As with every other living object on the planet, marijuana plants are susceptible to unique hazards and illnesses. No matter how much TLC you lavish on your crop, you are eventually going to encounter some type of threat or problem to your plants. This article is designed to give you an idea of the problems you might face as either an indoor or outdoor grower, and to offer some hints on how to overcome the most common obstacles.
Attrition rates for fledgling plants are often terrible for outdoor growers—more than 50 percent in some cases—and whether most plants live or die is a daily challenge until they’re at least two months old and a foot high or more. Even then there may be dangers from slugs, whose very passage across foliage leaves it dead and fouled with mucus, or even deer, elk, and moose that can (and do) wipe out more than a dozen large plants in a single night. And then of course there are people; reapers who will make off with all the plants that they can carry, regardless of their stage of growth, and sharp-eyed drug cops who live to be on the evening news next to a pile of burning marijuana plants (not a bad place to stand, actually). Outdoor growers contend with a number of natural threats to their crops, but most threats can be dealt with effectively. Following are some of the more common pests a grower can expect to encounter.
Grasshoppers and leafhoppers are among the few leaf-eating insects that seem able to subsist on hemp foliage. Beginning at the edges, both of these insects eat away narrow strips of leaf until sometimes there’s nothing left except the heaviest veins. In fact, though, these bugs don’t usually eat much, and it’s almost worth it to see the little critters so stoned that sometimes you can pluck them from a leaf. If either should become a problem, I lightly spray a perimeter of permethrin-based insect repellant onto the ground around the plant out to a radius of 6 feet. This synthetic pyrethrin—a product of the daisy family—is relatively safe to humans, and is the active ingredient in pet flea sprays, but kills insects and arachnids on contact. I don’t recommend using it on any part of the plant, but a perimeter of permethrin on the ground is sufficient to discourage most plant-eaters (even mammals) for a period of at least three days. Permethrin has no lasting effects on a plant or to the environment.
Slugs can be a serious threat to marijuana plants, especially during wet, warm summer nights when they emerge from beneath damp, rotting logs and stumps to find food. Left unchecked, just a half dozen of these lowly foragers can climb the stalk of a 3-foot plant, consume a surprising amount of leaf material, and inflict real injury to the plant. For a reason I don’t know, the slime trail a slug leaves across marijuana leaves is sufficient to kill those leaves by itself. Worse, slugs seem drawn to tender branch ends, which their browsing kills. And since branch ends are where most growth occurs—including buds—it becomes imperative to stop invading slugs before they kill entire plants.
Once again, a skirt of permethrin sprayed onto the ground around plants is an effective wall against slugs, which react immediately and violently when they touch it. If possible, visit your plants after dark, when slugs are most active, and use a dim flashlight or headlamp to pluck any that you find from plants using your fingers. You might drop the repulsive creatures into a jar for relocation to a less objectionable location, but I generally pull them from a plant and fling them into the woods. The airborne relocation does no harm to the slug, but I know it won’t be back tonight, while a shield of permethrin ensures that it won’t return after that.
Deer usually leave marijuana plants alone, but like squirrels that regularly nibble hallucinogenic mushrooms, there are a few fervid stoners that enjoy being high, and these individuals may strip an entire plant. Lacking upper incisors, deer (cattle too) with a taste for pot take branches between their lower incisors and hard upper palate and pull backward, stripping off all the foliage from that branch. Left unchecked, a single deer can completely defoliate two or more 4-foot plants in a single night.
Some large-scale growers for whom healthy plants mean income claim that the very best repellant for deer in the dope field is deer blood—and as a bonus you get to eat the deer that was made an example of. Less violent repellents can be as simple as depositing your own urine onto trees and shrubs to demonstrate that you are the dominant predator (vegans aside, all animals regard humans as meat-eaters). This is my preferred animal-repellant method. Should deer become too numerous and too pushy, urine scents sold in hunting supply stores and marketed for hunting work well if used judiciously; white-tailed deer will avoid the scent of a dominant moose or a sow bear, as will most animals.
Other animals, from mice and chipmunks to some birds, eat marijuana shoots and tender young plants before their stalks become woody and their THC content gets too high. Smaller animals also respect human scent-posts of urine, but the best method for frightening off the little guys is the scent of a predator built by nature to hunt them, like fox or coyote scent, which is sold online at Buckstopscents and other manufacturer Web sites.
Spider mites and other parasitic bugs that feed and reside on growing marijuana plants are seldom a problem when growing outdoors. Should mites or aphids invade your plants, it goes without saying that conventional spray-on insecticides are not a viable option for foliage that will find its way into a human body. Closet growers have had good luck spraying leaves and branches of infested plants with a solution of fresh garlic and water steeped to make a tea that does no harm to plants but is apparently repugnant to most parasites.
Gypsy moth caterpillars, known colloquially as “armyworms,” are common across America, and their large webbed habitats in tree crotches are a common summer sight, especially in cherry and other fruit trees, and particularly when populations are at their cyclical high. The caterpillars, which have almost no natural enemies because they jus taste bad, may become voracious during these peaks, because they have literally eaten themselves into mass starvation.
Population “seasons” that wax and wane like this are found throughout nature, and when starving overpopulated caterpillars are driven to feed and propagate under conditions that are simply inadequate to support their numbers, plants and trees that are not normally food will be razed as well. Again, a perimeter of permethrin sprayed onto the ground for a distance of about 6 feet all around is an effective deterrent, but it might need to be reapplied every two or three days and soon after every rain. By experimenting, I’ve also had good results from surrounding plants with a thin layer of fresh cow manure; for whatever reason, armyworms seem unwilling to crawl across fresh poop. Worms that do make it onto your plants are most effectively dealt with by plucking them off by hand the way farmers of old did before insecticides and crushing them underfoot-that practice might not set well with some folks, but what are you going to do, relocate the caterpillars?
Black widow spiders seem to have an unusual affinity for pot plants. I can’t say why, but I can relate several midsummer experiences when I’ve found an elegant female black widow perched on a web she’d anchored to the branches of a large marijuana plant. I leave them alone until harvestable, but I advise plucking a widow-inhabited plant with caution, especially when a protective mother is guarding a roundish egg casing suspended in her web.
If you order marijuana seeds from my webshop and you get problems with pests or diseases, I’m here to help you. There are guides about plant care where all pests/diseases are discussed and you can contact me by mail. Please like or share this article