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How to Control Stretching: Reigning in Cannabis Growth Spurts

Stretching is a natural growth-spurt for cannabis plants, but is known to adversely affect the outcome of a crop. We delve into what causes stretching in the first place and how to prevent it from taking over your grow room.

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There are few things more frustrating than an overcrowded grow room. “Stretching,” the term used to describe the rapid growth of cannabis stems, is a natural part of the vegetative stage and is influenced by plant genetics, among other variables.

Unfortunately, stretching is a common perpetrator of low yields and lanky plants, resulting in teetering, physically unstable crops. Despite the threat it poses, stretching can be controlled by first examining the factors influencing its growth, then creating protocol to sidestep these issues. Controlling cannabis stretching requires an understanding of how plants interact with both their internal and external environments.

WHY DO PLANTS STRETCH?

There are several reasons why plants stretch, one of which has to do with the strains themselves. Genetics plays a pivotal role in determining the eventual height of sativas, indicas and hybrids. Whereas most indica strains are bred to grow shorter and bushier, sativas often experience significant stretching, sometimes growing six feet or higher in some breeds.

While stretching isn’t harmful in and of itself, dramatic stem growth carries the potential of significantly lowering the crop’s final yield between 20-30% overall. When considering strains for your next home grow, research the common height of each variety you plan to grow in order to gauge whether or not the strain experiences a dramatic flowering stretch.

LIGHT EXPOSURE

One of the main culprits of inordinate stretching of cannabis plants is lack of light exposure. Plants that are too far away from an adequate light source will respond by spurring stem growth to move themselves closer to the lamp. Especially in the presence of many other plants, sparse lighting can easily cause a grow room to become severely overcrowded, throwing off the steady progress of your crop and the ratio of distance between bulbs and plants.

In response to this issue, be sure to provide enough light to quell stem growth. At the same time, you don’t want to position your lights too close to your plants, as this will also result in overstretched stems and in some cases, lost yield.

In addition to distance between the source and the plant, the kind of light being used also holds influence over the amount your crop will stretch. Orange and red light encourages stretching and results in thinner, taller stems. Conversely, blue light stimulates thicker stem growth and a shorter height therein. When a strain is ready to enter the vegetative stage and undergo its most drastic period of stretching, metal halide lamps can be used to discourage extra-long stems.

Heat is another pivotal element that can determine how much plants will stretch during vegetation. Temperatures reaching over 27 degrees push stems to grow longer and longer and will propel sativas to their full height potential. Heat lamps that are positioned too close to the plants creates an intolerable environment that will rouse tall, wobbly plants, which are likely to fall over and potentially lose flowers in the process.

OTHER CAUSES OF STRETCHING

There are numerous variables that can lead to plants stretching beyond what is normally expected from the strain. Significant environmental stressors resulting from transplant may cause the plant to go into shock. This will then trigger a reaction, causing it to stretch. Cannabis plants that are not properly cultivated under decent conditions or aren’t receiving satisfactory nutrition will respond in a number of adverse forms, including stretching.

Crops that are not spaced far away enough from one another are likely to stretch as a result of competition for resources. Due to extreme proximity, plants will fight each other to reach the light, forcing growth throughout the crop.

HOW TO CONTROL FLOWERING STRETCH

Flowering stretch, you guessed it, takes place when you switch your plants over from veg to bloom. This is a completely normal response as your plants prepare themselves to support the weight of their buds.

How much a plant stretches at the beginning of flowering varies a lot; some plants might double in height while others only grow a few centimetres. There are two main variables, however, that can give you at least some idea of whether your plants will stretch during bloom, and how much. These are:

• Strain: Genetics have the biggest impact on your plant’s size. Generally speaking, sativa strains stretch more than indicas and tend to develop long, lanky stems.

• Lights: Certain lights, like HPS, are more likely to encourage stretching. Moreover, large distances between your canopy and your lights will cause plants to stretch as they attempt to get closer to the light source.

The flowering stretch usually lasts for the first two weeks of the phase. To minimise stretching, keep your lights between 10 (for CFL) and 30 centimetres (for HID) from your canopy. Also, stick to indica strains if you’ve got a smaller grow space.

HOW TO PREVENT STRETCHING

In many cases, you can account for at least some stretching to occur in the flowering stage of your cannabis plants. With this is mind, it’s helpful to establish a plan in case you’re strain ends up growing higher than you imagined. If the information is available, obtain the average reported height of your strain and compare it to your grow room dimensions, considering the necessary distance between bulbs and plants.

CIRCULATION

While outdoor grow-ops benefit from the presence of the open air to naturally limit stretching, indoor operations are tasked with the tough assignment of maintaining air circulation inside a closed space. Sufficient airflow helps cannabis stems strengthen and grow thicker, rather than taller. This way, plants still grow vigorously, without reaching nerve-wracking heights.

MANUAL INTERVENTION

For those cultivators looking to get their hands dirty, there are physical intervention methods that can be taken to reduce over-stretching. Similar to the natural effect wind has on cannabis plants, manually bending the leaves and stems will cause tiny tears in plant tissue that will cause the plant to focus its attention on stem regeneration instead of vertical growth.

TOPPING

Topping is a form of manual intervention on cannabis to affect its yield, shape or size. In essence, topping is the process of cutting of a new, actively growing node from your plant in order to reduce its size and create a “v” shape that will then form two colas. Topping can be an effective measure for combating stretching, but it’s important not to top once the flowering stage begins.

In most cases, unexpected stretching from cannabis plants won’t destroy your yield entirely. In fact, stretching can actually help to increase your yield. For cultivators with sizeable grow rooms that can sustain tall plants, stretching may actually increase a strain’s total yield with more vertical surface area for colas to form. Either way, stretching can be easily prepared for in advance and there are a number of different solutions to help with this all-too-common problem.

Cannabis stretching is a growth spurt experienced in the vegetative phase. Here is how to control it.

How to Prepare for the Flowering Stretch

by Sirius Fourside

Right around the time when kids start hitting middle schools, short awkward teenagers go through puberty and turn into taller awkward teenagers…or was that just me? I’m sure you knew of at least one kid who grew so much, they looked almost as if someone grabbed them by their head and feet and just stretched them out.

When cannabis plants start to mature, they also go through a brief transitional period; a sort of cannabis-puberty if you will. And just as humans undergo a burst of rapid growth, cannabis plants will also create a large portion of their final height in a very short period of time. This explosion of vertical grow this commonly called the cannabis ‘Flowering Stretch’.

What is the Flowering Stretch?

The best way to give you an idea is with pictures. Here’s a cool animated .gif Nebula made showing the flowering stretch in action.

This makes for a great example of how small or large a stretch can be. Notice that the plant on the left (818 Headband) gains only a few inches while the plant on the right almost doubles in size! And even this picture is misleading once you realize that by the end of the .gif, the plant on the left is actually raised an extra foot higher off the ground! Note: This was done to keep the plants somewhat equidistant from the lights.

In short, the flowering stretch is potential burst of rapid growth that happens shortly after switching to the flowering phase. It can be massive, or it can be next to nothing. The important thing is to expect that it might happen and have a plan ready in case it does. Luckily, most strains you buy from a seedbank are stabilized, so it’s known if the plant will stretch a lot or a little. This allows us to not only plan for a stretch, but take advantage of it when it happens!

What to Expect

Depending on the strain you’re growing, you can expect a pretty wide range of stretching. Some strains can double their height or more, while some do almost nothing. However, if you know the name of the strain you’re growing, you can look up the breeder recommendations to get a good idea of what might happen. Additionally, there are other factors that can give you insight:

  • Sativas – Sativas tend to grow taller, lanky stems even when trained; inch-for-inch they tend to stretch much more than indicas.
  • Strain height – Many breeders and seedbanks have the average height of their strains listed. Sometimes it’s as vague as “short – medium – tall”, but even that is enough to give you an idea of its growth characteristics. Expect “tall” strains to stretch!
  • Light type – Certain light types are more conducive to stretching than others. You can use the right light color spectrum to help control growth characteristics. More on that later…
  • Light distance – Plants also stretch when they’re trying to get more light (like this plant to the right). As a result of this type of stretching, plants grow taller, but with fewer bud sites. Note that this is not the same thing as the flowering stretch, rather it’s a symptom of too-little light. Most growers want to avoid stretching caused by not enough light as fewer bud sites means less bud.

One of the main reasons it’s so important to know about stretching is that it can rapidly reduce the amount of space you have in your grow tent/area. If you’re growing in a space with limited height, a good stretch can make it so you no longer have room to raise your lights. This leads to light/heat stress and lost buds.

I personally had a stretch where there was no longer any room to raise the lights. I used a CFL/LED combination at the time, and the LEDs absolutely cooked the buds underneath it until I got desperate and cut the stuff that was too close. I would handle that situation differently now, but it shows that no one is immune if they’re unaware.

Planning For the Stretch

Planning for a stretch is easy to do when you know it’s a possibility. There are two things growers can do that will give a lot of control over planning for a potential stretching situation:

Research your strain
If you know what strain you’re going to grow, you can usually find its growth characteristics online. Seedfinder.eu is a pretty good resource for reviews on strains submitted by other growers. While it isn’t necessarily 100% accurate (what is?), it should give you a good range of what to expect. Knowing the strain you’re growing gives you the power to decide how much stretch you’ll deal with. Since stretching is often accompanied by a ‘tall’ final height, you can reduce stretch by getting plants that are characterized as being ‘medium’ or ‘short’.

Plan for a tall plant
In my opinion, the best way to be prepared for any stretch is to simply plan out the space as if you know that your plant will double in height by the end of flowering. Don’t worry, it’s much easier than it sounds! Here’s how you do it:

  1. Take the total amount of vertical (height) growing space you have in inches/cm.
  2. Take the total from #1, and subtract the height of your lights when it’s at its highest point.
  3. Next, subtract the height of the pot/container your plant will be in.
  4. Finally, subtract the amount of space you’ll need between your light and your plants.
    1. This varies depending on the lights; CFLs only need about 4″, HIDs need 12+”.
  5. When your plants height reaches half of the number you’re left with, initiate flowering!

Not too bad right? It’ll seem even easier with an example! Here’s how it would work for my tent (math is below the picture):

  1. I have a 7 foot tall tent which comes out to 84 inches. Total: 84 inches
  2. My lights and the stuff to hold them take up 21 inches. Total: 63 inches
  3. My container is 13 inches tall. Total: 50 inches
  4. I’m planning to keep 16 inches between my plants and the light. Total: 34 inches
  5. I flower my plants when they are about 17 inches tall.

You don’t always need the space when you plan like this, but it allows you to easily deal with the times when your sweet little babies turn into monsters like this:

Pre-Stretch

Post-Stretch

Reducing the Stretch

Since we know the factors that can lead to more stretching, reducing how much a plant stretches is just a matter of acting on those factors.

Genes – Indica plants tend to be shorter, so if you prefer Indicas and would like a short plant, you’re in luck. If you like Sativas and have limited height, there are many strains available that have a shorter finishing height. Again, you can usually find the heights of strains at the websites of breeders and seedbanks.

Light type – Did you know that HPS (High Pressure Sodium) lights encourage stretching? If you use HID lighting, you can combat this extra stretching by leaving your MH (Metal Halide) bulb in for the first 2-3 weeks of flowering before switching to a HPS bulb.

  • This also goes for CFL growers who are utilizing vegetative (6500K) and flowering (2700K) colored bulbs. You can leave the vegetative (6500K) colored bulbs in for the first 2-3 weeks of flowering before switching to your flowering bulbs.

Light Distance – Keeping your lights the right distance away will ensure that your plants don’t have to stretch to get more light. Use the ‘hand test’ to make sure your lights are as close as they can be without harming your cannabis.

Hand test: Place one of your hands right over your plant with your palms facing downward. If the heat from your CFL or HPS lights is uncomfortably hot or painful to your hand, move the lights away an inch and try again. This test does NOT work for LED lights.

Some General Rules About Light Distance:

  • CFL and fluorescent grow lights should be kept a few inches from your plants.
  • For MH/HPS lights it depends on the size of your light.
  • For LEDs it’s different for each model so you should always check with the manufacturer, but it’s generally recommended to keep most LEDs 12-18″ away, though some of the most powerful LED grow lights need to be kept even further away.

Plant Training – Although training itself does nothing to slow down stretching, it does give growers a good way to manage it. For example, if branches are set to grow vertically by low-stress training, a stretch can actually be beneficial since so much of the plant will be receiving light. Plant training can come in the forms of low-stress training, manifolding, topping, fimming, and more. These techniques are used in the vegetative stage to get your plant to grow the way you want in the flowering stage.

Making the Stretch Your Friend

The flowering stretch is not necessarily a bad thing! As long as you know what to expect, you can use it to your advantage.

For those with taller grow spaces, such as tents or custom made grow spaces with 6+ feet of vertical grow room, big stretching strains can actually be a part of your growing plan. A taller grow space lets you choose from taller strains, so you have more strain choice. Plus if you’re growing a short strain in a tall space, it could take a long time to fill up the whole tent. Since you increase yields by filling up the space under the light with many long bud sites, a taller strain can help you do that more quickly and efficiently in a big space.

On the flip side if you have a short space, you can choose a strain that doesn’t stretch much, if at all. This might let you maximize your grow space by spreading your plant out wide under the light during the vegetative stage, without having to worry about a plant that keeps getting uncontrollably taller and taller when you switch to the flowering stage.

The ‘Flowering Stretch’ is another piece of knowledge that makes growing cannabis much more easy once you know about it. I’ll leave you with a picture of when I got blindsided by a stretch and had no idea what to do (yet again). Don’t let this happen to you, too!

Cannabis plants grow a lot taller right after they start making buds. This is known as the "flowering stretch" and you can use it to your advantage to make longer buds!