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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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94 thoughts on “Marijuana Vegetative Growth Pruning

  1. kimmie says:

    first time grower how do you know when to start to flowering have 3 2-2half feet tall looking good.seem to be doing good have also 6 little one 6-8in. just top them for hight and more bud all in home made in the corner of basement two walls covered with foil dull side showing and top 2 walls instalation duct taped together makes nice 4x6x8 room have heater,fan,humidfer,temp.at 75 degree and 60% humid leaping frog soil (good loose drain soil like it..) should i make another box put the 3 bigger one in bud or just wait till the 6 get bigger and do at the same time 6 week and 3 weeks old .trying very had p.s. only using 4-6-3 dr. earth organic tomato,veg and herb fertilizer.just put seed in soil keep damp all grew.

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