cannabis grow schedule

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?


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.

Vegetative Stage

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.

Flowering Stage

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.

Cannabis Grow Schedule

Robert Bergman
August 31, 2020

This schedule is suitable for people growing in pre-fertilized soil. The schedule starts on the day you place your cuttings or seedlings in your grow room. Seedlings therefore already need to have a number of internodes and cuttings must be well rooted. Read the entire schedule, so you know what to expect.

In this guide you will learn

  • Week 1 vegetative stage
  • Week 2 vegetative stage
  • Week 1 flowering
  • Week 2 flowering
  • Week 3 flowering
  • Week 4 flowering
  • Week 5 flowering
  • Week 6 flowering
  • Week 7 flowering
  • Week 8 flowering
  • Week 9 flowering


  • 3′ x 3′ grow tent
  • 4 x Amnesia haze – 9 week flowering
  • 400 watt MH during veg stage
  • 600 watt HPS during flowering stage
  • Humbolt soil
  • Marijuana booster nutrients

Week 1 vegetative stage

During the first week the seedlings have to get used to the light. You should use an Metal Halide lamp during the grow phase if you have one. MH lamps provide much more white/blue light than HPS, promoting their growth. Fluorescent cool whites are also suitable but a lot less powerfull. I always use a 400 watt MH lamp during the vegetative stage.

For the first watering, give that much water that it starts to come out the bottom of the pot and wait with the next watering until the plants used all the water. You don’t need to provide them with nutrients yet, because this is present in the soil. This soil from FoxFarm or Humbolt is perfect for growing marijuana. Spray your plants every day with a plant spray to keep the humidity high.

Week 2 vegetative stage

Turn on all light if you have more and decrease the distance between the lamp and the plant to 20 inches, but make sure it doesn’t get too warm. Water your plants from underneath once, so the roots will start searching for water and they’ll quickly grow towards the bottom of the pot. You can provide a bit of nutrients, preferably with a high nitrogen content, such as grow booster or Grow fertilizer from Flower Power.

And this is the right time to top your plants if you want to. They have enough time to keep growing. Read more about topping marijuana plants. Put a fan on the lowest setting and aim it just above the plants, so they all ‘dance’ a bit. This gives them a firm stem. You can switch to flowering once the tips of the leaves of the plants start touching each other, so you know you’ll get a nice, green blanket of leaves.

Week 1 flowering

By shortening the duration of light from 18 to 12 hours the plant thinks fall is coming, and it will start to flower. It takes several weeks before the first flowers become visible and the plant will keep growing nicely until then. The need for nitrogen therefore remains high. Make sure the rotating fan is aimed between the lamp and the plant. This transports the hot air underneath the lamp and you don’t damage the plants too much with the hard wind.

Check the bottom of the leaves for any damage or discoloration because of bugs or nutrient deficits and make sure the tips of the leaves aren’t ‘burning’ = becoming yellow. This could indicate a surplus of nutrients. More about nutrients in the Plant Care section on this link.

Week 2 flowering

You shouldn’t top the plants anymore, but pruning some useless side shoots is perfectly fine. The removal of these side shoots at the bottom, makes the plant use its energy solely for the production of thick buds. Read more about pruning in the article How to prune marijuana plants. They’re growing very quickly now, so you have to check the distance between your plant and lamp every other day and adjust it if necessary. If the top leaves are starting to curl or show discoloration, it’s too warm and the extractor needs to be turned up or the distance between the lamp and plant needs to be increased.

Increase the EC with 0.1 every week and see how the plants react to this. If you make sure all the abovementioned values are correct, you should have healthy, green plants by now with thick stems and large leaves that can carry the weight of many heavy buds.

Week 3 flowering

The root system of the plant has grown out and should come out of the bottom of the pot by now. The first flowers will become visible and the plants will grow slowly for a while. They now use the maximum amount of water, so make sure they always have access to water by always having a moist soil. Feel halfway down the pot with your hand and lift them often to check the humidity.

If you’re growing with regular seeds you should have removed the males by now, before they can fertilize your females and endanger the quality of your marijuana. End this week with a good rinsing. Don’t add any nutrients to the water when rinsing, but make sure the pH is 6.

Week 4 flowering

You’ll see that little buds are starting to form all over and that the plants are getting a nice smell. The need for phosphor (P) increases, so move on to a good flowering nutrients, such as Bloom by flower power. You should’ve given your plants a good rinse at the end of last week, without any nutrients. This is to lower the nutritional value with a high nitrogen concentration in the soil and plant, so the plant can immediately absorb a good dose of phosphor to quickly let the little buds develop.

Week 5 flowering

The plant won’t grow anymore and will only focus on the production of buds. Make sure the distance between the plant and lamp is optimal; as much light as possible, but not too warm. The buds are even more sensitive to heat than the leaves. Attach a thermometer to a plant stick and place it under a lamp to see how warm it gets. Keep feeding your plants and look out for possible damage to the leaves by bugs or nutritional problems. Red stems are a sign of stress and can indicate certain problems. Check the Symptom Checker if you have any problems

Week 6 flowering

The buds continue to develop and the plant will use maximum amounts of water, nutrients and CO2. Make sure your fans keep spreading the fresh air and that your plants never get too dry. This is always a long phase and not much seems to be happening. It is a challenge to keep the environmental factors such as temperature, humidity and ventilation optimal right now. Rinse your plants again at the end of the week with water without nutrients, but with a pH of 6. This is to stabilize the EC and pH values of the plant and the soil again.

Week 7 flowering

Your buds are starting to become really big and the plant still needs nitrogen, phosphor and all other nutrients, but needs potassium more than anything. Use a product such as THC booster or Flower Power Boost to provide the plant with an optimal diet. You’ll see that a sort of white layer will arise on little leaves around the buds; this is THC. The whiter, the better, so give your plants maximum nutrients. You’ll see that the first hairs will start to turn orange.

Week 8 flowering

Your buds are making their final growth spurt, but will mainly focus on the compactness and production of THC. The bottom leaves might start to turn yellow and slowly die off, but that’s normal. It’s no nutrient deficit, so you can keep maximally feeding them. Check the buds every other day for possible fungi and yellow/brown leaves. Immediately cut off suspicious parts or post a picture on the forum and you’ll have an answer to your question in no-time.

Week 9 flowering

In order to get rid of all nutrients, you can drastically lower the EC. Provide your plants the last one or two times with only water of pH 6.0. This also rinses the soil, so you could use it for your next harvest. Keep all factors under control and properly check for rot or bugs. Discoloration of the leaves is normal and all hairs on the buds are slowly turning brown. If you feel like your plants can still use normal amounts of water and you’re not at 80% brown hairs yet, you could let them sit a bit longer. It’s also a matter of taste. Read more about harvesting in the article When to harvest marijuana plants

Follow our Marijuana soil grow schedule and maximize your strains' potential for more potent buds. We show you how to grow weed efficiently! ]]>