Cannabis Light Schedules: Vegetative Stage vs Flowering Stage
Cannabis plants keep getting bigger and bigger with long days, and start making buds when you give them long nights.
Cannabis is a “photoperiod” plant, which means the amount of light received each day decides when the plant starts flowering or making buds. This article explains how much light a day your photoperiod cannabis plants need to grow and start budding, so you get to a happy harvest day. What about auto-flowering strains?
Vegetative – Seedling or clone leads to Vegetative Stage –
Give 18-24 hours of light a day
Flowering – Flowering (Budding) Stage leads to Harvest –
Give 12 hours light & 12 hours dark each day
Seedling or Clone
While not technically a “stage,” all grows start with cannabis seeds or clones.
Plant your seeds or clones when you’re ready to start growing! What are clones? https://www.growweedeasy.com/cloning
Some outdoor growers start their plants indoors to give them a headstart before putting plants outside.
If you’re growing cannabis outdoors with seeds, you should wait until a few weeks after the spring equinox to put your seeds outside. In the northern hemisphere this means seeds go outside in-or-after April, In the southern hemisphere seeds go outside in-or-after October.
For growers starting with cannabis clones, generally you should wait a few weeks longer than with seeds. Cannabis clones are more prone to flowering early outdoors than seeds, so you might want to put your clones out in late Spring or early Summer. (What are clones?)
If you live in a cold climate, you must also wait until after the last frost before putting your plants outside. Freezing temps will kill cannabis plants. Strain choice is very important. Some strains flower earlier than others. For outdoor growers in cold climates, it’s important to make sure you grow a strain that is matched up with your local weather, so that plants are ready for harvest before temperatures drop.
The vegetative stage is one of the most important parts of the life of your cannabis plant.
The vegetative stage is the growing stage of the plant. When in veg, cannabis plants grow bigger and taller, growing only stems and leaves. As a grower, you are able to control the size and shape of your plants in the vegetative stage using simple training methods.
During the entire vegetative stage the plant does not produce buds at all. It only grows stems and leaves. During the vegetative stage plants tend to grow very fast, especially when conditions are right.
What keeps cannabis in the vegetative stage?
Short nights keep cannabis plants in the vegetative stage. You can keep a cannabis plant in the vegetative stage for basically forever as long as the plant continues to get short nights (shorter than 1s-12 hours, depending on the strain).
Cannabis will stay in the vegetative stage as long as the plant gets short nights (less than 11-12 hours of darkness each day)
Whether you’re growing indoors or outdoors, you must make sure your cannabis plants get at least 13 hours of light each day to stay in the vegetative stage. If your plant gets a few long nights, it may start budding before you want.
The plant can receive as much as 24 hours of light a day while in the vegetative stage. Many indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules) during the vegetative stage to encourage faster vegetative growth.
Don’t want to worry about light schedules? For growers that don’t want to pay attention to light schedules, there are auto-flowering strains of cannabis, which will automatically go through their whole life in about 3 months no matter what light schedule is provided. For some growers, an auto-flowering strain may be more simple than a traditional (photoperiod) strain.
Most indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules). Giving your cannabis plants more hours of light each day in the flowering stage will encourage faster growth.
Lingo: When a grower provides 18 hours of light a day and 6 hours of darkness, this is commonly known as the 18/6 light schedule. For 24 hours a day, this is referred to as the 24-0 light schedule.
As long as your plant is getting plenty of light a day, your plant will automatically stay in the vegetative stage from late spring until late summer. Every strain is a bit different.
Cannabis starts budding when plants get at least 12 hours of uninterrupted darkness each night. After plants start budding, they must continue to get long dark nights until harvest or they may revert back to the vegetative stage.
Indoors most growers put their plants on a 12-12 schedule to initiate flowering. Outdoors the plant will naturally start budding in late summer when nights are growing longer and longer as winter approaches. Just make sure plants aren’t exposed to light during their dark period!
What is 12-12 Lighting?
The indoor grower will need to artificially induce flowering/budding in plants by changing the light schedule so the plant receives only 12 hours of light a day, and 12 hours of uninterrupted darkness.
Once the plant is changed over to the flowering (12/12) light schedule, there is generally another 6 weeks-5 months (average 2.5 months) before the plant’s buds are ready for harvest.
Outdoor growers wait until their cannabis plants start naturally flowering on their own, usually after mid-summer when days start getting shorter than 12 hours.
It’s important to make sure plants aren’t exposed to light at night during their dark period, even street lights or spotlights, as this can prevent cannabis plants from flowering properly.
Growing Indoors? Not Sure When To Switch To Flowering?
So indoor growers have a choice to flower their plants whenever they want… When is the best t ime to start flowering your cannabis indoors?
The real answer is that it’s a matter of personal preference and also depends on what end result you’re looking for. There are two major considerations when choosing the right time to switch to 12/12, the age of the plant and the height of the plant:
Age: Some growers feel that a marijuana plant which has been grown from seed will not produce as many buds or have enough resin production if the plant is not given at least 60 days in the vegetative stage to mature before it’s changed over to the flowering stage. This is not true. many growers initiate flowering soon after germinating a seed in order to keep plants small and short. This is often called “12-12 from seed.” Just remember, no matter what you do, a young cannabis plant will not start flowering until it is 2-3 weeks old. Even if you put a seed on a 12-12 schedule from the beginning, it will not start properly budding for about 3 weeks. When growing with cannabis clones, age is not an issue and growers can switch directly to flowering once your clone has established roots. This is because even though a clone may be small, it’s still a ‘mature’ plant since it is made of a piece from a mature plant. Rooted clones tend to grow much faster for the first few weeks than plants grown from seed. In any case, age is not much of an issue, and you should switch your light schedule at the time that best fits your needs.
Height: A general rule is that your marijuana plant will double or triple in size during the flowering stage from the point where you first change over the light schedule to 12/12. Some plants will grow more, some will grow less, but a good rule of thumb is to change your light schedule over to flowering when your plants have reached half of their final desired height. Bending, known as “LST” or “low stress training” can be used to control colas that get too tall. Simply bend too-tall colas down and away from the center of the plant. Some growers will even slightly break or “supercrop” branches to get them to bend at a 90 degree angle. For those growing in a small space, height may be the primary concern. However, there are many techniques available to grow a short, bushy weed plant or basically train your cannabis plant to grow into any shape you want.
Here’s an example of LST to keep a plant short:
In optimal conditions if height and space is not an issue, you would probably want to vegetate your cannabis plant for 60 days or more before switching it over to flowering. This gives your plant plenty of time to grow big (so you get bigger yields), and allows new growers to dial in their grow before plants enter the sensitive flowering stage. In the vegetative stage, it is easy to recover from problems, but problems are a lot more serious in the flowering stage, where mistakes can dramatically hurt your final yields.
Giving cannabis plants more time in the vegetative stage, and taking time to train them to fit your space, will give you the best final yields. However, if space is tight, then it’s better to switch when the plant is half the final desired height, or even to just attempt to flower your cannabis plant straight from seed.
After the vegetative and flowering stage are over, it is time to harvest your plants!
What do I need to know about light cycles and flowering my marijuana plants? Plants keep getting bigger and bigger with long days, and start making buds when you give them long nights.
Stages of the marijuana plant growth cycle
Cannabis plants, like all living things, go through a series of stages as they grow and mature. If you’re interested in cultivating cannabis, it’s especially important to understand the changes a plant undergoes during its life cycle, as each stage of growth requires different care.
Different stages call for different amounts of light, nutrients, and water. They also help us decide when to prune and train the plants. Determining a plant’s sex and overall health rely on stages of growth as well.
How long does it take to grow a marijuana plant?
Generally speaking, it takes anywhere from 14-32 weeks, or about 4-8 months, to grow a weed plant.
The biggest variability in how long a marijuana plant takes to grow will happen in the vegetative cycle—if you’re growing indoors, you can force it to flower after only a few weeks when it is small, or after several weeks when it is big. If you’re growing outdoors, you’re at the whim of the seasons and will have to wait until fall to harvest. The plant will develop buds in the last 8-11 weeks.
The life cycle of cannabis can be broken down into four primary stages from seed to harvest:
- Germination (5-10 days)
- Seedling (2-3 weeks)
- Vegetative (3-16 weeks)
- Flowering (8-11 weeks)
Seed germination (5-10 days)
Light cycle: 18 hours of light
The first stage of life for a cannabis plant begins with the seed. At this point, your cannabis plant is dormant, patiently waiting for water to bring it to life.
You can observe the quality of the seed by its color and texture. The seed should feel hard and dry, and be light- to dark-brown in color. An undeveloped seed is generally squishy and green or white in color and likely won’t germinate.
To begin growing from a seed, learn more about germination here. This stage can take anywhere between 5-10 days.
Once your seed has popped, it’s ready to be placed in its growing medium. The tap root will drive down while the stem of the seedling will grow upward. Two rounded cotyledon leaves will grow out from the stem as the plant unfolds from the protective casing of the seed. These initial leaves are responsible for taking in sunlight needed for the plant to become healthy and stable.
As the roots develop, you will begin to see the first iconic fan leaves grow, at which point your cannabis plant can be considered a seedling.
Seedling stage (2-3 weeks)
Light cycle: 18 hours of light
When your plant becomes a seedling, you’ll notice it developing more of the traditional cannabis leaves. As a sprout, the seed will initially produce leaves with only one ridged blade. Once new growth develops, the leaves will develop more blades (1, 3, 5, 7, etc.). A mature cannabis plant will have between 5-7 blades per leaf, but some plants may have more.
Cannabis plants are considered seedlings until they begin to develop leaves with the full number of blades on new fan leaves. A healthy seedling should be a vibrant green color. Be very careful to not overwater the plant in its seedling stage—its roots are so small, it doesn’t need much water to thrive.
At this stage, the plant is vulnerable to disease and mold. Keep its environment clean and monitor excess moisture.
Vegetative stage (3-16 weeks)
Light cycle: 18 hours of light
The vegetative stage of cannabis is where the plant’s growth truly takes off. At this point, you’ve transplanted your plant into a larger pot, and the roots and foliage are developing rapidly. This is also the time to begin topping or training your plants.
Spacing between the nodes should represent the type of cannabis you are growing. Indica plants tend to be short and dense, while sativas grow lanky and more open in foliage.
Be mindful to increase your watering as the plant develops. When it’s young, your plant will need water close to the stalk, but as it grows the roots will also grow outward, so start watering further away from the stalk so the roots can stretch out and absorb water more efficiently.
Vegetative plants appreciate healthy soil with nutrients. Feed them with a higher level of nitrogen at this stage.
Flowering stage (8-11 weeks)
Light cycle: 12 hours of light
The flowering stage is the final stage of growth for a cannabis plant. Flowering occurs naturally when the plant receives less than 12 hours of light a day as the summer days shorten, or as the indoor light cycle is shortened. It is in this stage that resinous buds develop and your hard work will be realized.
If you need to determine the sex of your plants (to discard the males), they will start showing their sex organs a couple weeks into the flowering stage. It’s imperative to separate the males so they don’t pollenate the flowering females.
There are a number of changes to consider once your plant goes from its vegetative stage to flowering:
- Your plants shouldn’t be pruned after three weeks into the flowering stage, as it can upset the hormones of the plant.
- Plants should be trellised so that buds will be supported as they develop.
- Consider feeding plants with blooming nutrients.
What week of flowering do buds grow the most?
Buds typically grow the most toward the end of the flowering cycle, around week 6-7. You probably won’t notice much budding out at the beginning of flower, and it will slow down toward the end of the cycle, when buds become fully formed.
Once the buds have reached full maturation, it’s time to harvest.
This post was originally published on July 18, 2017. It was most recently updated on January 17, 2020.
It’s important to understand the changes a growing cannabis plant undergoes during its life cycle, as each stage of growth requires different care.