When To Begin Flowering Indoor Cannabis Plants

Timing is everything, and knowing when to start flowering marijuana plants can make or break an indoor harvest. This may seem remedial for many experienced growers, then again, I’ve seen plenty of “pro growers” with burnt plants growing into their lights.

First, let me say that there is no easy answer regarding when to start flowering indoor plants. There are a lot of variables including the strain, your grow room setup and your growing style. That being said, there is a lot I can share on the topic that will better equip you to evaluate your situation and help you decide when to begin flowering.

I should point out that the question of when to flower is usually measured by two metrics time and/or height. Let me explain.

Weeks of Veg = Worthless

Cannabis plants are often measured by their time in vegetative growth. For indoor growing this usually refers to the number of days or weeks that a plant has spent under 18 or 24 hours of light. You will often hear growers refer to plants as being in their 1st, 2nd or 3rd week of veg. This of course is a bit too vague to be of actual use, at least without further information.

Clones

A plant grown from a clone (cutting) is generally regarded as beginning its 1st week of vegetative growth when it has established roots and been transplanted into the growing medium. Most often this takes place when the clone is between 2 and 6 inches tall. While this seems pretty cut and dry, clones of the same strain can develop at drastically different speeds depending on root development. Differences in growth can become even more pronounced depending on the light, nutrients, co2, temperature and a myriad of other factors. After only a few weeks, two previously identical looking clones may look nothing alike.

Seeds

Plants grown from seeds make the measure of age in vegetative weeks a lot more confusing. Some growers refer to a plants 1st week of veg as when it emerges from the soil, some say it is when it produces its first set of true leaves. Still other growers will refer to week 1 as beginning when the seedling is transplanted, which in itself tells us very little. As if this did not confuse the matter, plants grown from seed often grow vertically much slower than clones, at least at first. Later plants from seed often outpace clones once their root systems have become established.

It should be obvious why telling someone to flower plants after some arbitrary number of week of vegetative growth is worthless. Yet, I constantly hear experienced growers coaching others to change the light cycle and begin flowering after 2 or 3 weeks of veg. A three week old seed start may be as little as 4 inches tall, while a clone with 3 weeks of veg could stand over two feet tall.

Plant Height

The second common way to know when a cannabis plant is ready to begin the flowering cycle is the actual height of the plant. This measure has significantly more relevance because the height of a plant is objective and universally understood. Combined with other known quantities like the strain and the size of the grow room, height is a good indicator of when it’s time to flower. A plants height can be measured by stretching a measuring tape from the top of the growing medium to the highest point on the plant. It’s likely that there will be some variation from plant to plant, more on this later. While plant height is useful information, the key is knowing how much room you actually have to work with.

Grow Rooms Size Restricts Maximum Plant Height

The size and style of your grow room will ultimately determine the tallest possible plant height. From this you can deduce the tallest height to begin flowering. Most rooms in a residential home have 8 foot ceilings but many growers use attics, basements, outbuildings, grow tents and I’ve even seen grow rooms built into filing cabinets. What’s important is the distance from the top of the growing medium to the bottom of the lights reflective hood. Many grow rooms are set up so the height of the grow light is adjustable. In this case measure from the lights highest possible setting.

Once you know the distance from the growing medium to the light you need to take into consideration your lighting setup. High Pressure Sodium (HPS) and Metal Halide (MH) grow lights get very hot and can damage the vegetation and even start fires if the plant grows to close to the bulb. It’s important to keep the plants some distance from the light so they don’t burn, but not so far from the light that the lumens are severely diminished. For further explanation on this, read Grow Light Intensity Principles. Light movers and ventilated ducting on your lights can reduce the heat and allow the plants to grow closer to the lights. As a general rule, plants should not be allowed to grow closer than about 12 inches from the bottom of the HID lights reflective hood. For those using fluorescent lighting which produces much less heat, the tops of the plants can grow as close as 4 inches from the bulbs. For more ideas on managing the heat from grow lights read Heat Management in Grow Rooms.

Assuming that you are using one or more 600 or 1000 watt HPS or MH lights, take the distance from the growing medium to the bottom of the reflective hood and subtract 1 foot. This will give you the absolute maximum height of your tallest plant at the end of flowering. This does not necessarily mean you want your plants to reach this maximum, but it does give you a concrete parameter to work within.

Strains

The strains your are growing plays a huge part in how tall to grow your plants before switching to the 12/12 flowering light cycle. Find out as much about the genetics and typical growth characteristics of your strains as possible. Indica and sativa dominant strains often display very different growth patterns and behave differently after the  light cycle is switched to flowering.

Indica Strains

Most pure indica strains slow their vertical growth very quickly once switched to a 12/12 light cycle. It’s not uncommon for an indica strain to add only 25% to 50% to its total height after switching to a 12 hour light cycle. Most of this additional growth will take place in the first week or two of flowering. If an indica plant is switched to flowering when it is 12 inches tall it can be expected to finish flowering at about 18 inches, give or take. That math is pretty consistent, so a 2 foot tall indica will likely finish flowering around 3 feet tall.

Here is the equation for indica strains. The vegged plant height (V) multiplied by 1.5 equals the plant height at harvest (H) or 1.5V = H

Sativa Strains

Pure sativa strains are much more difficult to grow indoors and are becoming increasingly rare in general. I don’t recommend them for novice indoor growers for several reasons. What is important for this topic is that their growth after being switched to a 12/12 light cycle is unpredictable. Pure sativas often continue to produce significant vertical vegetative growth many weeks into the flowering cycle. Sativas often double in size while flowering, and in some cases, will finish at three times the height they were at when the light cycle was switched. A 12 inch plant switched to the 12/12 light cycle could end up as tall as 36 inches, sometimes even taller. This additional vertical growth can quickly become a problem in a normal grow room. I’ve seen growers flower sativas at 2 feet tall only to end up with 6 foot plants at harvest. Most grow rooms just can’t accommodate plants this tall, not to mention some other real problems with tall marijuana plants.

Here is the equation for sativa strains. The vegged plant height (V) multiplied by 2.5 equals the plant height at harvest (H) or 2.5V = H.

Hybrid Strains

Hybrid cannabis strains are crossbreeds of various sativa and indica plants that have been bred selectively to promote specific characteristics. Some hybrids strains grow similar to sativas, other grow more like indicas. Most of the strains sold by seed and clone vendors are some form of hybrid. Find out as much about the strain as possible so you can guess how much vertical growth to expect after the flowering cycle beings. As a general rule, leave enough room for the plants to double in size. Once you are familiar with the strain you can let them grow a bit taller if there is room. Don’t forget, at a certain point, taller is not better.

Here is the equation for hybrid strains. The vegged plant height (V) multiplied by 2 equals the plant height at harvest (H) or 2V = H.

These equations are meant as a general aid and are by no means exact. Actual growth patterns may vary considerably.

Growing Style

Your growing style is a big factor in determining when to begin flowering. You may have heard terms like Sea of Green (SOG), Screen of Green (SCROG), Lollipopping, Super Cropping etc. I will go over these very briefly to point out how growing styles affect when to begin flowering.

Sea of Green (SOG)

Sea of Green is an indoor growing method where the plants are flowered very small and usually produce only one large bud. The plants are packed close together and occupy about half a square foot at full maturity. The plant count higher than other methods so clones are usually preferred. Indica dominant strains work best and should be flowered when they reach between 6-12 inches tall.

Screen of Green (SCROG)

The Screen of Green method utilizes a screen of wire or string mesh stretched horizontally above the plants to create an even plant canopy and help support the flowering tops. When the tops are bent down under the mesh the lateral branches grow up through the net and become tops. This method requires less plants per square foot than the SOG method but still requires clones because males can’t be easily removed. Plants must stay in vegetative growth for several week longer than with the SOG method and usually are allowed to continue vegging for one week after the tops are bent down with the netting. In most cases, the screen is placed between 1 and 2 feet above the growing medium.

Lollipopping

With the Lollipoping method the plants are usually switched to flowering around 1.5 feet for sativas and as much as 3 feet for indicas. The lower ⅓ to ½ of the plants branches are removed before the light cycle switch to direct the plants energy to the tops closest to the light source. This produces plants that resemble lollipops with long bare lower stems and multiple bushy tops.

Super Cropping

The Super Cropping method requires one of the longest veg periods but is designed to produce heavy yields with a minimal number of plants. While the plant is in in the 18/6 veg cycle the main stems are bent down to retard their vertical growth and promote the growth of lower branches. This decreases the overall height and results in bushy wide plants with tons of tops. If done properly vertical height does not become a problem and vegging can continue until the plants fill your horizontal space.

Plant Height Variation

When growing multiple plants you will often find a sizable variation in height of your plant. To maximize light exposure and therefore yield, it is ideal to flower plants of uniform height. Several methods can be used to help create an even canopy of tops before you switch to the flowering cycle.

Topping

By pinching off the tops of your taller plants you will not only create twice as many flowering tops, but you can buy some time for your shorter plants to catch up. This does stress the plants to some degree and I would suggest vegging for 1 week after topping instead of immediately switching to flowering.

Training

A less stressful approach is to train the tops of your taller plants down. This requires more skill and patience, but allows the lower branches to catch up. Several methods including screens (SCROG), plant tie, and stakes can help achieve the even canopy you are looking for.

Rotating Plants

Even with light movers the plants directly under the the lights will tend to grow the tallest and most robust. Rotating your plants each week from the outside edges of the grow area to better lit areas directly under the lights will help to create an even canopy. This only works if your growing method allows you to move the plants so it doesn’t won’t work with the SCROG method.

Plant Health

Half dead plants rarely make a sudden turnaround just because they are switched to the flowering light cycle. The veg cycle creates robust vegetation to support the plants while they flower. Without healthy foliage the plants ability to produce flowers is limited and its resources will be diverted to heal itself and fight off disease. It is much easier to treat nutrient and environmental deficiencies as well as pest problems while the plant is still in veg. A flowering plant that is stressed by poor health is also prone to reaching maturity early. This shortened flowering period gives the plant less time to produce the large trichome rich buds and will greatly affect the yield. Remember, veg usually takes only a few weeks while flowering can take from 7 to 14 weeks. Sometimes it makes sense to cut your losses and start a fresh round of plants rather than trying to flower sickly plants or coax them back to health.

Cliffsnotes

• The number of weeks your plants have vegged is a bad indicator of when they are ready to flower.
• Use the height of the plant as a gauge for determining the height they will finish at.
• Know your strain and how much additional growth it will produce while flowering.
• Know your grow rooms limit for vertical plant height and plan accordingly.
• Choose a growing method that works for your vertical space limitations and don’t over veg for your method.
• Create an even canopy by pruning, training, and rotating your plants.
• Flower only plants that are healthy and happy.

As usual, comments and quick questions are welcome below. If your questions are more involved and require a bit of back and forth, or are not related to this article, please ask them in the forum.

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