPS) lamps, rich in the yellow-red parts of the lighting spectrum.
  • Increase the air-exchange rate. Flowering plants tend to prefer a gradual lowering of relative humidity to around 40-60 per cent (high humidity can cause rot and mildew). General optimum temperature is 68-82°F (20-27°C). Also, carbon dioxide consumption is reaching its peak and is therefore often the weakest link.
  • Avoid lamp burn by keeping foliage at a safe distance. Do not keep plants too far away, though, as this will encourage stretching.
  • Avoid stressing plants by pruning during flowering unless there is over-crowding or damaged growth. Major structural pruning should be conducted during veg.
  • Don’t get complacent with bugs or diseases such as powdery mildew. Regularly check both sides of foliage on all your plants — especially those that sometimes get overlooked in the corner.

Chart of lights to foliage gap readings depending on light specifications.Avoiding the Stretch

Many growers complain that their plants stretch excessively, especially in the first two to three weeks of flower. Along with potentially reduced yields, lower foliage will be more difficult to light efficiently, and top foliage is at risk of being burnt if lamps cannot be raised any higher. Plants can also be gangly and weak and therefore susceptible to breaking once loaded with buds.

Although flowering-stretch is biologically unavoidable, there are ways of minimizing it:

  1. Source a strain that is prone to being short.
  2. Ensure plants receive enough light. Lack of light causes an additional stretch response whereby plants grow taller and with fewer buds. Lack of light can be caused by:
    • Shading, due to plants being positioned too close to one another. It is generally more productive to plant fewer plants, rather than more.
    • Lamps being too far from foliage will cause plants to stretch to find light. Note, avoid having lamps too close as this will also induce stretching.
  3. Use a metal halide lamp (MH) for the first two to three weeks of flower, then switch to HPS. The orange/red spectrum of HPS and other flowering-specific lamps encourages stretching. Conversely, the blue spectrum of MH tends to inhibit stem stretch.
  4. Keep the night (lights-off) temperature cooler but within about 9°F (5°C) of the day temperature. Ensure the day temperature is kept below about 82°F (27°C).
  5. Maintain sufficient air flow. This tends to promote shorter, stronger plants.
  6. Topping and LST are ideal for modifying the plant to ensure all foliage receives enough light. The majority of this should be done during veg.


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Written by Andrew Taylor | Head of Manufacturing, Research, & Development at FloraMax

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Andrew Taylor is an analytical chemist with additional qualifications in plant function and nutrition. He has more than 25 years experience in agricultural product design, research and development, and commercial manufacturing. Taylor is the head of manufacturing and research and development for FloraMax, an Australian manufacturer of hydroponic nutrients.

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