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The apex of cannabis growth happens in the bloom room. Changing light cycle induces major hormonal changes that cause one last major growth spurt. Plants surge upward and nearly double in size before vegetative growth ends! Within two weeks new growth stops and most energy diverts to flower production. Cannabis needs to be pampered and pushed to perform in the bloom garden.
When flowering begins, female or male flowers emerge where new shoots used to form. In about a month the flowers start to take shape. After two months flowers become dense and ripen. Senescence begins—growth slows, then stops. Leaves turn yellow on the dying plant, like trees in autumn. That’s when it’s harvest time.
Intelligent bloom plant care ensures high quality and high yield at harvest time. Big yields can’t happen without the right conditions:
proper photoperiod
bright light
temperate conditions
abundant CO₂
effective foliar canopy
adequate fertilizer
Cannabis is an annual plant that cannot survive winter. Instead, flowers are its mechanism to perpetuate its genes as death approaches. Sexual reproduction is evolutionarily advantageous because it creates vigorous new hybrids of parental genes packaged in a sturdy case: the seed. Without male pollen, female flowers remain seedless and ooze THC from sexual frustration.
Critical Photoperiod
Cannabis blooms when the light is right. Short days and long nights signal that summer is over and winter is coming. Photoperiod is the only environmental trigger for flower production. In response, cannabis stops growing new shoots and begins sexual reproduction.
The length of night necessary to trigger flowering is called critical photoperiod or critical night length. You read that right—length of night, not length of day! It is actually the hours of uninterrupted darkness that triggers cannabis to stop getting big and start making buds. This genotypic variation creates individual plants that flower earlier, or later in the season. Awareness of critical photoperiod enables cannabis breeders to create strains better adapted to local climate and latitude.
The transition from vegetative growth to sexual reproduction is not gradual. Cannabis will not begin blooming until nights are just long enough—not a minute less. Most strains flower with 9-12 hours of darkness. Indoor growers bloom with a 12/12 light/dark schedule because it is enough darkness for any photoperiod sensitive plant to induce flowering.
Hermaphroditism
If the dark cycle is too short, inconsistent, or interrupted, it will trigger an extreme survival mechanism: hermaphroditism. Even small interruptions of light during the dark period cause male and female flowers to appear on an otherwise gender conforming plant. To survive a hostile environment, cannabis can have sex with itself and create seeds without a partner! Eek! Unfortunately, hermaphroditic seeds are especially prone to hermaphroditism and thus undesirable.
It doesn’t take much light leaking into a dark garden to cause hermaphrodites. Hormonal changes can be triggered by a distant street light, passing headlights, or a smartphone screen. It only takes a minute, or less, of rogue light to interrupt the dark cycle! Routinely inspect the grow room at night for light leaks or timer malfunctions. If you need to work in the garden at night, use a green light. Green light is not photosynthetically active and will not disturb “sleeping” plants.
Quality & Intensity of Light
Outdoor cannabis needs bright, unobstructed light, all day long. Avoid shadows from trees and fences. Seek out southern exposure for your outdoor garden.
When cannabis blooms indoors, it needs brighter and warmer lights than during the veg phase. For this reason, High Pressure Sodium lights work great in flower. Fluorescent and Metal Halide lights are for veg use only. They make fluffy, low-density buds when used for blooming cannabis.
Position lights above the canopy at the manufacturer recommended height, or personal preference. Higher installations create a larger footprint of light but eventually a diminishing return on quality. Lights that are too high promote lanky, weak stems and small, low-density buds.
If lighting efficiency is not a concern, then place lights as close as possible without causing heat stress to leaves. Air conditioning, exhaust ventilation, air cooled light hoods and oscillating fans all mitigate heat damage. We prefer our lights as close as possible. Even better, sunshine!
Environmental Conditions
Mold & Humidity
Cannabis flowers are very susceptible to mold. Wet, humid, swampy conditions will cause fungal problems in the bloom garden. Mold and mildew can destroy an entire crop overnight so it is critical to keep flowering cannabis dry. Avoid spraying plants. If you do, make sure they dry off quickly afterward. Keep relative humidity in the air below 50% to prevent fungal problems.
One common systemic fungus is Powdery Mildew. It covers affected foliage with spores that look like baby powder. Even worse is bud rot, a grey mold that attacks at harvest time. Bud rot turns the biggest, ripest buds into a grey mush of mold.
Cold air holds less moisture, so humidity levels spike as the temperature drops. Outdoor crops are susceptible to bud rot because of cold nights around harvest time. The temperature drop causes humidity to saturate and condense into dew drops. A climate-controlled greenhouse is the best way to protect your crop from dew, rain, snow, hail and wind.
Good grow room design uses redundant design and technique to remove moisture from the environment:
Precise irrigation applies just the right amount of water without spraying leaves or creating puddles.
Porous grow mediums drain quickly and prevent stagnant water in the root zone.
Wastewater plumbing quickly removes runoff from the garden space.
HVAC stabilizes temperature and eliminates relative humidity spikes.
Dehumidifiers dry the air.
Oscillating fans mix air to prevent pockets of high-humidity within plant canopy.
Plant spacing enables air circulation to remove pockets of humidity.
Canopy management creates space for air circulation between the grow medium and foliage.
Exhaust fans remove wet, stale air.
Intake fans bring in fresh, drier air.
Temperature
The temperature of the foliage and grow medium affect bloom performance. Foliage grows well around 75°F, or warmer. The root zone is healthiest around 68°F. Root disease increases with temperature. Insect pest activity also increases with temperature. Ideally, cannabis would have warm air and a cooler grow medium.
Plants grown in the ground have their roots geothermally cooled. Hydroponic reservoirs depend on reservoir chillers to keep the roots cool. Warm water holds less dissolved oxygen and causes brown, slimy roots. If your grow medium is the same temperature as the air around it, then run your entire garden between 68-72°F.
Pest Management
Integrated Pest Management is best applied in the vegetative garden. Some IPM strategies contaminate flowers; others cause unnecessary stress. Flowering plants are large and finicky and usually better left alone.
Do not spray anything on buds during bloom. Pesticide sprays leave a chemical residue that can poison consumers. The wetness can also cause fungal problems. Apply pesticides and other treatment as early as possible in the veg phase.
Certain cultural, mechanical, physical and biological controls are ok to use in the bloom room. Check out our post on IPM for more information.
Air Quality
Plants breathe in CO2 and exhale Oxygen during photosynthesis. In fact, most plant mass is carbon derived from the air, not the soil! Therefore, indoor plants need fresh air or else they will suffer poor growth.
Atmospheric CO₂ is around 400ppm outdoors. Garden performance increases dramatically when bloom CO₂ levels are artificially increased to 1,100ppm. Artificial CO₂ supplementation speeds up plant metabolism. Plants can grow bigger and finish faster with extra C0₂. Enriched plants also tolerate even warmer air temperatures, up to 80°F.
Gardens without supplemental CO₂ need constant air exchange. Keep fresh air replenished in the grow room with carbon-filtered exhaust and intake fans. Use in-line duct fans with cubic feet per minute (CFM) ratings large enough to exchange the air in the room every few minutes. This is also a great way to mitigate odor.
Odor Control
Cannabis is a stinky crop. Indoor flower cultivation usually requires odor control. Without it, your grow can attract unwanted attention. In some places odor mitigation is the law. There are a few technologies that control smell. It is best to use multiple, redundant odor control techniques.
Carbon filters are the main strategy to scrub odor from garden exhaust. They are also installed to recirculate air within the room. These filters use activated carbon to neutralize odors and remove pollution like mold spores, pollen and dust. Carbon filters are the most effective odor control on the market. Choose a filter that matches the CFM rating of your fan.
Ozone generators chemically break down odor-causing molecules. Ozone is an oxygen molecule with three oxygen atoms, not two. That extra O makes Ozone a toxic and unstable substance. When Ozone breaks down, that extra oxygen is released and become a free radical. This is both dangerous and useful. They attach, oxidize and destroy organic molecules such as odors, pathogens, plants and people.
Ozone is a powerful tool. Use it safely. Never operate ozone devices inside a living area. Only install ozone generators near the outdoor exhaust port. Ozone generators are great for mopping smell at the point of discharge and are safe if used properly.
Stretch
Cannabis plants have a final “stretch” of veg growth in the first two weeks of bloom light cycle. The plant nearly doubles in size before switching all energy to flower development. No more vegetative growth occurs after stretch.
The end of stretch is a great time to remove light-deprived sucker-shoots from the base of stems. Use clean, sharp scissors to remove any lower growth shaded by more than 18” of canopy. This will concentrate the plant’s resources to the tip of branch where the top colas are forming.
Canopy Management
The microclimate within the leaves is healthier when you take care of the plant canopy. Canopy management affects the structure and shape of plants. Some common techniques are:
Pruning
Super-cropping
Trellising
Tying and Staking
Removing dead and diseased leaves.
Canopy management creates space for fresh air to flow through the foliage. It also focuses growth to where the light is. These techniques create more bud sites and provide structure to support heavy flowers. Check out our post on pruning and canopy management for more information.
Fertilizer and Irrigation
Cannabis needs lots of water and fertilizer during bloom. Watering technique and frequency is the same as the vegetative phase. Plants in bloom use more water and fertilizer because they are mature and under bright lights. Do not let plants wilt; they may never fully recover.
Cannabis grows vegetatively during the first two weeks of flower. Plants still need nitrogen during this period of stretch. The grow medium usually has residual nitrogen built up from weeks of veg formula fertilizer. An immediate switch to bloom fertilizer during stretch helps flowers form sooner. Just make sure there is some nitrogen available during stretch.
In mid-bloom cannabis needs lots of phosphorus (P) and potassium (K). Commercial fertilizer schedules decrease N and increase P and K during bloom. Follow the week-by-week feeding schedule for your fertilizer. It is designed to make sure your plants are getting what they need in bloom.
Stop all fertilizer 2-3 weeks before harvest. This is the“final flush.” It improves the flavor and purity of harvested flowers. Flushing plain water through the grow medium removes fertilizer salts from the root zone. Without added fertilizer, plants cannibalize nutrients from their lower leaves to preserve their flowers. The entire plant yellows, except for the buds. Flushed plants taste and burn better.
Preparing to Harvest
Harvest preparations begin with the final flush. Fertilizer salts buildup in the grow medium because it acts like a sponge. Plan as much as 3 weeks of flushing to thoroughly cleanse the soil and plant tissue. To flush, use plain water or a commercial flushing agent. Irrigate as usual; occasionally in excess to dissolve salt from the grow medium by passing a large amount of water through it. Flushing causes leaves to yellow and die. Remove them because they are habitat for fungal problems. A clean and dry garden denies mold habitat and refuge..
Measure ripeness by examining the resinous crystals on buds, called trichomes. Use a 60-100x microscope to reveal the mushroom-shape of the crystals. Ripe trichomes have a well-formed bulbous cap on a slender stalk. The mushroom is at first clear, like glass. As it ripens, it turns cloudy white. When a trichome is completely ripe it turns amber. Not all trichomes change color at the same time. Cannabis is harvestable when most of the trichomes are cloudy.
Textbook ripeness is when half of the trichomes are cloudy and half are amber. Under-ripe buds are less potent. Over-ripe flowers are prone to bud rot.
Optimum flowering plants grow from the healthy roots established in a stable veg environment. Enough hours of darkness causes a plant to bloom, but the care of a knowledgeable gardener enables a plant to produce quality buds.
Enjoy your bountiful harvest!