Editor's note: This story was updated at 12 p.m. ET on Friday, Oct. 8, 2021 to remove specific references to 2019 rainfall records in Portland, Ore.
Heavy rainfall during harvest season can leave outdoor cannabis and hemp cultivators across the country scrambling to save their crops.
When rain is in the forecast, what’s an outdoor cannabis grower to do?
Heavy rain can cause physical damage to crops, but the real threat is mold, according to Ryan Douglas, owner of cannabis consulting firm Ryan Douglas Cultivation, LLC. After heavy rainfall, cool night temperatures can create the perfect conditions for spore germination and propagation, he says, and almost nothing can be done to cure an infection once it sets in. Therefore, preventative measures are key.
“Powdery mildew may take advantage of wet conditions to germinate spores, but the real killer is Botrytis cinereal,” Douglas tells Cannabis Business Times and Hemp Grower. “Also referred to as gray mold, this disease thrives in cool, wet conditions and can completely destroy a mature flowering cannabis crop within one week.”
Jade Stefano, owner of Puffin Farm, an outdoor cannabis cultivation operation in Washington State, echoes these concerns.
“Choosing cultivars that are resistant to Botrytis is the single best way to guard against issues,” she says. “Otherwise, there is not much that can be done, as we cannot change the weather. Covering plants is not usually effective because during heavy rain, the ambient humidity is so high, condensation still gets onto flowers, and covering reduces airflow, which can make problems worse.”
However, selecting genetics for disease-resistance can be tricky, according to David Bonvillain, owner of Elite Botanicals, a CBD-hemp cultivation, extraction/processing and product development business based in Colorado.
“It makes me think of the genetics quandary with regards to the recommendation for resistance,” he says. “Outside of very few, I would think almost no breeders out there are in the game of disease resistance. ... Most are only working on hype, flavor and potency. And [if they are breeding for disease resistance, you have to ask if they] have run extensive test populations outdoors and experienced the specific issues. But that is where it is going in the drug-variety hemp space, and, ultimately, where it will have to go in the THC space too.”
Rain can also cause nutrient deficiencies, root rot, overwatering problems and, depending on the growing medium, fluctuations in soil pH, according to Melissa Beseda, co-owner of Wildwood Flower Farm, an outdoor cultivation operation in Washington State.
“Excessive rain on field grown crops can indeed leach nutrients,” says Dr. Brian Jackson, associate professor and director of the Horticultural Substrates Laboratory at North Carolina State University. “Different soil types can cause nutrient leaching or waterlogged plants, which could cause root rot. It also depends on if plants are on plastic or bare ground as well.” When growing in containers outdoors with soilless growing media, nutrients can be replaced with supplemental fertilizer additions after heavy rains, he adds.
Here are experts’ tips for guarding outdoor cannabis crops against heavy rainfall, from proactively choosing the right cultivars to acting quickly after a weather event to save the plants.
1. Consider preventative fungicides.
Regular applications of preventative fungicides that contain Bacillus subtilis as the active ingredient can help prevent fungal infections that may occur after periods of prolonged heavy rain, Douglas says. The naturally occurring bacterium produces antibiotics that suppress fungi, which can minimize damaging outbreaks of Botrytis and powdery mildew.
“This product is most effective when used preventatively, so begin applying it in the vegetative stage to guarantee that the Bacillus has a chance to colonize the plant prior to periods of frequent rain events,” Douglas says.
Jackson agrees “that it is the period of wetness that promotes fungal growth and not necessarily the ‘heaviness’ of the rain (amount).”
2. Gently shake water off the plants.
While there is little that growers can do during a heavy rainfall to protect their crops, it may be feasible to give plants a gentle shake after a heavy rain event, depending on the size of the operation.
“While this seems like a futile activity, it decreases the amount of water on the plant and the amount of time it will take for that water to evaporate,” Douglas says.
“This reminds me of what the cherry growers in Washington State sometimes do when the fruit are ripening and rainfall comes; they hire helicopters to hover above orchards to dry the trees/fruit before the water soaks in and causes the fruit to split thereby ruining them,” says Jackson.
3. Apply potassium bicarbonate.
An application of potassium bicarbonate following a heavy rainfall can help alter the pH of the water-leaf surface, Douglas says. This can help kill fungal spores or make the plants less susceptible to new infections.
“There are scientific reports on the benefits of foliar/fruit applications of potassium bicarbonate, especially for organic operations,” says Jackson. “The disease I am familiar with it mitigating/lessening is powdery mildew.”
4. Don’t pack wet plants together.
When harvesting a rain-soaked crop, Douglas advises minimizing the amount of time that wet flowers stay packed together between the field and the drying area. Ideally, wait until plants are dry before harvesting if feasible.
“Squishing wet, ripe flowers together for an extended period of time will increase the possibility of mold infection,” he says.
5. Dry rain-soaked plants at higher temperatures.
If harvest has to occur before the plants can naturally dry in the field, it is suggested that once the plants arrive to the drying area, cultivators should remove as much leaf mass as possible, and then dry the flowers at higher-than-ideal temperatures for the first 48 hours. While 65 to 75 degrees Fahrenheit is a preferred range for slow-drying cannabis over the course of seven to 10 days, cultivators should initially aim for temperatures of 75 to 85 degrees Fahrenheit if harvesting after a heavy rain. Growers should also add multiple fans to the drying area to ensure plenty of air circulation, he adds.
“This will help remove the majority of moisture on the plants and allow you to safely lower temperatures back into the ideal range after the first few days,” Douglas says.
6. Harvest immediately if there are signs of Botrytis.
Crops should be scouted daily for signs of Botrytis, Stefano says, and if any plants are affected, the entire crop should be harvested immediately. While buds infected with Botrytis must be removed and destroyed, unaffected plants can still be salvaged.
“Once it starts, it is unstoppable, and harvest and drying is the only way to save the unaffected plants,” she says. “They may not be great for flower, but early harvested plants are still good for concentrates.”
7. Consider weather events when choosing genetics and scheduling harvests.
The best way for farmers to guard their crops against damage from prolonged and/or heavy periods of rainfall is to start with good genetics, preferably ones that are mold-resistant, according to Beseda. Cultivators should maintain healthy plants with healthy roots grown in well-draining soil, she says, and they should also practice harvest diversification.
In addition to laying a good foundation with strong genetics, Beseda says that growers should always plan around the weather.
“It’s imperative that outdoor farmers know the weather forecast and plan their watering and harvesting schedules accordingly,” she says. “The weather dictates so much of our growing season, so you have to be adaptive. Ensure that the crop is properly trellised and staked so that heavy branches do not break. Be prepared to harvest trouble spots early and get really good at identifying the first signs of problems.”
8. Stay vigilant.
Identifying problems early means knowing what to look for, Beseda adds.
“Know the early signs of bud rot and check larger colas for signs of mold and mildew,” she says. “Be sure to have repair materials on hand to fix any damaged plants/branches.”
After heavy rainfall, cultivators should look for mold in places where it might not typically show up, Beseda adds, and they must act quickly to prevent it from spreading.
“The mold sets in very quickly and impacts colas facing away from the sun more often, and in areas water might stagnate,” she says. “Remove and quarantine any bud rot and monitor other branches from affected plants. When hang drying, consider drying at a lower humidity than normal to prevent spreading."