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Transition to Bloom Phase: When & how to switch

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. NOTE: For outdoor grows, light deprivation techniques will need to be employed to trigger flowering when nights are too short.

When to switch?  There are 2 main considerations:

Age of 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 2-3 weeks old). For “seedlings”, if space (and time) permit, better yields can be obtained from a 6-8 week veg period. Alternatively they can be switched straight after germination, however they are biologically incapable of budding until about 3 weeks of age.

Height of 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% of the ‘available’ height (remember to consider the minimum gap distance between lamp and foliage). Topping and LST (see section) are very useful techniques for keeping plants short and bushy and will help enable a long veg (6-8 weeks) if required.

Bloom Phase (Powering the Flower)

As a rule-of-thumb, in the first 2-3 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:

1. 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.

2. Increase the air-exchange rate. Flowering plants tend to prefer a gradual lowering of relative humidity to around 40–60% (high humidity can cause rot and mildew). General optimum temperature is 68-82 deg F (20-27 deg C). Also, carbon dioxide consumption is reaching its peak and is therefore often the weakest link (Fig 7.1.

3. Avoid lamp burn by ensuring to keep foliage at a safe distance (Table 3). Do not keep plants too far away though as this will encourage “stretching”.

4. Avoid stressing plants by pruning during flowering unless there is over-crowding or damaged growth. Major ‘structural’ pruning should be conducted during veg.

5. 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.

Avoiding the Stretch!

Many growers complain that their plants ‘stretch’ excessively, especially in the first 2-3 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 which 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 an MH lamp for the first 2-3 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 deg F (5 deg C) of the day temperature. Ensure the day temperature is kept below about 82 deg F (27 deg 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.

© Andrew M Taylor (FloraMax)