Whether you have an abandoned lot or a weedy garden that you want to turn into a productive ganja-growing paradise, you need an effective way to get rid of weeds, pathogenic bacteria and fungus or nematodes. Outdoor cannabis gardening can be a rewarding hobby (free weed or income), and if you do it in healthy soil with a climate-adapted strain, it’s an easy one as well.
This environmentally friendly process takes about one year, but it will improve soil fertility, kill pests and keep them at bay for years to come. Preparing organic soil requires patience but comes at a fraction of the cost of digging and buying new soil. Native wildflowers and grasses planted around your garden—and as cover crops post-harvest—have been scientifically proven to suppress weeds and attract beneficial insects that consume pests.
A technique called soil solarization can turn a weedy, pest-infested lot into a naturally nutrient-rich plot of soil. “Soil solarization” sounds a lot more complicated than it is; it basically consists of covering the ground with a tarp over the summer and waiting. The high heat under the tarp cooks off soil-borne pests and weed seeds without the use of pesticides or herbicides. Use soil solarization as the first step in creating truly organic and environmentally friendly cannabis.
This technique will kill and help get rid of weeds, some nematodes and fungus but will leave most beneficial organisms intact. For example, earthworms can escape the heat by burrowing further down, and mycorrhizal fungi can quickly readapt to solarized soil once pathogens have been eliminated.
Pick Your Plot
You might not have much of a choice, but try to grow on land that already has plenty of topsoil. If not, you might want to start first by layering organic material down to help develop more soil. Check out our instructional on soil-layering by Dragonfly Earth Medicine.
Solarization works best in warm sunny areas, but cooler climates can use a modified technique to reap the benefits as well.
Prepare The Soil
Mow or mulch the soil over to get rid of whatever weeds are growing there, and then rake the area to get rid of sticks and stones that can rip the plastic. Make it as level as possible; the more even the soil is the less air movement between it and the plastic, which will provide better heating.
Before placing the plastic on top, drench the soil until soaked 12 inches deep. Wet soil conducts heat better, making pathogenic organisms more vulnerable. Don’t irrigate once solarization is under way, because this will lead to cooling through evaporation.
Pick Your Plastic
A cool or partially sunny climate needs almost a whole summer’s worth of solarization, while sunny climates can complete the process in just five weeks. If you plan on solarizing through the whole summer or on a large scale, you will need UV-stabilized plastic, the kind used in greenhouses. If solarizing will only be a five to six week process on a small scale, you can use a generic “painter’s plastic” from a hardware store.
Though it may seem counterintuitive, solarizing plastic should be white or transparent. Black plastic does not work as well. Clear plastic creates a greenhouse effect over the soil, cooking it under the hot sun to temperatures up to 140°F.
How thick should the plastic be? Thin plastic generates more heat, but will tear more easily. Thicker plastic can be used for small solarization areas. You can use greenhouse repair tape or a similar product to patch any holes.
Cooler climates also benefit from a double layer of plastic, which can provide up to 10°F of additional heating. Use plastic water bottles or PVC tubing to put a few inches of space between the two layers to create an extra greenhouse effect.
Solarize That Soil
Cover the plot with your plastic, bury the edges and weigh down the center with a few rocks. If you know wind might become a problem, first dig a 4 – 6 inch trench around the perimeter, bury one edge of the tarp and stretch it over the plot like saran wrap and bury the other edge to keep it taut. This might be a two or three person job.
Keep the soil covered in a sunny season for at least five weeks. Deep-rooted weeds need extra time to perish in the heat, or even consider an additional cycle of site preparation the following year to fully eliminate pesky weeds.
Plant A Cover Crop
Now that fall is almost here, it’s way too late to sow any cannabis seeds, so plant a cover crop to suppress weeds and attract beneficial insects. Depending on the climate, the cover crop will die over the winter or stay alive through the spring. If the cover crop doesn’t die, leave strips of it alive on the edges of your garden to continue to attract beneficial insects.
Cover cropping is a science on its own; the right seed mixture needs to be selected for the right purpose and climate. If weeds are a problem, plant native brassicas that will absorb excess nutrients and prevent their growth. If insect pests are a problem, plant native legumes (like Crimson clover, good for warm climates of the West Coast and southern US) that provide pollen and shelter for beneficial insects. To add nutrients to the soil, plant a combined grass-legume cover crop to add nitrogen and organic matter and to prevent runoff.
Leave some cover crops or native wildflowers alive around your garden through the growing season to reap these benefits year-round.
Whatever you do, do not till. Tilling can bring up viable weed seed and disturb the underground habitats of earthworms, mycorrhizal fungus and beneficial insects.
Here we have outlined the basic steps involved in soil solarization, but read this great instructional as well, provided by the UC Davis Integrated Pest Management Program. For more information about conservation biocontrol check out the resources provided by the Xerces Society for Invertebrate Conservation. For more information about cover-cropping, you can download the free book, Managing Cover Crops Profitably, published by the USDA Sustainable Agriculture Research and Education program (SARE).