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Marijuana Vegetative Growth Pruning

Marijuana Vegging and Lighting

Growing marijuana has two major stages: vegetative and flowering.  During the vegetative phase your plants will establish a solid root system and full leaf development to prepare them for the final flowering phase, the actual marijuana buds.

Marijuana will stay in the vegetative growth cycle if it is given anywhere between 18 and 24 hours of light per day. The vegetative growth cycle is usually only two to four weeks long in most indoor strains and will be completed in the third to fifth months of growth depending on the marijuana strain and conditions.

Marijuana Vegetative stage

The marijuana plant can only grow as fast as it can produce energy required for new growth. The more large leafs it has the better it is a photosynthesis and so grows faster and faster. With perfect hydroponic conditions strong marijuana strains can grow up to six inches per day. Once you are past the middle of the vegetative stage the number of blades on each leaf will decline and the stem (phyllotaxy) changes with internodes, the space between two sets of leaves on the stem, will start to decrease in length.

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Lighting for vegging

Marijuana requires about 14% of the light spectrum to consist of blue light to prevent stretching. Insufficient blue will lead to spindly, weak cannabis plants with long internodes (the vertical spacing between branches.)

High Pressure Sodium (HPS) lamps generally only have 3-6% of their spectrum in blue which is why they are generally not recommended during the vegging phase.

The most popular choices are T5 fluorescents and Metal Halides (MH) and full spectrum LED grow lights. Micro growers often use compact spiral fluorescents (CFLs). They work fine, but are not nearly as efficient (light to heat ratio) as T5s, LEDs or MH. Relatively new to the scene are LEDs with the top models working brilliantly. Another new entry to the field is Ceramic Metal Halides (CMH) which display a fuller spectral distribution than standard MH.

Marijuana Vegetative stage MH lighting

How much light do you need?

That is not an easy question to answer because it depends on your plant spacing, genetics, state of maturity, room temperature, the distance of the lamp to your plants, the reflectivity of your walls, the efficiency of your lamp and so on. A general guideline for MH and T5 is to have a minimum of 50 Watts of power for each square foot but you can get away with far less using LEDs.

Your lights should be on from 18-24 hours per day. If you choose less than 24 hours, a timer is highly recommended to keep a regular schedule.

A big part of your lighting is capturing the photons that are not directed to your plants. Covering the walls with flat white paint, mylar film, or Reflectix all work well.

Marijuana grow room conditions

Be sure to keep your grow room between 70°F– 85°F (21.1°C– 29.4°C) degrees during the day. It is OK to have a 5-15 degree drop at night. Humidity should be in the 45 – 55% range. Get plenty of fresh air and keep it recirculating. More about the correct temperature for marijuana.

Cannabis nutrients

Cannabis requires three primary or macro nutrients for optimal health. These are Nitrogen (N), Phosphorous (P) and Potassium (K) or NPK.

All plant-food labels include the percentage of these three elements in numerical form and are presented as N-P-K levels. The optimal ratios for growing weed are up for debate. More about marijuana nutrients is here.

Marijuana Vegetative Stage nutes

Micronutrients are equally important, but only very tiny amounts are required and many, if not all, are readily available in potting soils without the need to add extra. If you are growing hydroponically, then these must be added regularly. These micronutrients or trace-elements  are: Calcium (Ca), Magnesium (Mg), Sulfur (S), Manganese (Mn), Boron (B), Zinc (Zn),  Iron(Fe), Molybdenum(Mo), Chlorine(CL) and Copper (Cu).

The second biggest damager of your weed after overwatering is over-fertilizing. No matter how many times this is stressed, new growers seem to have the need to ‘push’ their plants. More is not better. Excess nutrients will ‘burn’ your cannabis plants. A sure sign of ‘nute-burn’ is that the tips of most of the leaves will turn brown. This is how your plants attempt to void the extra chemicals.

marijuana nutrient burn
For very young plants (1-3 weeks) use nutrients at ¼ of the manufacturer’s recommended level and then s-l-o-w-l-y increase over time to full strength. Note that every cannabis strain has different requirements and tolerances and that the manufacturer’s recommendations are only a guideline. If you observe your plants carefully, you will learn what they need.

Less is better than more, as less may slow growth a little, but more can stunt or kill your marijuana plants.

When to switch to flowering (budding)

Once again, there is no correct answer. Some growers switch at two weeks to create small, single bud cannabis plants; others will veg their plants for up to four months to create monsters. The variation is enormous and depends largely on your goals and setup.

It is important to note that your cannabis plants will ‘stretch’ or continue to add height after the flowering phase is induced by switching your light cycle regimen to 12/12. The amount of stretch is strain dependent, but the general rule of thumb is that indicas will double in height and sativas will triple in height and hybrids will be somewhere in between. This is useful for determining the maximum height to ‘veg’ before switching. Let’s say that your HID light must be no lower than 16” above your plants and your container size is 12” and the highest the bottom of your lamp can be is 84”’ and you are growing a sativa. Your actual vertical plant growing zone is 84” (lamp height) – 16” (lamp distance to plant) – 12” (container) height = 56” / 3 = 18.7”. This gives you the maximum height for your sativa plant before switching to flowering.

In the same setup, an indica strain would require 56”/2 or a switch at 28” in height.

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93 thoughts on “Marijuana Vegetative Growth Pruning

  1. Luke says:

    Hi there, I’m a first time grower, I have 3 rooted cuttings that I have under a low energy light bulb in my growroom, I’ve got a 600w dimmable ballast, inline carbon fan/filter, but I don’t have an air cooled shade yet, just the ordinary MH, my wonder is, that my cuttings are more than ready to be put into 11ltr pots with my medium soil, will it be alright to put them straight under the 600w light when I have reported? Also how long is the growing phase with cuttings? I’ve had them under a low energy light bulb for a couple of weeks now and I’m unsure when to nip the tops and start budding, I’m new to this, so I’m sorry if I seem a bit dim, lol, any advice would be beneficial as I am very new too this, thanks very much

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  2. Garls says:

    Are the metal halide ‘uv block’ globes helpful in the growth of plant , running 7 150w MH , 2 are uv block globes , I think ..any help would be appreciated , cheers Garls ?

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  3. Brian says:

    Ok I got a situation and could use advice. I have a tent and my 10 plants are under 2 600 watt mh lights the issue is I used the timer I normally use for flowering in the veg stage so the plants have been growing for two weeks on a 12/12 light cycle.
    Should I just leave it and change the bulbs to hps soon or do I change the light cycle now and then back in a few weeks then change bulbs?

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  4. mark morang says:

    I have a 4x6x7 grow box I bolt in my basement. One side is open and the roof on the same side opens for easier access. I want to grow 4 kaya gold in there. The space used to be lighted with a boat load of CFL’s. Now using 1000 watt hid set up. Never used hid before any suggestions would be great. Thank you in advance

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    • Steve says:

      I have grown with HID for years. Lot of heat but great results. There are a number of bulbs with varying color temps. Get the right ones. You need the blues for veg and orange for flower. I switch bulbs when switching to 12/12

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  5. Alexander Kushington says:

    Would I stress or cause my plant to become a hermaphrodite if I switch the light cycle from 18/6 to 24/0 ? New to growing help please

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    • Artemis2318 says:

      Hi! I know this is late. But I think 24 hrs is okay if your plants are still on their veg period. Btw, Im also new at this, Id like to ask how did your plants turned out? And which light sched you think is best?

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  6. Devigod says:

    Hey guys im a first timer also and im using a 300w (rms) full spectrum LED med grow light 18/6. Its been about 2 months now they were beautiful in the tent i had them in with light about 15cm above 10 babies. I had to move houses all of a sundden and i lost half of them in the move. And the rest have been stunned for a while i slowly brang them back 2 sun for 3 days and now the remaining are back under the LED and also 2 100w 2700k fluorescent lights because i hearf i need more 2700k spec lighting for veg. They are just starting to grow again noticeably and they are about 3momths old now. Any advice on what to do from here? They are roughly 10inches high some are bushy and others not so much. I have also tried the pinch technique on half to the main top of the plants to promote 2 tops of goodness in flowering. I am always learning and gathering more info about growing and i am serious as f**k about it. Any help would be hugely appreciated. Please no time wasters… ☺

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    • Buddy Buddy says:

      Hi Devigod
      If you are serious as fuck about growing why not join our forum. There’s lots of growers there who may be able to answer your questions and the other questions that are bound to follow. http://forum.howtogrowmarijuana.com/
      Just a comment to make on your post. 2700K will help your plants grow but is warm white and mostly in the red spectrum and normally used for flowering. Around 6500K – cool white- is the one for veg. Why not get a dual spectrum bulb that will help with both.

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  7. christopher harvey says:

    first time grow ever and I’m having difficulty figuring out the lighting. I am usually good at understanding and implementing suggestions or ideas. But regardless what I try with the lights it doesn’t appear to b a proper height or I’m using the wrong light. Everything I read about lighting seems so technically and complex I need it broken down for me in plain simple English. Please help I’ve got white widow going

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    • Reoz5@aim.com says:

      I have had only 4 grows on my own but have helped others with outdoor grows over the last 5 years or so and have smoked herb since 2000. I am 31 now and have an associates in Horticulture err,, Close enough I’m about to grad here in about 2 terms. (just being honest) I am furthering my schooling in integrated pest management probably at Oregon State. Or I may move to California to finish my schooling. There can be a lot to digest when you first look into lighting. Don’t let the spectrum 410-450, 700-770, bring ya down. When you first start your grow just grow. The little tweaks are things you can adjust when you are familiar with your crop. Each variety of strand is different.

      Veg is blue light not actually blue
      flowering is Red light Not actually Red

      Red and blue refer to the spectrum of the light needed for the plant for optimum growth. But a 75W bulb would do through the entire grow. Poorly but it would do. So don’t beat yourself up just be patient. You really have to mess things up to really mess things up. And as long as your plants aren’t touching the lights than there will be plenty of other things that can hurt your grow before the lighting does.

      So first things first. Are you using a High pressure sodium (HPS) vegetative set up, LED, Or fluorescent. And why your at it if you can, you should probably tell me a little about your medium (what are you planting in) situation Soilless mix? Like a pre mix soil, or hydroponics water fed system ect. Then I can better answer your question

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  8. King smoke says:

    Sence a plant can go in revegitative stage an turned on to 18-6 or 24/7 wat cycle would be best to do it at cuz I’m a first time grower an have had sum great plants an I learn more an more…I read everything I can to learn but I try to find people that have a lot of experience in growing

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  9. denise says:

    I have thrips. Getting ready to flower. Any advice for spraying? Please and thanks.

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    Rating: +4 (from 4 votes)
  10. mario says:

    ok, first time grower, i’ve been growing for 2 weeks and 3 days in soil, light is 5 watt pure white diod (heard diods were good and cheap choice). 24/7 light, pouring a little water 1 time per 2 days, i keep the soil a little wet. Anyway, all is good, i’ve got the 2 bunny leaves, 1 set of blade leaves, and 1 set just started growing. The problem is the height..its leaves are developing, but it’s height is not, almost 3 weeks and its about 5 cm tall …. is anything wrong? thanks

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    • old timer says:

      Mario, you’re watering wrong. Marijuana likes to be wet then dry hence, you should water the crap out of them until there is visible water in the tray under the pot. Then do not water again until soil is thoroughly dry on top and just barely moist 2-3 inches down, now it is time to water again. Mine in 5 gallon cloth pots get watered about every 3-4 days, it depends on temperatures and humidity. I live in a dry climate so humidity is not much of a factor but temps. always are as it can get pretty cool at night here. If you keep the soil moist all the time you will develop fungus gnats (little black flies that live in the soil) guaranteed! Check the weight of your pot when bone dry then water it and observe the weight. When it is light add water. Also use a ph balanced nutrient like SensiGrow and SensiBloom. If you already have gnats, they will die out once the soil drys but you will need to kill the lava in the soil, do this by adding 16 oz. of hydrogen peroxide to a gallon of water and water with this the next time you water. This will kill the larva and oxygenate the soil.
      Good luck and Happy Growing

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