Seedlings light schedule?

I’ve recently had 8 seedlings sprout and I’m wondering what’s the recommended light/dark ratio at this stage? They sprouted about 3 or 4 days ago and vary between 2 - 4 inches. :slight_smile:


18/6 is the general rule, but you can shorten the photo period as much as 2-4 hours depending on personal habits, and whether you have Sativa, or Indica


16-18 hours of light and 6-8 hours of complete darkness is what is most often used for seedlings and larger plants during the vegetative or “grow” cycle, right up until you are ready to start flowering/fruiting/blooming them. A lot of people even use 24 hours of uninterrupted light on their seedlings and clones with no ill effects. Some argue with 24 hours of light that you’ll get more or faster growth. Other’s argue that the growth difference is negligible and the plants need some darkness anyway and you’ll save electricity by turning off the lights. Most seedlings are grown under florescent or low power LEDs and so the power usage is also negligible and so in the end it is mostly a personal preference. The most important thing in lighting no matter whether in the veg or bloom cycle is consistency, the lights need to come on and go off at the exact same time each day and so a timer of some sort should be used, don’t rely on yourself or someone else remembering to do it manually at the same time every day.



Thanks for a very helpful reply! i’d say for around the first week they were on 24 hours of light, can’t say one bad thing about it as the leaves are fatter and longer this time round than any other time i’ve tried, now they’re on 18/6 for about two days, second set of leaves are making they’re way up and we’ll see how it progresses from there :slight_smile:

Also I’ve tried finding metal halide lights and can’t find them anywhere, but I found Halogen lights with the same weird shapes inside the bulbs… will these work just the same as metal halide?

1 Like

No. You cannot use halogens. Don’t know where you are but the big box home supply stores carry 100-150 watt MH security lights.

As afr as phot period. As I said in other threads. Do not stress seedlings with a 24/0 photo period. You may get away with this for clones, but not seedlings. It is up to you.

Unfortunately some folks are only educated by pot forums. I learned from pot forums, then from real world agriculture experts and University seminars. It was then that I realized that a lot of info on growing pot is off base. All I can say is: You will learn as you go. Good Luck


Actually you can use halogens and you can grow seeds under 24 hours of light, especially if you are growing your seedlings under florescent lights in a controlled environment. Halogens are certainly not the best light to use and additionally you have to worry about too much UV light. Most Halogens lights have leaded glass in the bulb itself or in the housing’s glass to filter out the UV. Although a little UV might be good for your plants, too much is not good for them at all, they really won’t stand much more UV than our own skin. The question then really is more – why use halogen when they are certainly no more efficient than HPS or MH and the price of the bulbs and light systems are no more expensive to get the proper HPS or MH and as ‘latewood’ stated, HPS and MH are just as common and readily available at most hardware and garden supply stores.

As far as 24 hours of continuous light stressing young seedlings, it could be argued from a agricultural bio-chemistry perspective that the plants perform some important functions during the dark phase and overall are healthier when given some uninterrupted darkness. Ideally this light vs. dark photo period should match the strain’s natural environment. So, say for instance, in general – wild Sativa’s grow close to the equator and so they will likely never get less than 8 hours of darkness and average much closer to 12 hours of darkness, as all year long the day to night ratio stays near 12/12 being so close to the equator. On the other hand Indicas growing in more northern latitudes, generally, will have very short dark periods in the summer and very long ones in the winter, even to the point that the most northern growing cannabis, Ruderalis, may get 24 hours of light straight in the summer time, as many places in the northern hemisphere have months without any darkness during certain parts of the summer, and this is why the Ruderalis strain has evolved to “autoflower” despite what the light cycle may be. So in general you could say Sativa dominant strains need longer dark periods than other strains for their entire life cycle. And Indicas need much shorter dark periods during the “vegetative” cycle that is supposed to simulate Summer. Now this all sounds great in theory, however as all strains of cannabis have been continuously tinkered with by human breeders for hundreds, if not thousands of years, who is to say what the proper ratio is. Certainly breeders have noticed that some strains do prefer certain ratios of light vs. dark at different parts of its life-cycle that would simulate the changes in light that occur as the plant nears a 12/12 ratio as it nears winter. And again what you decide and seems to work best in your own grow with your strain or specific plant is often up to your own personal preference. However, most commercial growers don’t notice any significant difference in their yields and so don’t go to all the trouble and stick with the standard ratios of 16-18 hours light for vegetative “summer” growth with the respective 6-8 hours of darkness. and then go to the 12/12 ratio for the flowering period. I have never noticed any ill effects with seedling grown under a 24 hour light schedule, no increases in hermaphrodites nor changes in male to female ratio of “regular” un-feminized seeds. “Light stress” can cause hermaphrodites but this occurs usually when the 12 hour dark period is interrupted during the flowering cycle. As far as I know, most commercial growers will grow seedlings and clones under 24 hour florescent light, and rarely with the 16 or 18 hours of light with the alternative darkness to save on electricity(but many believe the 24 hour light gets the seedlings or clones growing a little faster having more “light” time to grow), and then they will move the plant – when it has reached a desired size – to the grow area under high intensity/output light such as High Pressure Sodium or Metal Halide and continue the “long light cycle/ short dark cycle” switching to 18 hours of light as opposed to 24 hours as the High Output lights will certainly make a difference in the electric bill. They will continue to “grow/veg” the plant for a couple of more weeks while growing the plant to the final desired size before initiating flowering with a 12/12 cycle later under the same light in the same grow area. To clarify, using 18 or 16 hours of light, or any ratio in between is mostly a matter of personal preference. Some people may prefer 17 hours of light and 7 hours of darkness for their “veg” cycle. As I stated before, the most important thing is consistency in the light schedule, so the light turns on and off at the exact same time of day for whatever schedule you are using. Keeping the light schedule consistent is the most important thing to preventing hermaphrodites. And the two things that most directly relate to the amount of your yield – would be the amount or intensity of light and secondly the proper supply of nutrients, water and fertilizer. In general the more lumens of light, the bigger the yield. And usually the biggest mistake most beginners do that will negatively effect the size of your yield at the end and the overall health of the plant is over watering and/or over fertilizing. Often less is more for the beginner and you will learn what works best for you.


Thanks again,

Ok so what would you say to 24 hours of light (under 2 90W CFL bulbs) during the seedling stage until they get to a half decent size, then when they hit vegetative stage switch them to 18/6, using the same two CFLs but with one extra low wattage halogen bulb?

The light schedule is absolutely precise every night the only time it’s been changed was when I changed from the 24 light to 18/6, because I read 24 hours is bad. But what about under CFL’s? Would changing the lights back to 24 lighting stress the plants seeing as they’re still fragile?

They’ve sprouted about a week and half now, second set of leaves should be fully up by tomorrow night or so, first set seems to be fat enough, is that fast enough progress?

1 Like

I would say leaving the schedule as you have it now is just fine. As I said, there is no real benefit from the 24 hour light cycle with the exception that you have a few extra hours of photosynthesis per 24 hour day. More often than not, the grower goes with one over the other as a matter of convenience to their style of growing more than for trying to squeeze out a little more growth per day. When you try and change the plant’s light cycle “backwards” is when you will more likely run into the “light cycle stress” type problems. So the plant, if you will, thinks it was sprouted at the wrong time of the year. The days seem to be getting longer again as if it was sprouted at the later end of winter instead of the spring/summer. So the plant kinda knows that it may have some winter freezes coming up soon or the drought of long summer and it will die before it has a chance to be pollinated by a male plant the next fall flowering season. This evolutionary stress is what has caused the plant the ability to the create hermaphrodite flowers as a reaction to this type of photo/light related stress. This way the plant will hopefully self-pollinate and create a mature seed that may survive the coming freeze and or drought.

Your idea for setting up your light schedule may work fine for the type of grow you are attempting. People who already use HPS and/or MH might not approve and they have their points, and besides it is real hard to go back after experiencing the significant increase in yield with HPS or MH, but for your setup it should be just fine. I would look into finding HPS or MH bulb if that is what your light’s housing was designed for. Wattage and ballast requirements will be different. If you are trying to use a halogen desk lamp or halogen shop spot light to supplement your CFLs, then that may work but be sure to keep all leaded glass covers in place. HPS, MH, and halogens all produce a whole lot of heat, so what ever you do keep this in mind. Your life and the destruction of your plants and or house are more important than trying to squeeze out extra yield.

1 Like

Ill keep it on 18/6 then so simply cos that made too much sense ha,

I don’t think I’m gonna go near any halogen lamps it was just a query because there’s a huge selection of them beside me.

Ok I’ve been back to the store AGAIN looking for HPS or MH lights… I CAN’T! They’re just not there!

I live in Ireland I’m wondering is there a different name on the bulbs here maybe? The store is Woodie’s DIY, they’re a big chain, so I’m sure they’re in there somewhere!!

1 Like

HPS lights are often used for lighting the outsides of buildings, in parking lot and city street lamp posts, and even public parks and athletic field lights, at least in the States. They usually do require a special housing that has a matching ballast built in or inline to/from the power supply. Sorry I don’t have any non-specialty garden supply ideas other than that. The gas inside is high pressure sodium and so I don’t think they are likely to go by another name except possibly High Intensity Discharge lights and then there should still be something specifying the specific type of HID lights, HPS or MH. In general MH is prefered for veg. and HPS for bloom/flower. But if you can only find one of the two or your budget restricts you and you have to choose only one, get HPS, that is what most experienced growers will use if limited to only one of the two.

1 Like

Being a first time grower, im gonna have ALOT of questions , so be patient with me , lol. I wish there was a way to show a picture of how im trying to do this … Pretty much the Northern light started a small root and i transplanted them to those small cardboard type little planters. 2 of them have little, tiny, tiny stems popping out of the soil and I have a led light bar for under cabinet lighting I want to use … Will this work or do i need to put them outside ?

It will work to start seedlings, but you will probably want much more light soon.

A picture and more details about the light would help.


It is a 10w led light strip. I read the ideal watts is only 3 . So I covered a few of the leds so not to have too much light and set a timer for 16/8 …

U can never have too much light as long as it’s the right distance from the plants. Good luck and grow on. People got very sciency on u. Just remember it’s a weed and will grow. What u do to it most like will affect yield and size more than life or death.

1 Like

So should I uncover all the leds’ ??? There isnt any heat coming from the light strip and its 10’’ away from the seedlings … Also is the 18/6 timer good or should I go 12/12 on the lights ?? Sorry to be a pain in the ass guys. I really need to get this right because my local buddy is gettin $75 for a qt !!! Back in the day it was only 20 bucks lol

I would use the full light and if they are seedlings I would have it about 4-6 inches until u have 2 full sets of leaves and then start raising it a few inches at a time til u reach around 16-18” for an LED u don’t want your plants reaching for it and u want good node spacing.

Is this normal ?? Is it too much light, over or under watering ???

Your soil looks very wet. Let them ask for water before watering. I would carefully remove the casing from the seed also so light can get in


3w? Someone was yanking your chain there, especially as only maybe 3 of your leds will be having any effect on the seedlings.

This little blighter has been under a 14.5w 6500K deglobed LED replacement bulb since she broke ground 9 days ago, and that lamp is half the distance away that your strip is, and this is how she looked on Sunday, 7 days after breaking ground. She’s obviously grown more since then so uncover those leds and get the strip closer or you’re going on a one way trip to Stretchyville.

1 Like

Also, you will need 60 of those if that’s all u plan on using. Please let @dbrn32 help u with lights. I wouldn’t want u to have major problems throughout your grow.

1 Like