Cannabis transitions through three distinct stages of flower development with different challenges and opportunities.
Whether you are growing a few plants at home, or harvesting with a combine, you will yield more weed if you understand the different flowering stages, and how to get the most from each.
Once the switch is flipped on a flowering light cycle your plants will be fully committed to bud production before you see the first signs of flowering.
The potential for production of potent flowers is dependent on your decisions, so it is good to be prepared for what’s ahead.
The photoperiod is the duration of time that a plant is receiving continuous light during each 24 hour period.
Outdoors, the daily photoperiod changes with the seasons. Cannabis plants measure this change in light cycle to determine the best time to start flowering.
Cannabis plants have adapted grow in many different conditions worldwide and have different flower photoperiods depending on their genetics.
When the nights get longer, photoperiod cannabis plants are obligated to flower. This makes them short day plants.
Outdoor Flowering Photoperiod
If you are growing outdoors where you can’t control the photoperiod, then you must use strains that will initiate flower at the right time at your location.
The seasonal daylength varies based on latitude and determines when different strains will start flowering outdoors.
If you live in a temperate area with a long growing season, many different strains may work in your garden. They will flower at different times, depending on their genetic response to the day/night cycle.
For growers living above about 40 degrees latitude, there are early flowering, or fast strains available that can be harvested before weather conditions deteriorate.
Autoflowers are also a great choice for northern growers as they evolved at high latitudes and flower profusely during long summer days.
The internet still thinks that Cannabis requires 12 hours of darkness to flower, but most Photoperiod strains will initiate flowering once the night period is reduced to 10 or 11 hours in length.
You don’t have to take my word for it, here is a recent research paper about cannabis flowering photoperiods.
Indoor Flowering Photoperiod
Being in control of your plant’s flowering stage is one of the big advantages to growing your favorite plant indoors.
Controlling the photoperiod allows growers to determine the best time to transition to flower based on plant health and size.
Cannabis plants love light and can consume it at much higher levels than most other plants. Photosynthesis is the engine of bud production, so you should provide as much light as your plant can handle if you want to maximize yields.
A plant’s potential is determined by the total amount of light received each day. This is known as the daily light integral (DLI). The DLI is determined by multiplying the light intensity by the photoperiod.
Unless you have a PAR meter, it will be difficult to measure your DLI accurately. The important takeaway is that an 18 hour photoperiod will provide 50% more light energy than a 12 hour photoperiod using the same light source. It is helpful to have lights with dimmers so that we can provide similar light levels during different light cycles.
Plants will perform based on their limiting factors, and additional light will not improve performance in sub optimal growing conditions.
In addition to greater yield, a high DLI supports dense flower formation, and elevated cannabinoid and terpenoid biosynthesis.
Growing from seed allows us to pick the plants that perform best in our specific environment. We can then keep this plant as a clean mom, and clone from her forever.
Young plants will do best in nursery climate conditions of 70-80f (20-26c) and 65-75% relative humidity (RH). When starting plants indoors you need to harden them off when you bring them outside. They need time to acclimate to the UV in sunlight, so keep them in the shade for a day before exposure to direct light.
Don’t waste your time moving plants to flower if they are struggling. It is better to start over and get the seedling stage right. If your plants don’t start out healthy, things are likely to get worse during flower.
Male cannabis plants produce pollen and need to be identified early to avoid pollinating our sinsemilla crops.
Pre flowers will form near the base of each growth node when your plants are mature and allow for identification of male and female plants.
Male cannabis preflowers look like little balls on short stalks.
Female cannabis preflowers will form a pair of white hairs called stigmas.
Outdoor plants grown from seed need to be carefully monitored once they reach maturity. At my latitude, most outdoor plants will start to show sex in mid July. During this period it is best to inspect your crop daily and cull any males.
Be sure to dispose of males far from your grow site, as the pollen sacs will continue to open long after the plant is cut down.
Plants have roots for a reason and are not fond of being moved between different environments. Good growers try to manage these transitions to be as smooth as possible to avoid shock.
Plants should be transplanted as soon as they are well rooted and hardened off.
When moving plants to a new environment for flowering it is important to match the conditions as much as possible until your plant settles in.
When you move plants between grow areas, there is an opportunity to interrupt the pest cycle. Small plants can be pruned, and thoroughly sprayed with an insecticidal soap or similar contact killer to ensure total coverage.
The soil that you are planting into should be moist and fluffy, and at a similar temperature to the root ball. Properly hydrated soil should clump together, but not leak water when squeezed. When planting into garden beds, irrigate thoroughly a day or two prior to transplant.
Once the transplant has been placed and filled, the new soil level should just cover the transplant surface. Water in your new plant generously to make sure there are no air pockets between the root ball and the new soil.
Healthy plants will quickly acclimate to positive growing conditions and take advantages of the resources available as you increase light and nutrient levels.
Quality liquid kelp has every micronutrient your plant desires, and contains plant growth factors. These are natural hormones that will benefit your plant.
Nematodes are microscopic worms that swim through the water in the soil and eat pests. They especially enjoy dining on fungus gnat and thrip larvae, but they will occasionally consume a root aphid if there happens to be one around.
Steinernema feltiae nematodes are not very expensive, they ship well, and will last for a week in the fridge, so it is one of the more practical biocontrols for small scale growers to access.
Weed plants are usually moved into their flowering area and allowed to root in and grow for a few weeks before being flipped to flower.
The amount of time that plants spend at this stage depends on plant size and density. The closer the plants are together, the quicker you can flip them. In the case of autoflowers, high densities and minimal pruning are recommended.
The goal during vegetative growth is to establish root and shoot structure that can support rapid bud production during the flowering stages.
Scrogging is a method of canopy management that involves training for an even canopy using a taut horizontal trellis. This method was pioneered in grow tents, and the concept works well at craft production scale. As plants grow through the scrog net, branches can be redirected to other holes in the trellis to fill gaps in the canopy.
If you plan to scrog, a layer of plastic horti trellis can be installed and pulled tight just above the plant tops when they are well rooted and growing vegetatively. It is best to prune plants first, then spray for total coverage before applying the trellis and complicating these jobs.
Growing a strain that is suited to your conditions will improve your success at canopy management.
Some strains naturally have a branchy, compact structure well suited to growing in areas with height limits. These strains often require more pruning.
Greenhouse growers with high disease pressure and no height restrictions will benefit from plants with longer internodes and a bit of stretch to increase airflow through the canopy.
Insecticidal soap sprays will kill most insects on contact.
Sulfur is an effective fungicide which can be sprayed on plants during the vegetative stage.
Sulfur will not stay suspended in water, so you have to shake the sprayer as you are using it.
Sulfur treatments and horticultural oils should never be used together.
These products are widely available and approved for use in organic agriculture.
Plants should not be sprayed under direct light, and sprays should be timed so that the plants dry off as quickly as possible afterward. Pesticide label requirements should always be followed for safety.
A strong focus on pest management during the vegetative stage will reduce pest issues later in flower. I usually spray plants once or twice a week during veg and early flower.
Outdoors, photoperiod plants will continue to veg until they begin flowering based on day length and maturity.
Clones are already mature plants when they are rooted and will flower if placed outside too early in the season.
When a clone starts flowering in the spring, it will spend the summer trying to revert to vegetative growth. This is disturbing to witness and will destroy your yields.
Seedlings can be transplanted, or direct sown earlier than clones without initiating flowers.
If you have a few pot plants at home, light dep is simple (but tedious).
Just move your plants into a dark area each evening (make sure they have ventilation), then move them back outside after dark.
The best time to start a light dep is when plants are mature, and outdoor lighting conditions are entering their peak. I usually start depping in early-July here at 50 North for harvest in early-September.
At larger scale, light dep systems can be automated. External roll up systems work well until they fail.
More sophisticated greenhouse operations have internal blackout screens. These systems affect climate conditions and are usually opened and closed gradually to prevent rapid swings.
Early flower - Stretch
The first signs that your cannabis has entered the flowering stage start about a week after the light cycle has changed.
Plants will start to stretch, and form additional pistils, or “hair up” The plant has now committed fully to flower.
If you return a plant to 24 hours of light once flowering is initiated, it can take a month or more for it to resume vegetative growth. With autoflowers, the first signs of females flowering usually occurs about three to four weeks after planting seeds.
Flowering starts outdoors when weed plants get enough interrupted darkness to trigger hormones and start producing buds.
The start date of flower will depend on the genetics of your strain, and the seasonal day length in your area. Most photoperiod plants will begin flowering when night length is 10 or 11 hours.
Outdoor plants sometimes transition more slowly into flower due to gradually decreasing daylengths. As plants move into flower they stretch rapidly to compete with the neighbors when the pollen starts to fly.
Plants can double in size during the second and third week of flower. They will need lots of resources to build this biomass. Nitrogen uptake is at its peak during the stretch.
Canopy management will increase your quality and yields, but be prepared, this is the flowering stage that requires the most work to get right.
Colas which start to overgrow the average canopy height can be tied down to the scrog net to give other tops time to catch up.
Supercropping is a good way to manage plant height during stretch. This technique involves squeezing a branch with enough force to compress the tissues in the stem that transport hormones. This slows down growth above the compression so lower buds can catch up.
Managing the stretch allows plants to focus on flower biomass and shortens their supply lines. For growers with limited vertical space, limiting stretch is mandatory.
Fortunately, it is not necessary to run a negative dif all the time to get this benefit.
If you can achieve a negative dif for the first couple of hours of the photoperiod, the plant signal is established for the day, and you can return to normal operating temperatures to optimize biomass growth.
Temperature and relative humidity need to be controlled to get the best performance from your plant.
It is helpful to know what is happening when you are not in your grow room. You can buy a data logger for about a hundred bucks that will record and chart your temps and humidity. If your environment gets too far out of range, you get an alert.
It is important to measure temperature the way the plant feels it. The leaf surface temperature can be read with a laser thermometer.
Temps will vary a bit from leaf to leaf, so it is good to measure a few and average.
Mid flower - Bud Growth
The mid flowering stage is a time of bud growth.
Plant uptake of Nitrogen will slow as plants stop making leaves and focus on reproduction. Flower expansion requires lots of cellular energy, and Phosphorous uptake usually peaks in mid flower.
By mid-flower the plants hormone balance has fully shifted to flower production.
Increasing the daily light integral during this time of expansion can increase your yields if all other plant requirements are met. The photoperiod can be increased to 13 or 14 hours during mid flower and the plants will not re-veg.
When plants are getting more light, they will uptake more water and C02 for photosynthesis.
Increasing the dry back of your media after each irrigation can cause a generative stress response that promotes flowering.
Selective removal of leaves during the mid flower stage can boost your yields. The benefit that flower sites get from direct lighting outweighs the loss of leaf photosynthates. It is best to deleaf regularly during early and mid-flower so that you don’t shock the plant by removing too much foliage at once.
As flowering progresses, reproductive frustration builds, driving production of dense fields of trichomes.
Predatory mites like Californicas are good generalists and will consume multiple pest species including mites and juvenile thrips and aphids.
Specialists like Persimilis dine on spidermites at all life stages with single minded voracity. They consume nothing else and will die if spider mites are not available.
Larger bodied bios like Feltiella can navigate dense trichome fields when hunting during late flower.
It is important to scout for and kill caterpillars and loopers during the growing season.
They can be managed with bioinsecticides containing BTK. In addition to eating your plants, the manure from these pests creates a perfect niche for bud rot to establish during late flower.
Your solar aspect can make the difference between success and failure.
When cannabis crops receive light very early in the morning, they quickly dry, with less opportunity for mold to establish. Some dedicated growers will blow the dew off their outdoor ladies each morning with a leaf blower.
Late flower - Ripening
The late flowering stage is when the bulk of psychoactive cannabinoids are synthesized. Between 20 and 30 percent of the dried flower weight is added during the final ripening phase.
Plant priorities have shifted from making carbohydrates to making babies. When reproduction is frustrated by lack of pollination this energy is directed to synthesis of complex biochemicals in the trichome head.
Photoperiods can be returned to 12 hours and light intensity ramps down during late flower.
Temperatures are reduced and humidity lowered to slow the terpenes from vaporizing and protect our dense buds from rot. Nighttime temperatures can be dropped to 60f (16c) to enhance anthocyanin production. This will cause purple coloration for many strains.
Your plants are starting to senesce (die) at this point. They know the end is near and will start to pull resources from fan leaves that are not needed for reproduction. Leaves should be removed as they yellow so that dying tissue doesn’t become an entry point for disease.
Most growers will stop feeding synthetic nutrients, and flush water through their plants for the last week or two. This is not necessary when growing in organic soil, but there is usually not much need to feed your plants for the last few weeks.
But there is no question as to why cannabis produces THC in such abundance.
Cannabis makes THC for us.
The THC rich cultivars we enjoy today are the result of our ancestors selecting seeds from the most potent plants starting about five thousand years ago.
During late flower, there is lots of chemistry happening in the trichome head. CBG is the precursor cannabinoid, and can be refined into CBD, THC, and a litany of others.
The synthesis of cannabinoids is a continual process, and THC will degrade into other non-psychoactive substances like CBN if the flowers are harvested too late.
Flowers are usually ready for harvest when most of the white pistils have turned brown.
Some strains will continue to produce white pistils late in flower out of desperation to procreate.
Harvesting too early will result in loss of cannabinoid yield and quality will decrease if you wait too long.
The best way to determine if a cannabis flower is ready for harvest, is to examine the trichomes under magnification. Trichomes will be clear early in flower and become cloudy and start to amber as the flower ripens.
If most of the trichomes are cloudy, your plant is probably ready to harvest.
Outdoor flowers are usually harvested as soon possible to avoid loss to bud rot. The top flowers on a cannabis plant are usually the first to ripen and can be harvested as soon as they are ready.
The plant will allocate resources to quickly ripen lower flowering sites that are exposed to higher light when tops are removed. It is best to harvest buds when they are dry, but don’t wait for the rain to stop if you need to get your crop in.
Congratulations on your bumper crop! Now it’s time to get to work on harvesting and processing all of that weed. Great cannabis can be ruined by an improper dry and cure, so be sure to be prepared.
Check out the outdoor grow guide below for drying and curing tips. Happy Flowering!