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How Many Plants to Maximize Grow Space?

If you’re about to start growing cannabis, you have a decision to make: How many plants do you want to grow?

This is one of the most common questions I hear from new growers, and it’s great if you’re asking this question because it’s actually pretty important.

If you grow too many or too few plants, you may not get the yields you want, and it can possibly take longer than needed until you get to harvest. It’s a little bit of a balancing act of finding the right amount of plants to achieve your goals.

The short answer is that for one grow light, you should plan on growing 1-8 plants. If you grow more than 8 plants under a single grow light, chances are you’re hurting your yields by not giving each plant enough space to thrive.

Don’t grow more than 8 plants under a single grow light

In the following pic, there are too many plants to grow all the way to harvest under a single grow light – each plant won’t get enough light and space to produce a significant amount of dense bud. The small pot size needed to fit this many plants in a small space will also eventually stunt each plant even if they did get enough space and light. This grower would likely get bigger yields by growing fewer plants in bigger pots.

There are several factors that make a difference when it comes to how many cannabis plants you should grow including the size of your space and what grow lights you’re using. I’ll explain each factor so you can grow the perfect number of plants for your setup to maximize yields and harvest as soon as possible!

Factors to Consider

Size of grow space

Type/Size of containers

Ease of growing

Desired timeline (How soon do you want to harvest?)

Size of Grow Space

The total size of your grow space affects the number of containers you can fit in your grow space. If you have small containers you will physically be able to fit more in your grow space than if you were using big containers. If you’re growing with extra big containers (for example a DWC reservoir) you may only be able to fit one or two plants in your grow space.

Because the DWC reservoir is large, there really isn’t enough room for another container in this grow tent

Type/Size of Containers

Going along with the size of your space, the size of your containers affects how big your plants will get. Smaller containers can support smaller plants, but if you want big plants you want big containers. Which size container? (full guide)

The max number of plants is determined by how many plant containers you can physically fit in your grow space

You will likely end up growing less plants than this max, but you won’t be able to grow more! Once you know what size container(s) you plan to use, you’ll be able to figure out the upper limit of how many can physically fit in your grow space.

As a general rule, for hand-watered plants if your final (desired) plant size is…

Too-small pots can stunt plants and cause symptoms that look like nutrient deficiencies. Learn how to transplant plants to a bigger pot.

The space around each plant container needs to be big enough to support the desired plant size! Keep in mind that each plant is likely going to be bigger than the width of its container, and needs enough room to spread out!

Think about how big you want each plant to get, and leave enough room around each container to support the plant you want.

Grow Light Part 1 – Figure Out Your Light Footprint

The grow light you use determines your total available light coverage (how much space can actually support plants). If you have a small grow light, it will only have a small amount of coverage even in a big grow space. You can only grow vibrant plants within a grow light’s “light footprint” – where the plant is getting direct light.

Therefore the size of the light indirectly determines how much space you can use to grow plants. If desired, you can actually measure your light footprint directly by using a cheap lux meter. Or use the following general guidelines…

For CFLs & Other Fluorescents (like the T5), the light footprint includes only the space directly under the bulbs, as the light (usable by plants) from fluorescent lighting does not travel much further than a few inches. All parts of the plant should be within a foot of a bulb, otherwise they’ll be outside the light footprint. Therefore the total footprint depends on how you configure your CFLs or fluorescents.

CFLs or fluorescent grow lights should be kept just a few inches away from your plants

For MH/HPS , your light footprint is as follows:

  • 150W – covers 2′ x 2′ (0.6m x 0.6m) area
  • 250W – covers 2′ x 2′ (0.6m x 0.6m) area up to 2.5′ x 2.5′ (0.8m x 0.8m)
  • 400W – covers 3′ x 3′ (0.9m x 0.9m) area up to 3.5′ x 3.5′ (1m x 1m)
  • 600W – covers 3.5′ x 3.5′ (1m x 1m) area up to 4′ x 4′ (1.2m x 1.2m)
  • 1000W – covers 4′ x 4′ (1.2m x 1.2m) area up to 5′ x 5′ (1.5m x 1.5m)

A 150W HPS grow light can cover up to a 2’x2′ area, so all the plants in this picture are getting good light levels. However, any plants outside that main area would be starved for light. You can actually see the corners of this tent are in shadow.

For LEDs , the light footprint is different depending on the model, and you can get this information from the manufacturer. Feel free to check out a short list of some cannabis-tested LED grow lights which includes their light footprint information.

The Kind “K3 L600” LED grow light (like the one pictured below) has a light footprint of 3’x4′. That means any plants growing inside that area under the lamp will be fine, but plants outside that footprint won’t be getting enough light.

Grow Light Part 2 – Make Sure There’s Enough Total Light Power for Each Plant

When it comes to the size of the grow light vs how many plants, you never want to give a plant less than 75W worth of light per plant for HPS/LED, and 150W per plant for CFL/T5. A plant that receives less light than this likely will never get big enough to produce a significant amount of bud, or buds will be airy. So make sure to consider the size of your light so you don’t grow more plants than your grow light can handle!

HPS/LED: At least 75W per plant

CFL/T5: At least 150W per plant

As an example, if you had a 400W HPS grow light, you use the formula this way: Take 400÷75=5.3. That means you probably shouldn’t grow more than 5 plants under a 400W HPS grow light.

If you had 400W worth of CFLs, you would take 400÷150=2.6. That means you really shouldn’t grow more than 2-3 plants under that many CFLs.

It’s important to keep in mind that these figures are bare minimums! For example, in my current setup each plant I grow gets 250W to itself, while Sirius’ plants get 300W per plant.

Desired Ease of Growing

The number of plants you grow affects the overall ease of growing. Ultimately, more plants generally means more work and effort. Plants often need to be watered and trained individually, plus if you have more than one row of plants it can be really difficult to tend to the plants that are all the way in the back.

Growing just one plant at a time is the easiest, but you end up only harvesting one strain. When you are growing lot of plants of different strains at once there’s more of a chance that some of the plants will grow very differently from the others, which can be frustrating. If one plant is 3x the height of all the others, it becomes much more difficult to give every plant what it needs!

If you choose different strains, you may be surprised at how differently they grow in the exact same environment!

When it comes to desired ease of growing, opt for fewer plants for an easier grow, and opt for more plants if you really want extra strains and/or are willing to spend the extra time it takes to watch over and tend to many plants at once.

Desired Timeline + Yields (How soon and how much do you want to harvest?)

The amount of yield you can achieve with a good grow is determined less by the number of plants and more by your strain, skill and grow light (learn more about what determines your yields). Generally for any strain you will get about the same total yields from one or many plants, just as long as you fill up the space under the grow light.

But the timeline (how long it takes until harvest) is partially determined by how many plants you choose to grow, and this indirectly affects your yields.

As an example, if you’re growing in a 3’x3′ space, it might take 8 weeks in the vegetative stage to get just one plant to completely fill up that space. A single plant needs enough time to grow big enough to support all the buds you want to harvest. If you switched to flowering earlier than that, it would ultimately hurt your yields because you’d be wasting some of the space under the light.

This single plant needed almost 8 weeks in the vegetative stage to get big enough to fill in most of the grow tent

But if you were growing 2 plants in the same setup, each one would only need to grow half the size in the vegetative stage to achieve the same amount of coverage under the light. This is because a single seedling has less leaf mass and simply can’t use as much light as two seedlings. Less wasted light = more total growth. If you were growing 4 plants, each one only need to grow 1/4 the size to achieve the same coverage in the vegetative stage.

This fact can be used to shorten your veg stage because whether you’re growing one plant or many, once you’ve achieved that coverage in the vegetative stage you’ll end up getting similar yields at harvest. In other words, with more plants you may need less time to fill up your available growing space, which can save you a few weeks of veg time, leading to a quicker harvest!

Together, these 4 plants have created a canopy that is a similar shape, length and width as the single plant above. This means the yields will be similar (if the light and strain are the same). However, they only needed 6 weeks in the vegetative stage to achieve this size (2 weeks less veg time than the single plant above) so were able to switch the flowering stage sooner.

Ultimately, growing more plants (up to a point) allows you to have a shorter vegetative stage since your grow space will fill up much more quickly. And a shorter vegetative stage ends up saving you electricity and time! That being said, for some growers a few extra weeks of veg time is worth the fact that you have to spend less time tending your plants every day.

So in some ways when it comes to timelines/yields and number of plants, it’s a matter of deciding how much personal time you want to put in to your grow on a regular basis vs how much time you are willing to wait to get to harvest. If growing several strains and getting to harvest as quickly as possible is most important to you, opt for more plants (up to 8 under each individual grow light). If you want a grow that’s easier and less time-consuming, opt for fewer plants.

Start with More Plants Than You Plan To Grow

If it’s your first grow (and honestly in general) it’s a good idea to start with more plants than you need. This lets you choose the best plants of the bunch and not be stuck with any weaklings that just aren’t growing as well as the others. Plus you never know when a seed won’t germinate or come with some other problem like a bad mutation.

This grower started with 6 plants even though they only planned on growing out two. This way if there’s a weakling or one of the seeds doesn’t germinate, it doesn’t affect your grow!

Start with Feminized Seeds

With non-feminized seeds, about half of your plants will end up being male (which need to be thrown away) and that can seriously mess with your plant numbers. To make things worse, due to the random nature of genetics sometimes you can end up with something like 5 males and 1 female out of six seeds (just as you can get 5 females and 1 male). This can make it hard to plan for the number of plants, because on average you would expect to get 3 females from 6 non-feminized seeds.

To make things a LOT easier for you as far as plant numbers, choose to start with feminized seeds (available for most strains). With feminized seeds, all your plants will end up being bud-bearing female plants, which means you get to keep all your plants.

Get feminized seeds from a trusted breeder

Maximize Yields by Filling Up Your Grow Space via Plant Training

The best yields indoors are achieved by using plant training to force cannabis plants to grow flat and wide. This is true for just about every indoor growing setup that uses grow lights. Plant training lets you fill up the entire space under your grow light with an even canopy of colas, and this can increase yields up to 40% compared to not training at all. Plus it’s free! If you don’t want to learn any new techniques but still want all the benefits, check out our page on no-technique plant training.

Example of the manifolding technique in action, which is a specific way to train the plant in the early vegetative stage to create multiple big colas instead of just one.

Ultimately, no matter what technique you use (or even if you use our no-technique plant training) your goal is to to grow a plant with many main buds like this.

When a plant is not trained, it only produces one main bud per plant. You don’t want this! It will reduce your overall yields indoors because only the single bud closest to the light will get big and fat. You want many main buds to get the most from your grow lights, and the best way to accomplish that is with plant training!

Now that you know how many cannabis plants you want to grow, time to start growing!

Growing Cannabis in Raised Garden Beds

Over the last few years in the United States, public perception of the use of marijuana for recreational and medicinal purposes has softened dramatically, accompanied by varying degrees of emerging legality. As of May 2017, federal law still prohibits the possession, use, sale and cultivation of marijuana; however, it also provisions the allowance of a state to individually mandate its own laws regarding recreational and medicinal use, so long as there is a regulatory system in place.

For instance, California has legalized both recreational and medicinal use, whereas Florida only allows for medical use. We encourage everyone to seek and fully understand the laws and regulations regarding marijuana in their state.

There are more ways than one to skin a cat, just as there are methods to cultivate marijuana. Indoor cultivators rely on lighting methods to induce and control the different stages of growth, from seedling to flowering to harvest time. This approach certainly has its advantages, especially in regions that receive little sunlight. Some, however, may find the purchase of costly specialty bulbs and higher electricity bills to be a deterrent. Those pursuing a more organic and inexpensive growing method might consider outdoor cultivation.

Cultivating Cannabis in Raised Beds

Enter raised bed square foot gardening, a method celebrated by those who enjoy its simplicity, structure and economy. By dividing a raised bed gardening space into square-foot planting sections, gardeners can create small but intensive vegetable gardens. In contrast to row gardening, where gardeners leave walking paths between rows of plants to access them, raised bed square foot gardening employs area planting within the sections mentioned above. Rows are not needed, since properly designed raised garden beds always have a dimension of 4ft or less, providing gardeners arm’s length access to all plants from one side of the bed or another.

Novices and experts alike have enjoyed successful yields of all types of vegetables and herbs using the raised bed square foot gardening method and can certainly do the same with marijuana. Consider the following advantages of raised bed square foot gardening when planning your outdoor marijuana grow:

Ideal Soil Conditions

A strong, healthy marijuana plant requires plenty of nutrients throughout its lifetime, namely nitrogen (N), phosphorous (P) and potassium (K). Unfortunately, the soils found in our backyards tend to be deficient in one or more of these nutrients and can be difficult to work with. With a raised garden bed, you are not limited to local soil conditions. A soil recipe high in nutrients will go a long way in ensuring the success of your plants. Pre-mixed, nutrient rich soil blends are widely available at home and garden stores, however when selecting/creating your growing medium consider the following ingredients noted beneath the following ‘PRO TIP’.

PRO TIP : Matthew Sokolowski, Operations Manager at True Plant Science, suggests a soil recipe that comes from TGA Subcool Seeds, a marijuana genetics breeder out of Oregon. It’s full of quality nutrients that will sustain the plant throughout its entire lifespan. Below is a general recipe that can be adapted to the size of your planting space:

• (8) 1.5 cubic ft. bags of high quality base soil
• 33 lbs. bag of Worm Castings
• 2.5 lbs. Fish Bone Meal
• 5 lbs. High Phosphorus Bat Guano
• 5 lbs. Blood Meal
• 2.5 lbs. Bone Meal
• 3 cups. Oyster Shell
• (3) cups. Kelp Meal
• (3) cups Alfalfa Meal
• (3/4) cup Epsom Salt
• 1 cup Dolomite Lime
• 2 cups Azomite
• 2 tbsp powdered Humic Acid

Allow a mix like this to break down over 6-10 weeks prior to planting. This mix yields roughly 14 cubic feet.

A quality soil blend, such as the one mentioned above already contains the necessary nutrients to feed marijuana plants. So, the need for fertilizers and pesticides is greatly reduced if not all together eliminated.

Pro tip: The only time that supplemental nutrients may need to be added is during the flowering cycle, as the plant consumes much more Phosphorous and Potassium than it does Nitrogen. Compost teas would be a good option for adding organic nutrients into the soil as needed. A good flowering tea should consist of Seabird Guano, worm castings, Indonesian Bat Guano (or similar high phosphorous guano), kelp and molasses.

Structure and Spacing

Marijuana plants tend to adapt to the space they are given, but generally grow well in 4-6 gallons of soil. Employing the square foot gardening method in a typical 8-inch- tall raised bed will create equal square foot sections of .667 cubic feet, or about 5 gallons — ideal for one marijuana plant per section.

This intensive spacing system is advantageous when you have limited room to work with. Taking this plant spacing method and expanding it to a common size garden, you could grow 16 marijuana plants in a 4ft x 4ft raised garden bed. (1 plant per square foot = 4 plants across and 4 plants deep).

Marijuana Strain Considerations

There are two primary strains of marijuana, Sativa and Indica. Often they are cross bred but each has growing characteristics to consider. Sativa strains are more prone to growing up/elongating than Indica strains do, making them the a more ideal marijuana strain for 1 square foot plant spacing.

Indica strains tend to grow more bushy or outward. Raised bed square foot gardening is not limited to one square foot spacing as a maximum though. You can grow 1 plant per 2 square feet to accommodate wider growth. Plant spacing as such would allow for 8 plants in a 4×4 raised garden bed.

Despite the common characteristics of Sativa or Indica strains, most any strain can be cultivated in a smaller space with the right kind of plant training techniques and proper trimming maintenance.

Leaf Icon Credit: Nikita Golubev via Flaticon

PRO TIP : The more room marijuana plants have to grow, the wider and bushier they will become. When kept to a more confined root system, the roots are still able to grow downwards but are restricted laterally, resulting in a more conical plant. Some light plant training may be needed to keep the branches of other plants from entangling and preventing the growth of powdery mildew or other disease.Tomato cages would be useful in this situation.

Improved Drainage and Reduced Watering

A well-composed raised garden bed also enjoys excellent soil drainage. This is key to the health of the root systems of your marijuana plants: excess moisture drains out, allowing the roots to absorb oxygen and thrive. At the same time a raised garden bed improves drainage, planting marijuana intensively in a square foot fashion reduces the overall water needed to sustain your garden.

As plants grow in close proximity, they shade the soil and minimize moisture loss. Ultra-efficient ground level watering systems and spacing guides can be easily adapted to the raised bed structure as well. All this means you’ll need less water less often, minimizing your footprint and conserving resources.

PRO TIP : Cannabis plants in an outdoor environment will generally require water every 2 to 3 days.The amount of water the plants require and application rate will increase as the plant transitions into the flowering stage. Also consider the climate. A Hot and dry climate will necessitate more water than hot and humid climate.To gauge general watering needs for your marijuana plants, ensure that your garden bed’s soil is moist an inch below the surface. Keep a close eye on the plants early on to get a good idea of your water schedule.

Acknowledgements

A special thank you to Matthew Sokolowski, Operations Manager at TruePlant.com and consultant to multi-licensed, commercial cannabis cultivators out of Washington State, for providing his expert insight and ‘PRO TIPS’ to create this guide.

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Bryan Traficante co-founded GardenInMinutes in 2013, turning a passion for home gardening and innovation into a family-owned venture to make starting a quality garden, easier. GardenInMinutes invented the Garden Grid watering system which combines square-foot planting principles with efficient ground-level irrigation. GardenInMinutes also crafts tool-free, modular raised gardens and provides time saving gardening insights on their blog and social media pages. Find Bryan and GardenInMinutes on Facebook , Twitter , and read all of his MOTHER EARTH NEWS posts here .

All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Guidelines, and they are responsible for the accuracy of their posts. To learn more about the author of this post, click on their byline link at the top of the page.

Published on May 1, 2017

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