marijuana plant spacingHow close should your marijuana plants be to each other? It makes a big difference in harvest yield and plant health. © Gary Anderson, 2016

Marijuana Plant Spacing for Maximum Production & Plant Health

Marijuana plant spacing for maximum production and plant health in your indoor garden is a serious topic we need to talk about right now.

One factor affecting marijuana plant spacing is the differing ways reflectors, hoods, and various types of hydroponics bulbs affects the amount of square feet that have enough light intensity for your plants.

Read this great article on reflectors and hoods.

Some hoods have a narrow angle, are too deep, or they have crappy reflective materials.

Instead of distributing light, they eat it.

They may only give you 11-15 square feet of usable well-lit space per 1000-watt bulb.

Others are better engineered. Maybe they have better reflective materials or their angles are more effective at delivering more light to a wider area.

Using the right reflector or hood, and a quality bulb, you could get 16-22 square feet of usable, well-lit space per 1000-watt bulb.

Double-ended bulbs add a new dimension. They run so hot, you have to put them way higher above your plants.

If you use high-quality double-ended bulbs, your light intensity is as much as 35% higher than conventional HID bulbs at canopy level.

You could get 19-27 square feet of usable, well-lit space using the best 1000-watt double-ended bulbs.

Marijuana plant spacing should allow delivery of enough direct light through the canopy and onto each plant.

It should also prevent plants from touching each other.

If your plants are touching each other, the leaves and buds where they’re touching are extra-humid, and unable to transpire properly.

Your goal is to have as many plants as possible under each light, but not to have them touching each other.

Overly crowded marijuana plant spacing, especially combined with problems with ventilation, humidity control, temperature control, and air movement, creates ideal conditions for gray mold, spider mites, thrips, and other problems.

Using net supports is increasingly popular, but growers using sea of green and SCROG sometimes find it hard to use trellis nets without creating too-crowded marijuana plant spacing.

Also realize that individual plants might be forced by nets or general crowding to have their branches compressed together so that even if plants aren’t touching other plants, they’re still being harmed.

Crowding creates leaf on leaf, branch on branch, and bud on bud conditions favorable for pests, diseases, and inferior growth.

On the other hand, you can skillfully “weave” individual stalks through the netting to separate the buds and leaves, providing the right amount of spacing for light penetration and air movement.

Take a look at my grow op photo with this article, and you’ll see an example of using trellis netting to properly separate plant stalks and plants.

And even that example is a bit too crowded for my liking. I had to go in and remove leaves, and have some of my oscillating fans only 1-3 feet off the floor, to keep the air movement flowing.

Here are some general guidelines for marijuana plant spacing:

  • Keep cannabis plants far enough away from each other so they don’t touch each other at all.
  • Use at least one 12-inch oscillating aeration fan per 1000-watt light.
  • Make sure you can feel a gentle breeze from top to bottom of plant canopy. Dead air harms plants.
  • Use exhaust fans and other ventilation techniques to exchange your entire grow room air every hour, unless you’re using added C02.
  • Be sure to keep humidity between 54-57%.
  • Use carbon filtration and microfilters on air intakes, and with in-room odor control equipment, to keep the air ultra-clean.
  • Routinely check your plants. If you see plants touching each other, trim back or tie back the branches so the plants are no longer touching.
  • Use a PAR light meter and measure PAR (photosynthetically active radiation) and general light intensity to see the effective area profile of your hydroponics lighting equipment.
  • Make sure that light can penetrate from to to bottom and side to side of your each marijuana plant’s perimeter. If you see shade and other darkened areas, those parts of your plant aren’t being fed light, which is an essential nutrient.

My general advice is to have larger and fewer plants, each with just enough space so they don’t touch each other and so light penetration can get in from top to bottom.

I realize people want formulas like: you can have 6-8 plants under a thousand-watt regular HID light if they’re in 7-gallon buckets, or 4-5 plants under a 1000-watt if they’re in 13-gallon buckets, but formulas are too generic.

For double ended you might be able to put 6-10 13-gallon bucket plants under a 1000-watt double-ended bulb.

But generic formulas do you no good. There are too many factors that affect exactly how many plants you can fit under each light.

Factors affecting how many plants you can have under a 1000-watt grow light include: type of bulb and reflector, pot size, marijuana strain, how the plant is grown, how the plant is trimmed, the quality of your grow room environmental controls (temperature, air movement, air exchange, humidity).

The best rule is this: put as many plants as you can fit under the light as long as all the plants have air space around them, aren’t touching each other, are getting full-spectrum light intensity, and you can feel good air movement throughout the entire plant space.

When you do marijuana plant spacing correctly, you aren’t wasting light or space, but you aren’t crowding either.

You’re using the geometry of grow room plant placement to get the most buds per watt!

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