Cannabis Seeds Light Cycle

Learn how to use the best light cycle for weed during the seedling, flowering, and vegetative stages to help your cannabis plant produce excellent yields. 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. How much light cannabis seeds need depends on where they are in their growth. If they have not yet germinated, then they need…

Ideal Light Cycles for Weed’s Growth Stages

Cultivating cannabis can be a challenging adventure, especially for beginners. However, you can produce a gratifying yield if you understand and master one critical element in the growing process: the light cycle for weed.

Marijuana plants need light, lots of it, to grow big and strong. When you cultivate indoors, you must mimic the amount of light that plants get in nature.

Timing is also essential. Ensure your plant gets the optimal amount of light at the right time during its growth stages.

The weed light cycle you implement will determine the health of your cannabis plant and the yield it delivers.

In this straightforward guide, you’ll learn how light affects your plant’s growth, how much light to give it, and the best time to do it.

The science behind the light cycle for weed plants

To understand the science behind the light cycle for weed, it’s essential to know that while cannabis plants need light for growth, it’s the amount of darkness that influences their development.

Unlike autoflowers that automatically transition from the vegetative stage to the flowering phase when they reach maturity, photoperiod weed requires a certain number of hours of uninterrupted darkness to do the same.

When you change the weed light cycle of a photoperiod marijuana strain from 18 hours of light and 6 hours of darkness (18/6) and give the plant 12 hours of the latter (12/12), it begins to flower.

You can adjust the schedule of your grow lights for indoor plants to mimic what happens in nature when the seasons change and the number of daylight hours get shorter.

Why marijuana lighting cycles help you get bigger yields

Your cannabis plant needs light to thrive, and using optimal marijuana lighting cycles will maximize the quality and quantity of your harvest. Knowing when to apply the schedules also matters.

There are two weed plant stages where light is critical, namely:

The first phase is when your plant focuses on growing bigger and taller, and the amount of light it gets directly corresponds with the size it grows to. More light means larger plants.

By manipulating the cannabis light cycle and giving your crop more light during this period, you’re helping it grow better and potentially increasing its future yield.

The minimum amount of light to encourage healthy growth is 18 hours. That’s how long you should keep your grow light switched on every day.

If you want larger plants, you can switch to a 24/0 marijuana light schedule, giving continuous light throughout the day. However, this schedule may cause your crop to develop too fast.

Weed light cycle for each growth stage

What makes photoperiod marijuana plants unique is that they need different light levels during each growth stage.

Here’s a brief explanation of the light cycle for weed at each growth stage:

Light schedule for seedlings

If your climate is suitable for seedlings, you can cultivate them outdoors. Alternatively, you can grow them indoors and wait till it’s warmer before you move them outside.


At the seedling stage, you can use inexpensive grow lights such as LED or fluorescent bulbs. What matters is that you provide your seeds with lots of light.

The optimal light schedule for seedlings is 18/6. Therefore, leave your grow lights on for 18 hours and switch them off for the next six. Having a timer will save you the trouble of doing this manually for the duration of the stage.


Nature should do its job when your seedlings are outside. If they don’t get enough sunlight for 18 hours, use artificial lighting for the rest of the period.

Cannabis vegetative lighting

During the vegetative stage, the cannabis light cycle you use should focus on giving your plant plenty of light to help it grow big and healthy.


When cultivating indoors, make sure you use at least the 18/6 light schedule for weed. Keep your grow lights on for a minimum of 18 hours and off for six.

You can give your plant up to 24 hours of light if you want a larger crop. However, we discourage using the 24/0 schedule because your plant can develop too quickly with that much light.


The marijuana light schedule for outdoor plants is based on the available number of hours of daylight. So, be sure that your plant will get at least 18 hours of sunlight if you cultivate outside.

Alternatively, you can keep it indoors under grow lights until the climate is suitable before moving it outside.

Flowering light cycle

The flowering light cycle refers to the amount of darkness that stimulates marijuana plants to transition from vegetating to producing buds.

In nature, this phenomenon occurs when seasons change. As summer moves into fall, the days get shorter, and there are more dark hours, signaling cannabis plants to start flowering.


When growing indoors, you can nudge your plant to begin budding by changing the weed light cycle, mimicking the change in seasons.

Set the timer for your grow lights to switch off and on every 12 hours. Using this 12/12 cycle, you can expect your plant to start bud production for 8–12 weeks. Some strains have a shorter flowering period.


There’s very little to do except to let nature take over. The flowering light cycle begins automatically when fall approaches.

In late June, the shorter days and longer dark hours will cause your cannabis plant to flower, and within weeks, you’ll be able to harvest beautiful nugs.

Autoflower light cycle

Marijuana cultivars that are autoflowers will begin producing buds when they attain a certain maturity point. They don’t depend on any light cycle for weed to signal them when to bloom.

So, it’s not critical to have an autoflower light schedule if you cultivate these strains.


You should be aware that autos are typically smaller than photoperiods because of their shorter vegetative stage. As such, if you want a robust plant and better yield, give it lots of light.

The 18/6 flowering light cycle is the optimum schedule to implement. Growers have also seen excellent results when giving their crop 24 hours of light.

Some cultivators stick to the 12/12 cycle, which triggers flowering in photoperiods to save energy costs, but this schedule produces smaller buds than the 18/6 and 24/0.

There’s no hard and fast rule. We suggest you experiment with the various weed light cycles and choose the one that meets your requirements.


It’s best to leave it to nature if you cultivate outdoors. Alternatively, you can decide on a light cycle that you think will give you the results you desire.

If there’s not enough sunlight for your chosen marijuana light schedule, you may use grow LED lights for cannabis to make up for the lost hours.

FAQs related to light cycles for growing weed

If you have specific questions about marijuana lighting cycles, the following section may have the answers you want.

Alternatively, you may get in touch with us if you have additional inquiries.

When should I change my light cycle to 12-12?

As cannabis plants switch from vegetative mode to flowering when there are more dark hours than light, you should set your weed light cycle to 12/12 when you want your plant to start producing buds.

Can a weed plant get too much light?

Although marijuana plants love light and need it for their growth and development, too much light is not recommended in some cases.

For example, while an 18/6 marijuana lighting schedule during the vegetative stage can boost your plant’s growth, giving it 24 hours of light may cause it to develop too quickly.

Too much light can also hurt your cannabis plant if you place your grow lights too close to it. The intense heat can burn its leaves and potentially destroy your crop.

How long can seedlings go without light?

Seedlings need at least an 18/6 marijuana light schedule to develop into a healthy plant. As such, it’s advisable not to let them go more than 6 hours without light at this stage.

What’s the best weed light cycle?

There’s no best light cycle for weed per se. It depends on your cultivation goal and when you implement it during your plant’s growth stages.

However, if you apply the fundamental principle that marijuana plants need lots of light to grow big and strong and at least 12 hours of darkness to start flowering, you’ll know which cannabis light cycle to use and when.

See also  Cannabis Seeds Washington State

Check out our blog for more cultivation tips from expert growers.

About the author: Derek LaRose

Also known as Kronic from The Cannabis Kronicles, Derek LaRose is a young ambitious cultivator and a staple educator for indoor cultivation.

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.

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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!

How Much Light Do Cannabis Seeds Need?

You want to make sure they sprout, because any seeds that go to waste cost you a lot of money.

To maximize the chance of success, seeds need the right conditions.

They need the right amount of light, the correct temperature and the correct amount of moisture.

If you provide what they want, the changes of successful germination skyrocket.

If you do not provide the ideal conditions, the chances of success plummet.

We’ll tell you exactly what you need to do to give your weed seeds the best chance at becoming a strong and healthy marijuana plant. And we’ll start with the question that likely brought you here: the light requirements.

Note: if you’re looking to buy seeds, we don’t sell them, but we recommend this store (they have a germination guarantee, which is a nice touch). We caution against using bag seed.

How Much Light Do Cannabis Seeds Need?

Cannabis seeds need no light when they are germinating. In fact, they require an absence of light. All of the methods below call for darkness.

Once they have sprouted, they will need a lot of light—18 hours a day, to be exact (though you could even give them 24 hours of light per day).

How To Germinate Cannabis Seeds

There are several methods for germinating marijuana seeds, each with their pros and cons. We will cover the best methods below, with the first method offering the best chance of success and the last one offering the lowest chance.

These seeds are expensive, so we recommend using the method with the best chance of success, so you reduce the risk of wasting seeds as much as you possibly can.

Germinating Cannabis Seeds In A Propagator With Peat Pellets

You get the highest success rate when you provide the perfect environment for germination. A propagator ensures optimal control over the environment.

  • best chance of success
  • once you have the propagator, you can keep reusing it
  • need for equipment/higher startup cost
  • not as simple as some other methods

There are various types of propagator on the market: some just have a plastic dome, some have seed starter trays, some have heating pads and some even include a grow light. Here is a low cost propagator that includes a seed starter tray.

Fill each hole in the seed tray with a peat pellet. You can buy them at any garden store or get them online. This bag of pellets from Jiffy is a great deal.*

To use the pellets, simply soak them in water for around ten minutes. They will expand.

Once expanded, poke a little hole about half an inch (1.5 cm) deep into each pellet. Place one seed in each of the pellets and cover the seed up. It needs to be beneath the soil in darkness.

Make sure you keep the propagator warm, at a temperature of 68° to 82° F (20° to 28° C) and the seed pellets moist. They can never dry out or the seed will die.

Note that peat pellets are great for planting in soil or coco coir, but they do not work with a hydroponic setup. For that, you should use rapid rooters.*

Rapid rooters actually work great for any type of setup, but we prefer peat pellets for non-hydroponic setups, because they cost less and they do not dry out (since rapid rooters come in bags of 50 or more, if you do not use all 50 fairly soon, the remaining ones will dry out and become useless, though you can reseal them to give yourself a bit more time).

Whether using peat pellets or rapid rooters, this method ensures a high rate of success, but it does require the purchase of a propagator and the pellets or rapid rooters.

You can save a bit of money by skipping the propagator and just rigging something up yourself (or not using any type of covering at all), which is what we’ll cover next.

Using Peat Pellets With No Propagator (Or With A DIY Propagator)

If you are on a bit of a budget, you can forgo the propagator and just use peat pellets on their own. You could even make a DIY propagator by simply using some kind of plastic cover.

  • lower cost than using propagator
  • peat pellets (or rapid rooters) still offer the best chance of success, even without a propagator
  • less control than with propagator
  • if using a DIY propagator, requires time to construct

A great DIY solution is to use simple plastic cups, one for each peat pellet. Cut the top off a small plastic drink bottle and place it over the peat pellet with the seed as a dome.

But you don’t need any propagator at all.

Covering the seeds helps keep in the moisture and makes it easier to keep them warm, but it is not necessary. You can just as easily keep the peat pellets uncovered, as long as you ensure they stay moist and are kept in the correct temperature range.

Germinating Cannabis Seeds Using Paper Towels

This is probably the most written about method, but that does not make it the best. It is easy and you won’t need any additional equipment, but it requires handling the delicate seedling.

  • very easy
  • no additional equipment required (assuming you have paper towels)
  • requires transplanting the seeds, which risks damage
  • need to ensure the paper towel stays damp, but not wet

The only thing you need for this method is a paper towel, although I would also recommend using two plates.

For the paper towel, you actually want to use the cheapest brands. More expensive towels are more porous, which makes it easier for the delicate root to get stuck and tear off when transplanting the seedling.

For this method, place a paper towel on a plate and get it nice and wet. Drain off any excess water, though. It should be damp, but there should be no standing water or the seeds can drown.

Put your seeds on one half of the paper towel and fold the other half over them, so that they are covered. Then take the second plate and put it upside down on the bottom one, forming a dark cavern between the two plates for the seed to germinate.

Check once or twice a day to ensure that the paper towel never dries out. If you need to add water, make sure that you always drain out any standing water. Keep the seeds covered and at the correct temperature. They should sprout in a few days.

Once they have sprouted, you’ll want to transfer them to soil or a growing medium. See below for instructions on how to do this.

Germinating Weed Seeds In Soil

This method is the easiest, since you simply let the seeds germinate in the same place where they will grow afterward. Not having to transplant the seed after it sprouts means you don’t risk damaging it causing it shock that will slow growth. The main drawback is a lower success rate.

  • easiest method
  • no transplanting required
  • no additional equipment required
  • lower success rate than other methods

All you do for this method is poke a hole in the soil or growing medium that is about half an inch deep (1.5 cm). Place the seed in the hole and cover it up. Ensure that the soil or growing medium is moist, but not soaking wet. The temperature needs to be in the correct range as well.

Germinating Marijuana Seeds In Water

This is another easy method, but it does require transplanting the seeds once they have sprouted.

  • very easy
  • no additional equipment required
  • lower success rate
  • often seeds will not sprout in time and you will need to use a different method as a backup (or lose the seed)

For this method, simply fill a glass with warm water and drop the seeds inside. Store in a dark and warm place for 12 to 24 hours.

You should see the tap root poking out of the seeds by then. If not, you’ll want to continue germinating the seeds elsewhere, perhaps in soil. If they are submerged in water for more than 24 hours, there is a risk that the seeds can drown.

My Seeds Germinated, Now What?

Once the seeds have germinated, it is time to transplant them into soil or a growing medium. Be very careful not to damage the delicate taproot. Ideally, use tweezers to handle the seeds, to avoid any oils from your fingers doing any damage.

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Plant the seeds about 1 to 2 cm deep, so that it does not require too much energy for the stem and first leaves to pop up through the soil.

Make sure the soil is moist and the seeds are kept at the correct temperature of 68 ° to 82 ° F (20 ° to 28 ° C) . Your little plants should pop out of the soil within a few days.

At this point, the seeds will want light, and lots of it. Even if they are still beneath the soil, you can go ahead and turn on your grow light.

If you do not have a grow light, there are a number of different types you can consider. For seedlings, fluorescent lights or LED light bars (like the HLG Propagator Cloning Lamp) are ideal, unless you are germinating a lot of seeds at once. Then you might want to consider a larger LED grow light.

Best LED Grow Lights For Starting Seeds

The best LED grow light for seedlings is the NextLight Veg8 Pro. It was especially designed for seedlings, clones and vegging, with a separate “clone” mode for clones and seedlings. The main drawback is that it is made to cover a 2 by 4 foot area. If you only have a few seedlings, this light will be overkill.

LED bars are the best LED lights for seed starting when you don’t have enough seedlings to fill a 2 by 4 foot area. The Secret Jardin bar is a great choice. It is inexpensive and will give off enough light to get your plants through the seedling stage in no time.

Another great option is the HLG Propagator Cloning Lamp which you can get for a very good deal if you’re buying a few of them as they come in packs of 1,2 or 4. It’s an inexpensive option for a larger cloning area and HLG is one of our favorite LED Co’s

HPS Or MH For Seedlings

You can put seedlings under HPS or MH light, but I would only suggest this if you already have the lights. It is more cost effective to use LED or fluorescent lights.

The only time MH or HPS really makes sense is if you keep your plants in the same space from seed to harvest, i.e. you do not have a separate area for seedlings.

Metal halide light is better for seedlings than HPS light, since they need cooler light with more blue light than red.

When To Put Seedlings Under MH or HPS

You can turn the grow light on once the seeds have sprouted and they are in the soil or growing medium. Even if the plant is still not visible, the heat from the grow light will actually help warm the soil, which encourages the plant to grow.

How Long From Seedling To Vegetative

It usually takes from 10 to 15 days for seedlings to transition to vegetative growth, but it is difficult to give an exact timeline. It just varies so much from one strain to the next and from one growing environment to the next.

How Long Do Weed Seeds Last?

If stored correctly, marijuana seeds could last up to 5 or 6 years. That said, the older they get, the lower the chance of successful germination and the longer it takes, even if it is successful.

To store your seeds for the best results, keep them in a cool, dark place. A basement works well, as does a refrigerator. We recommend these storage containers to best preserve your seeds.

Where To Get Seeds

We often get customers asking to buy seeds from us. Unfortunately, we do not sell seeds ourselves, but we highly recommend the seed store from HOMEGROWN Cannabis Co.

They have a huge selection of quality seeds and their prices are excellent, especially when you snag one of their deals.


My seedlings have been in their peat pellets since Jan 10. Two have the two sets and two baby leaves below coming in. Two are a bit leggy.
I’m going to move to a larger pot. Probably a coco pot.
I’ll place the pellet inside the bigger pot and remove the netting first. I’m a gardener and know that it’s not really biodegradable. It may over the years but I always pull them out of my compost..
what type of soil should I put in the first bigger pot I put them in? Before last transplant to their final home?
Thank you!

How many lumens is a abundance.

So I did the water cup method and got a little bit of a tail after ~48 hrs, and popped the seed tail-down in a peat pellet. Do I want to put it under a light now before it sprouts or wait until it sprouts to put it under a light? If I put it under a light now what should the schedule be for the lights before the seed sprouts?

Thanks for the detailed info…much appreciated…

To answer some of your comments –
As soon as the seeds germinate (the shell cracks open and the tap root known as the radicle begins to show) you can pop them into your substrate, coco is superior to soil IME 20+ years growning.
Once propagating you can get them under blue light even if they have not sprouted on 18/6 is a reliable schedule until they start to veg, the light should be hung about 1m from your seeds.
Monitor temperature, humidity and substrate miostness as this will be crucial for your stouts to start off strong.
You will want you temperature to be between 20 – 25°C, humidity at 45-50% and your substrate should be consistently moist not wet or dry with the ability to drain. do not pack down the substrate and gently cover the germited seeds as this will give both the roots and plant space to push through the substrate and allow airflow which will help establish a strong root system.

Once the cotyledon (first round leaves) emerge they occasionally have the seed casing attached make sure you allow it to fall of naturally don’t pull it off.
At this stage you can drop your blue light down to 46 – 60 cm from the cotyledon depending on the light you are using. Maintain this and only water to retain moisture allowing top inch of substrate to dry before watering again until you have a couple of sets of true leaves.
At this stage you plant will be strong enough to survive transplant into larger pots.
If you are using Autoflowers plant the propagation plug in their final pot, depending on the strain grow space this will vary between 10L – 15L pots, I personally recommend Airpots if you’re after very healthy plants. (healthy plants yield bigger)
If you are using photoperiod this will also be different depending on strain, grow space and how long you veg for, they can be transplanted into gradually larger pots ending up in anything up to 20L.

Once your seedlings have been moved and they have developed 5-7 true leaves this is a good indication they are happy and beginning the veg phase. You can maintain 18/6 or move up to 20/4 on lighting schedules some recommend 24hr but again IME plants still need a few hours of darkness to recover.
At this point you can maintain the blue light at around 46-52cm height and begin feeding with nutrients. I recommend Canna Bio and make sure the first few times use a diluted solution just to allow the plants a chance to adapt to their new diet. After a week you can switch to full strength feed and growth will explode dramatically.
For Autos they will flower when ready or mature enough hense the “Auto” then you can switch to red or a combination of red and blue light all the way through until harvest.
Photo’s will stay in veg under blue light as long as you keep them there and will just keep bushing out until you switch to red or combination light telling the plant that flowering will begin.
During flowering light height will depend on the strain you are growing and light you are using x square metre grow space but during flowering roughly 36 – 40cm from canopy is a good benchmark. With LED units you can go a little closer as heat is not as much of an issue but make sure your whole plant is getting equal amounts of light.

Maintaining adequate light, humidity, temperature, substrate moisture, nutrients and root air flow are what will help give you your plants full potential, big dense nugs and beautiful smell and flavour.

Hope this helps clarify a few things for the first timers, 19oz on one auto and at least double that on a photo dry weight were some of my best yields so look after your babies and they will look after you