Grow Hack: Plastic Mulch For Higher Yields and Better Flavor

As the California Drought transitions into its fifth year the situation has gotten dire. Indoor, hydroponic gardens that are fairly water-efficient grow the majority of the top-tier cannabis on the medical and recreational markets, but lower-overhead outdoor fields grow most of the pot people smoke all over the country. Covering the ground around plants with clear, black or colored polyethylene, a practice known as plastic mulching, can save up to 50% of the water used in growing while increasing yields and flavor.

Tree-ring analysis showed 2015 was the driest year in California in the past 5 centuries, a situation that has forced farmers to be incredibly water conscious due to drying wells and increasing costs of water delivery to remote, mountain-side ganja farms. A technique known as plastic mulching can save water and reduce the need for herbicides when used properly.

Plastics come in handy on outdoor farms, and the practice of using plastics in agriculture is called plasticulture. Clear plastic is a cheaper option for covering greenhouses than glass, and offers benefits increased insulation and light dispersion. Soil solarization is the practice of covering the ground with plastic for an extended period of time to kill weed seeds and harmful pathogens.

One way or another, the more ground the plastic cover the less evaporation the ground will face saving water for the farmer. The color of plastic the farmer chooses will affect the soil temperature underneath, weed growth, and reflected light. Contrary to what you might think, black plastic mulch only increases soil temperature by 5 °F at the 2-inch depth, and 3 °F at the 4-inch depth. On the other hand, clear plastic mulch increases soil temperature by 8 to 14 °F at the 2-inch depth and 6 to 9 °F at the 4-inch depth. To gain a reduction in soil temperature, use a white-on-black or silver reflective mulch which can reduce soil temperatures by 2 °F at the 2-inch depth and -0.7 °F at the 4-inch depth.

Clear mulches can be great for cold climates like New England or British Columbia, but the passage of light to the ground underneath allows weeds to grow. Clear mulching requires the use an herbicide, or better yet, soil solarization to eliminate weed seeds before sowing your crop.

Different color plastics can be used to gain intermediate effects between black and clear. Blue-green and brown mulches block photosynthetically active radiation (PAR) but transmit infrared radiation (IR). By blocking PAR, these colors of plastic don’t allow weed growth, but the transmitted IR warms the soil almost as much as clear plastic, providing a compromise between black and clear plastic.

Another aspect to consider when choosing a color of plastic is the reflected radiation above the surface. In one experiment from Penn State, twice as much PAR was measured above clear plastic than black plastic or bare soil. Clear plastic could increase foliage growth during the vegetative growth period of a plant, while heating soil at the same time, a great combo for farmers based in chilly, cloudy climates.

Red and black plastics reflect around the same amount of total PAR and raise soil temperatures by the same amount. Unsurprisingly, red plastic increased the ratio of red: far red light the plastic reflects. This difference in light reflectivity has been shown to increase yields in tomatoes by to 20% when compared to black mulch. Other sources say red and plastic mulches used to grow tomatoes also have a positive benefit on flavor and taste.

With the use of any color or shade of mulch, the farmer needs to make sure the mulch makes good contact with ground in order to ensure adequate heat conduction to the ground. A close contact area will also prevent a strong gust of wind from ripping up the plastic potentially compromising your crop. Use rocks or ground staples to fix the polyethylene to the ground.

Plasticulture has a great potential for making cannabis cultivation more eco-friendly. Reflective mulches are also part of California’s legal pest management strategy for flea beetles and whiteflies as an alternative to azadirachtin pesticides. Always think about the environment before you grow cannabis outdoors or indoors, and if you get into the habit of using the least amount of resources as possible you’ll save money and contribute minimally to the destruction of the environment.

Photo Credit: Stephanie Curtis

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