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Nothing makes cannabis growers’ hearts sink more than discovering mold on their plants. Sometimes referred to as “bud rot,” mold can quickly turn a once-nice plant into a worthless, decaying eyesore.
While there are ways to effectively treat plants with mold, the best way to avoid it is to prevent it from occurring in the first place. Like mold’s nasty twin sister powdery mildew, sound cultivation practices will all but eliminate it from impacting your grow.
Common Causes of Mold
Although it can occur in both indoor and outdoor settings, mold usually results from high humidity. Areas with persistent rain can make it a serious problem for outdoor growers. Indoors, it can occur when other environmental factors are at play and lapses in sound cultivation practices in the grow room occur. As well as high humidity, the main causes of mold are mild temperatures, poor ventilation, dense foliage, dense flowers, and plants with poor vigor and weak immune systems.
All cultivators should be certain to monitor temperature and humidity, keeping temperature above 68 degrees Fahrenheit and humidity at 40 to 50 percent when plants are in flower. Air circulation is critical while plants are flowering. Oscillating fans or a few mounted, stationary fans will provide good air flow throughout the room. If necessary, install forced-air ventilation in the grow room to remove stagnant, humid air on a steady basis. If you’re growing strains with an abundance of leaves, remove any leaves that are touching bud sites, since water can build up under the leaves and result in mold on the buds.
Mold will kill plants and ruin flowers, so its impact on a grow can be severe. Unfortunately, it often reveals itself well into the flower stage, when small, yellow leaves begin to appear on them. The yellow leaves are a sign that mold is occurring at their base, within the interior nooks and crannies of the flowers where moisture can hide and get a foothold. Left unchecked, mold will quickly destroy the entire bud and spread to other areas of the plant. If you detect mold—often found on the biggest, densest, and best buds—clip the bud from the plant immediately and remove it from the grow.
As well as following sound cultivation procedures without any lapses, there are other steps you can take to prevent mold from striking your grow. One of the best preventive measures is to grow mold-resistant strains. Some strains are more resistant to molds and fungi than others. To learn more about strains that are resistant to this malady, consider including some of the following strains in your next grow—particularly if you’ve had problems with mold in the past.
Growing Mold-Resistant Strains
The following strains are more resistant to mold than many other commonly grown strains on the market. If you’ve had mold problems in the past or live in an area with a high potential for moldy cannabis outdoors, consider investing in one or more of these strains. Growing them—combined with following the sound cultivation practices mentioned above—will help minimize the changes of mold impacting your harvest.
Some mold-resistant strains are landrace strains that have been grown in the same region for many years and thrive it wet environments where persistently rain-soaked buds can present the perfect environment for mold growth. These strains have not been crossed extensively and have been inbred, retaining certain qualities over multiple generations.
Many of the strains that grow well in Hawaii are known for their resistance to mold. Hawaii has mild temperatures and copious amounts of rain present an excellent environment for mold to get a foothold and become a pervasive problem.
Landrace strains with a long history of growing well outdoors in wet regions like areas of Africa, Asia, and Central and South America, may do well in areas with a propensity for mold in marijuana crops. Many mold-resistant strains are sativa-dominant or hybrids with genetics that go back to mold-resistant landrace strains.
Here are some good mold-resistant strains to consider:
- Moby Dick. This highly coveted strain is known as an easy one to grow. As well as being resistant to mold, it’s also very potent when grown under ideal conditions. This sativa-dominant strain is the result of crossing White Widow with Haze, and has a sweet peppery flavor and aroma with a touch of dank skunkiness present. Moby Dick is known for its quick finishing time and bountiful yield, two qualities that make it a popular favorite among cultivators.
- Strawberry Cough. Another flavorful sativa-dominant strain, Strawberry Cough has enticing creamy strawberry flavors that make it rank high as far as unique terpenes go. A descendant of an American strain called Strawberry Fields, Strawberry Cough created a lot of excitement in the cannabis space when it was released several years ago. Although the hoopla has died down a bit, Strawberry Cough is notable for being resistant to mold and makes a worthy addition to any cannabis garden, either indoors or outdoors.
- Durban Poison. This South African classic sativa has been inbred for multiple generations, and the pure version of this strains is still 100-percent sativa, looking and tasting just like it has since this strain first gained notoriety and popularity. Durban flowers mature more quickly than many other saliva, which is just one of its pleasing qualities. Quickly maturing buds also are desirable for mold prevention since mold doesn’t have as much time to get a foothold. The buds are highly coveted among cannabis connoisseurs and have excellent aromas and flavors that capture hints of pine spice and mint.
- Colombian Gold. This landrace strain has legendary status, although it’s not as popular or seen as often as in years past. Commonly imported in the 1970s, often with abundant amounts of seeds present, its origin is the Santa Marta Mountains of Colombia, Colombian Gold genetics have been preserved and used to create other strains, like Skunk #1, that now eclipse their Colombian parent in popularity. “Lumbo Gold” can grow to huge proportions outdoors and produces buds with earthy, spicy, and slightly sweet flavors. The pure Colombian Gold is an outdoor strain, but many strains with its genetics thrive in an indoor environment as well.
- Super Silver Haze. This immensely popular sativa-dominant strain is the creation of Green House Seeds and, as its name implies, contains quite a bit of Haze, which, in turn, has a healthy level of Colombian Gold in its genetic background. Although it’s sativa-dominant, Super Silver Haze’s lineage includes sativa strains Skunk and Haze with Northern Lights, a decidedly indica-heavy strain. The results are quite balanced and pleasing, with a happy, uplifting high that transitions into relaxed-but-functional effects. The taste is a tasty combination of woody, citrus pine.
- Thai. Another classic sativa heavily imported in decades past, Thai is a landrace strain from Southeast Asia, and it will grow to massive proportions outdoors, although most breeders have used it to create new strains rather than cultivate the pure form. In the 1970s, a variety of Thai buds became very familiar to American consumers. Not only were there Thai sticks, but Chocolate and Juicy Fruit Thai as well. The strains created by D.J. Short, like Blueberry, Flo, and Blue Moonshine, nearly always contain decent amounts of Thai. Some of these Thai-heavy strains are known to resist the development of mold.
- Sweet Tooth. Another classic that’s been around for many years, Sweet Tooth has a devoted following, particularly on the West Coast. As you might have guessed from its name, Sweet Tooth has a pleasing candied sweetness and a floral berry flavor. Its genetics include landrace strains from Nepal, Afghanistan, and Hawaii, which contribute to its mold-resistant reputation. Although it’s a sativa-dominant hybrid, Sweet Tooth has a relaxing effect, without the racy high so common to some sativa hybrids. Sweet Tooth is still available in either clone or seed form, and makes a worthwhile—and mold resistant—addition to any indoor or outdoor garden.
- Holland’s Hope. This is a decidedly outdoor strain that’s been around for decades and finishes fairly early, as soon as late September or early October. Readily available and known for huge yields and good potency, Holland’s Hope would make a great choice for any outdoor grow in wet climes where mold can be a problem. This is the only indica-dominant strain on our list of mold-resistant strains, and it can grow quite large for an indica. Created specifically for Holland’s wet outdoor environment where mold can be a problem, this strain has a good reputation for its resistance to mold and its tasty woody and floral terpenes.
If you grow outdoors in a wet climate where mold can get a foothold, you may want to consider growing one of these strains. Indoor growers who have experienced an issue with mold or powdery mildew may want to consider one of the ones that grows well indoors, as well. When it comes to mold, all strains are not created equally, and some hold a higher rank when it comes to mold resistance.
Since landrace sativa strains from wet regions of the world are usually the best for wet climes, the majority of mold-resistant strains lean to the sativa side when it comes to genetics, but some of them with indica phenotypes live up to the indica stereotype as far as effect goes. There’s no reason to let a fear of mold hold you back from enjoying a bountiful harvest. Just choose your strains carefully!
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Luis Cordova is a distinguished author, and renowned expert in cannabis cultivation, who possesses a Master's degree in Plant Biotechnology and Pharmaceutical Science. As a valued contributor to highly esteemed publications such as Cannabis Training University and Maximum Yield Magazine, Luis has emerged as a trusted source of guidance and knowledge in the cannabis industry. Having written thousands of informative articles, Luis is widely recognized for his comprehensive expertise on cultivating cannabis, both indoors and outdoors.