In nature, cannabis grows throughout the summer and flowers mid-fall when the days get shorter. When grown indoors, it’s a different story. Growing indoors means that you can grow year-round without worrying about seasons, but it also means that the grower is solely responsible for initiating the plant’s flowering phase.
Begin too early, and your plant may grow smaller than you hoped or suffer from a root system that is not completely formed.
Begin too late, and you may bite off more than you can chew with a plant that outgrows your grow room. Here’s when exactly to switch your plants to flower, and how to tell when it’s time.
How do I switch my cannabis plants to flower?
If you’re growing an autoflowering strain, your plant will start flowering automatically whenever it reaches a certain age. If you’re growing a photoperiod strain indoors, you’ll need to “trick” the plant into thinking that winter is approaching. To do this, you have to control the amount of light your plants receive.
During cannabis’ vegetative stage, they should get 18 hours of light every day and six hours of darkness (referred to as an 18-6 schedule). To cause your plant to begin flowering, switch to a 12-12, or equal amounts of light and darkness every day.
With winter on its way (as far as your plant can tell), your cannabis plant will go through a rapid growth spurt, roughly doubling its size over the next two weeks, and then it will expend most of its energy into growing buds.
Switch to flower nutrients
Once you switch to the 12-12 light schedule, remember that it’s also time to switch to a fertilizer that is balanced for the flowering stage, such as Reefertilizer Bloom. Also, keep in mind that flowering plants generally should not be exposed to temperatures above 28°C (or 82.4°F).
Flowering plants are also more susceptible to mold growth, so aim to keep your grow room’s humidity at 40-50%. Different strains like different conditions, of course, so do some research on what you’re growing to make sure your plant stays happy and healthy.
How do I know when to switch cannabis to flower?
Getting your cannabis to flower is easy, but choosing the perfect time can be more difficult. Marijuana plants can start flowering super early if they are left in the dark – even within the first few weeks. Some growers do this on purpose by starting their plants off on a 12-12 light schedule so that they’ll flower as soon as possible.
In addition to shortening the time until flowering and harvest, this this ensures the plants will stay small and manageable, which means that growers can plant lots of plants at once. This is known as the “sea of green” method.
On the other hand, as long as your plant gets 13 or more hours of sunlight per day, it’ll just stay in the vegetative stage and continue growing pretty much indefinitely. The bigger your plant grows, the higher the potential yield will be.
Fortunately, the plant’s yield and resin production should more or less correspond to the plant’s size, not its age, so you don’t have to feel obligated to grow the biggest monster cannabis plant you can to maximize yield – you’d be better off growing a plant the size you can manage, harvesting, and planting again.
How long should I let my cannabis plants veg?
Most growers recommend keeping your plants in the vegetative state for 4-5 weeks. At that point, the plant will be large enough to support all of the bud sites but it will still be at a size that’s easy to maintain. 4-5 weeks is only a general guideline though, and it may not always be the right decision. When you’re preparing to switch your cannabis to flower, make your decision according to these factors:
- Age. Some growers believe that a seed-grown marijuana plant doesn’t reach its full resin production potential until it’s been in the vegetative stage for at least 60 days, but this isn’t true. You can switch to flowering at any age.
- Is your plant cloned? If you’re growing a clone, you can start the flowering stage as soon as your plant establishes roots.
- Desired yield. Bigger plants mean higher potential yields, but they will also require more light and nutrients. Plan ahead, do your calculations, and make sure you don’t grow a plant that’s bigger than you can sustain until harvest time. Larger plants may also be more difficult to maintain, so make sure you consider the time investment as well.
- Height. This is the major factor here. Depending on your plant’s genetics, it could grow a lot more once it starts the flowering stage. Pure indica plants might only grow another 25-50%, but sativas often double or even triple their height. Hybrid strains may do something in between. In any case, a good rule of thumb is to assume that your plants will double their height – so, figure out how big you want your plants to grow, and switch to 12-12 when they’re halfway there. Remember to leave some extra space between your plants and your grow lights so they don’t get burned!
Photoperiod cannabis strains can begin flowering at any time, and the choice of exactly when is up to you. That may seem like a difficult decision, but it really only comes down to one factor: think about how big you want your plants to grow, and start their flowering stage when they’re a little less than halfway there.
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