Finding the Balance Between Cannabis Plant Density and Crop Health

Ryan Douglas Cannabis Photo plant density

Photo: Ryan Douglas

One of the goals of facility optimization is to maximize crop yields. This is especially true with cannabis. Every square inch of flowering space needs to be as productive as possible. Chronically low yields can place a cultivator at risk of going out of business.

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Plant density directly correlates to crop yield — the more flower sites per square foot, the greater the output. But there is a trade-off when a grower stuffs as many plants as possible into a grow room. Dense canopies encourage foliar diseases such as powdery mildew, and cannabis growers are limited in the kind of chemistries they can use to prevent and control disease outbreaks.

While there is no universally recommended planting density for cannabis, most indoor and greenhouse bench production programs utilize flowering plants at an average of .65 to one plant per square foot. That means for every 4 feet by 4 feet of canopy, growers will average 10 to 16 flowering plants.

Less densely populated canopies are common among cultivators that are limited in their plant count. Long vegetative growth periods followed by multiple soft pinches before flowering is one of the best ways to maximize production from very few plants.

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On the other extreme, multiple plants per square foot are more typical with a technique colloquially referred to as “sea of green.” This cultivation method limits the vegetative growth period to only a few days and, in some cases, eliminates it altogether. The plants are not pinched during production, resulting in a uniform, packed canopy that can exceed standard yield expectations.

What kinds of yields should cannabis growers expect? Start-ups should anticipate an average yield of 35 grams of dry cannabis flower per square foot per harvest. Established operations that have refined their genetics and cultivation protocols should expect closer to 50 to 70 grams per square foot per harvest.

Growers realizing yields of 100 grams or more per harvest are few and far between. These outcomes result from high-yielding genetics grown in finely tuned cultivation environments with true pros at the helm of the operation. Most growers should expect 35 to 70 grams of dry flower per square foot per harvest from their cultivation program. More is just icing on the cake.

If you’re hitting these numbers, but still spying the occasional outbreak of powdery mildew, consider these five tips for decreasing disease pressure without reducing your yields:

Confirm There Aren’t Extreme Swings in Temperature

The difference between the warmest point of the day, or “lights on” period, and the coolest point of the night, or “lights off” period, shouldn’t exceed 10°F. Large fluctuations in temperature create large fluctuations in relative humidity and can result in micro condensation that encourages mold spore germination. Ensure your environment is on point.

Install Under Bench or Benchtop “Air Socks”

Increase airflow by delivering fresh air right where the plants need it most: within the canopy. Small air socks that run the length of the grow bench can be installed on the benchtop or under these structures if you’re cultivating on a screened platform or gutter system. Fresh air will help to break up stagnant air and decrease the likelihood of foliar disease.

Avoid Late Afternoon Irrigations

Most water transpires or evaporates within the first 12 hours after irrigation, so growers should plan heavy watering events for the morning or early afternoon hours. With late-day irrigations, moisture is more likely to be trapped in the canopy and aggravate disease outbreaks. If you must irrigate late in the day, apply just enough water to get the crop through until the following morning.

De-leaf

Periodic de-leafing can help ensure adequate airflow and light penetration throughout the crop. De-leafing removes large fan leaves that congest the canopy or block light to underlying flowers. Many growers use a 20/40 approach: a thorough crop de-leafing at day 20 and day 40 of the flowering cycle. Others prefer a constant light de-leafing throughout the first half of flowering to avoid the potential stress of a plant being stripped naked of all its fan leaves at once. Either option is acceptable, but if you’re removing massive amounts of leaves, it’s time to revisit your production program. As you remove those leaves, you’re also removing all the time, money, and energy that went into growing them.

Get a Little Help From Your Friends

Implementing weekly or bi-weekly fungicide applications can help prevent pesky outbreaks of powdery mildew.

  • Green Cure and Milstop are both made from potassium bicarbonate. They dehydrate the mildew and change the pH of the leaf surface, making it difficult for germinating spores to take root.
  • Regalia is a biofungicide that can help protect high-density crops from powdery mildew. Its active ingredient, Reynoutria sachalinensis, is believed to stimulate the plant’s natural resistance and help it fend off foliar diseases.
  • Terra Vera is a promising chemistry that helps prevent powdery mildew outbreaks and is also proving effective as an insecticide.

Like any complex situation, there isn’t an easy answer to finding the right balance of plant density and crop health. But implementing the above recommendations should help create a path toward achieving a mildew-free crop while meeting or beating yield expectations.

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