Introduction: Average Four Minute Read
During the winter months we sometimes get distressed messages, largely from beginning growers, asking why their beautiful plants are suddenly showing signs of herming or, more accurately, intersexing.
Sex determination in Cannabis is controlled in three ways - by genetic factors, by non-genetic factors, and by hormones. While genetics and hormones are huge, in this article we’ll focus on the non-genetic, non-hormonal factors that affect herming and that are largely within the grower’s control.
Both science and our collective experience says that although intersex/herming is most often caused by stress, but in some varieties that tendency is built into the cultivar’s genetics and those Cannabis varieties will express intersex even if they are never stressed.
Some cultivars, and some plants within those cultivars, seem to resist intersexing under extreme swings of temperature, humidity and even lighting conditions. There are no studies we can find that specifically discuss how the natural ratio of males to females varies in outdoor populations of the same cultivars under different environmental conditions.
The bottom line seems to be that every grower has to deal with a complex interplay of genetic and environmental factors, along with hormonal wild cards, that can cause some plants to intersex and some not, even in the best-controlled indoor environment. If the seed line comes with a well-documented genetic heritage and provenance, then that’s one less unknown to deal with.
The sensitive varieties are also more easily tripped by even moderate stress from other things like light leaks during dark time, but in our experience the majority of non-genetically determined intersex/herming is caused by some stressful combination of poor VPD (humidity) control, cold air, cold water, or cold soil.
It doesn’t have to be a consistently cold air, water or soil environment either - a short dip in temperatures at the wrong time can be all it takes. Cold temperatures aren’t just a cause of intersex/herming either - they are directly involved in low THC and CBD production, in slower vegetative growth and poor bud development, in vulnerability to pathogens and molds, and in underdevelopment of key ‘minor’ cannabinoids and terpenes.
OBSERVATIONS FROM EXPERIENCE
“Back in 1996 I was growing Cannabis in Amsterdam with Old Ed Halloway and he taught me that if I watered with ice cold water during the first few weeks of flowering, it would usually cause the plants that are semi-tropical to stress and grow male flowers naturally, without chemicals. I did this because I wanted to bring home the genetics of my favorite plants as S1’s and he knew an organic way to do it.” (TM)
#1 on our list of undesirable intersex/herm-inducing cold stress is having the root zone too cold during emergence and the seedling stage, followed closely by having any of the other plant’s zones too cold at any stage. “Too cold” means outside of a specific range.
‘I like to keep the daytime temperature of my Mother-room between 75 to 80 degrees tops, with nighttime temps no lower than 60-65, while maintaining around 70% humidity. Then in my flowering room as my plants go into flowering stage I drop the humidity to around 60 while maintaining daytime temps between 72 & 75 and nighttime temps between 65 & 70. Stable temps and humidity are critical.’ (TM)
Varieties with tropical genetics can tolerate higher humidity levels because of their loose and spindly flower structure that lets air flow in and around the individual buds. However, plants from colder places of origin with densely packed flower structures like the Afghan varieties are more at risk from mold formation when high humidity combines with poor air circulation and creates stagnant air pockets inside the flower.
Some varieties are also more sensitive to root zone temps than others but all are cold-sensitive in the sense that they can be pushed toward intersex/herming later in life by exposure to cold temps during germination or any time afterwards.
The good news is that this #1 stressor is also among the easiest to manage given grower awareness. Here’s some advice Todd gave to a grower in the Rockies a while back that we hope makes the point clearly:
“I have also had the experience of growing the exact same cutting from the same mother plant in different rooms and had the very same cutting sometimes herm and other times not, and because of that experience I always look at the environmental factors before I look at the genetics.
“Some cannabis cultivars are more sensitive to their environment than others, but that is because they come from very different regions and are acclimated to very different climates than what thy are being exposed to, such as 5000 feet in elevation during winter.
“I would definitely recommend controlling your evening temperatures and not letting them drop past the mid 60s, I would also recommend monitoring your VPD which means you have to have pretty big humidifiers to bring up the humidity of the dry climate at altitude, and I would definitely recommend heating your watering tanks, because if you are watering with pretty cold water which I suspect you may be, you are definitely going to get intersex tendencies from plants that would be stable in warmer more humid conditions.”
Now, in no special order, here’s a look at some of the other known intersex/herm-inducing stressors:
- Photoperiod disruptions or irregularities, along with inadequate lighting, are common reasons for intersexing - especially light leaks during the dark cycle right after you switch over to the 12/12 flowering cycle which is a critical time in gender expression differentiation.
- A very important temperature-related issue is VPD - Vapor Pressure Deficit. There’s a “sweet spot” at every temperature level (see chart below) where the humidity is neither too high nor too low, which means the leaves can breathe. When the vapor pressure outside the leaf is greater than the pressure inside, the leaf can’t expel waste products and decay and mold processes are initiated inside of the leaf and external pathogens and insects can be attracted as well. Under this kind of stress any plant that survives is being pushed toward herming.
Growers can control this and a lot of other undesirable events like Powdery Mildew in their indoor grow simply by managing the relationship between temperature and humidity in their growing environment.
This chart shows you that VPD “Sweet Spot” you need to hit at every temperature level in your grow. Check this out - at an ideal 75.2 degrees F. your humidity has to be between 60-80% in order for your plants to breathe. But only 5 degrees higher at 80.6 F your humidity has to be kept between 70-85% - your plants need higher levels of humidity in a narrower temperature band. That takes management.
- pH - a growing medium too far out of the 5.8 - 6.4 zone in either direction creates progressively greater stress. The plant is progressively being deprived of the specific balance of nutrients it needs to cue the ‘decision’ to become male or female. In other words, the cues that tell it which part to play in the natural cycle become that much more difficult for a young Cannabis plant to interpret. As everyone knows, and this graphic shows, Cannabis has a narrow range for optimal development and that includes avoiding pressure to intersex/herm.
- Water stress caused by too much, too little, or too erratic timing is another major cause of intersex/herming. This applies as much to whole fields of Cannabis as to single plants in a container. This is another strong indication that intersex/herming is a survival response - when the environment is detected as hostile and reproductive survival is threatened the plant responds to ensure that survival.
- Fertilizer/Nutrient/Supplement issues - Be very wary of using any nutrients that contain growth hormones or PGR’s. You probably won’t get the results you want and you’ll definitely increase your chances of getting hermaphroditic plants.
- Cultivar selection - Science is establishing that the germplasm in the seeds of different cultivars carries different proportions of male & female genetic packages and that those proportions affect growth and development outcomes under stress.
“Durban Poison is known to herm, which has been part of the conversation with growing the variety since the 80s. I tell everybody not to breed with the plants that intersex, but Durban plants that don't herm are worth working with because of the short flower time and the great flower structure. It is also one of the most sensitive varieties to environmental problems. If anything in the room is even a little bit off, it will herm.” (TM)
END OF 4 MINUTE INTRODUCTION
REFERENCES & COMMENTS:
INTERESTING NEW SCIENCE BEHIND HERMING
Here are several current research papers, with our comments, that we think offer useful information and valuable perspectives on one or more of the important aspects of herming in Cannabis.“Hermaphroditism in Marijuana (Cannabis sativa L.) Inflorescences – Impact on Floral Morphology, Seed Formation, Progeny Sex Ratios, and Genetic Variation”
There’s lots of useful info in this research on stressors that induce herming but most interesting to us is their finding that Hermaphrodite female flowers produce only genetically female plants. The research doesn’t look at whether these all turn out to be healthy females, it just finds that they are all genetically females.
Skunkman Sam: “Male intersex plants are different because they are XY times XY so out of any seeds that result, 25% will be YY, 25% will be XX and 50% will be XY.”
We wonder if this research applies only to the cultivars these researchers were working with, or if this could be a more general phenomenon. If seeds from hermaphrodite plants of any cultivar are all genetically female - that might have some interesting implications for breeders and growers.
Here are some especially interesting quotes:
“Seedlings from hermaphroditic seeds, and anther tissues, showed a female genetic composition while seedlings derived from cross-fertilized seeds showed a 1:1 male:female sex expression ratio. Uniquely, hermaphroditic inflorescences produced seeds which gave rise only to genetically female plants.”
Note: We would really hesitate to put any work into growing seeds from an inersex female even with this research in hand
“In the present study, we observed spontaneous formation of hermaphroditic flowers on 5–10% of plants of three different strains of marijuana grown indoors under commercial conditions. In most cases, small clusters of anthers developed within certain female flowers, replacing the pistil. In rare cases (two out of 1,000 plants), the entire female inflorescence was displaced by large numbers of clusters of anthers instead of pistils The factors which trigger this change in phenotype have not been extensively researched.”
Skunkman Sam: “If these were DNA tested I suspect they would be XX just like females that are transformed by Silver thiosulfate (STS). They may look male after transformation but that is just sex expression not what their DNA is. DNA is not changed from female to male or vice versa by STS.”
“Physical or chemical stresses can also have a role in inducing staminate flower development on female plants of marijuana. For example, external environmental stresses, e.g., low photoperiods and reduced temperatures in outdoor production, were reported to increase staminate flower formation.”
“Such stress factors could affect internal phytohormone levels, such as auxin:gibberellin ratios, which could in turn trigger hermaphroditic flower formation in marijuana plants. In marijuana plants, environmental stress factors which enhance jasmonic acid (JA) production could potentially promote hermaphroditic flower formation but this requires further study.”“Recent advances in Cannabis sativa genomics research”
This quite technical but still very readable research discusses new discoveries in Cannabis genetics that will allow breeding of novel cultivars with desirable cannabinoid profiles, including pretty advanced discussions on genomic variations and their effect on Cannabinoid content. It also discusses the genetic basis of sex in some detail. Frankly most of the science is way beyond us but the information here is interesting and there are plenty of smart people in our community who may be able to apply it.
“The identification of sex chromosomes in the cannabis genome is another notable genomics172 driven achievement. Of the 565 sex-linked genes identified in the PK transcriptome, 363 were mapped to cs10 v. 1.0 chromosome 1 (cs10 v. 2.0 chromosome 10), indicating that this chromosome pair constitutes the sex chromosomes (. This enabled the identification of sex-specific molecular markers to aid cannabis breeding.”
Skunkman Sam: “This is using DNA to find an XY chromosome. There are no reliable DNA tests to find female or intersex that like monoecious plants that are DNA tested will test as female.”
“THCA and CBDA are produced at much higher concentrations in the inflorescences of female cannabis plants compared with males, hence female plants are economically more valuable.
“Having the capacity to identify male and female plants at an early stage enables yield improvement and better management of cannabis crops. Approximately 3,500 sex-biased genes have been identified, which are differentially expressed between female and male cannabis plants, with a subset being expressed in the flower buds. These are genes are not restricted to the sex chromosomes. Some are located on the Y-chromosome of male plants and are involved in trichome development, sex determination, hermaphroditism and photoperiod-independence.”
SKUNKMAN SAM: “Male testing can find males, but simply not testing male does not mean a plant is a female. It could also be intersex, and you do not want to use or breed with any intersex plant as it will give progeny that are intersex, either due to genetics or to increased vulnerability to stress.”“Architecture and Florogenesis in Female Cannabis sativa Plants”
While this research is not strictly about Cannabis hermaphroditism, and it offers some unusual explanations for flower induction, it introduced us to the remarkably useful concept of female Cannabis plants having an “architecture” that influences their flowering characteristics, which we want to share here because it is such a useful concept. Since architecture is intentional structural design to produce desired results, it makes a lot of sense to approach our plants this way and to consciously structure their architecture. This article discusses some very hands-on and practical ways that growers can influence and manipulate their plants’ flower yield.
“Short photoperiod induces intense branching, which results in the development of a compound raceme. Each inflorescence consists of condensed branchlets with the same phytomer structure as that of the larger phytomers developed under long day. Each phytomer consists of reduced leaves, bracts, one or two solitary flowers, and an axillary shoot (or inflorescence).”
“Understanding the morphophysiological and genetic mechanisms governing flower and inflorescence development is therefore of high scientific and practical importance. However, in-depth investigations of cannabis florogenesis are limited.
Cannabis producers and researchers consider long photoperiod to be “non-inductive” or “vegetative,” but under these growth conditions, the development of solitary flowers and bracts in shoot internodes clearly indicates that the plant cannot be defined as vegetative or non-inductive in the classical sense. Most probably, induction of solitary flowers is age-dependent and controlled by internal signals, but not by photoperiod.”
“Therefore, the effect of short photoperiod on cannabis florogenesis is not flower induction, but rather a dramatic change in shoot apex architecture to form a compound racemose inflorescence structure. An understanding of the morphophysiological characteristics of cannabis inflorescence will lay the foundation for biotechnological and physiological applications to modify architecture and to maximize plant productivity and uniformity in medical Cannabis.”
Skunkman Sam: “Males with XY can have a female plant architecture and they will have bigger and more male flowers. Females with XX male architecture give less branching with slimmer flowers and are not desirable.”“Dioecious hemp (Cannabis sativa L.) plants do not express significant sexually dimorphic morphology in the seedling stage”
This very early-days research looks into whether there is any way to physically distinguish plants that are on their way to becoming male or female during the seedling stage of growth, and whether any such signs would vary among different Cannabis cultivars. Obviously the earlier we could tell for sure which plants are going to be male, the less labor and expense there would be. We aren’t too excited about this approach compared with others like DNA analysis but eyeballing your plants for male/female signs is an old tradition among growers and every little clue helps.
“We asked: can we reliably differentiate males, females, and co-sexual individuals based on seedling morphology in Cannabis sativa, and do the traits used to distinguish sex at this stage vary between genotypes?
“Preliminary evidence suggests that co-sexual plants may be distinguished from male and female plants using short hypocotyl length and seedling height, although this relationship requires more study since sample sizes of co-sexual plants were small. In one of the cultivars, two-week old male plants tend to produce longer hypocotyls than other plants, which may help to identify these plants prior to anthesis.
“We call for increased research effort on co-sexual plants, given their heavy economic cost in industrial contexts and rare mention in the literature. Our preliminary data suggests that short hypocotyl length may be an indicator of co-sexuality. These results are the first steps towards developing diagnostic tools for predicting sex using vegetative morphology in dioecious species and understanding how sexual dimorphism influences phenotype preceding sexual maturity.”“The sexual differentiation of Cannabis sativa L.: A morphological and molecular study”
Like the previous article this research is focused on identifying physical characteristics that will allow growers to identify Male and Female plants at the earliest possible stage of development.
“Cannabis sativa L. is a dioecious species with sexual dimorphism occurring in a late stage of plant development. Sex is determined by heteromorphic chromosomes (X and Y): male is the heterogametic sex (XY) and female is the homogametic one (XX). The sexual phenotype of Cannabis often shows some flexibility leading to the differentiation of hermaphrodite flowers or bisexual inflorescences (monoecious phenotype).
“Microscopic analysis of male and female apices revealed that their reproductive commitment may occur as soon as the leaves of the fourth node emerge. Five of the several cDNA-AFLP polymorphic fragments identified have been confirmed to be differentially expressed in male and female apices at the fourth node. Cloning and sequencing revealed that they belong to nine different mRNAs that were all induced in the female apices at this stage.”“Microgametophyte Development in Cannabis sativa L. and First Androgenesis Induction Through Microspore Embryogenesis.”
The article contains many fascinating micro-photographs of deep structures within the Cannabis flower as the authors discuss pollen and seed development. Paraphrasing the research, it says that exposing Cannabis flowers to cold during seed development makes plants resulting from those seeds more likely to herm.
This response will vary since there are cold-adapted cultivars that are genetically programmed to germinate, grow and thrive in cold high altitude climates, but this research reinforces the principle that growers must avoid cold temps while managing their humidity.“The Complex Interactions Between Flowering Behavior and Fiber Quality in Hemp”
This article is focused on Cannabis Sativa as the hemp plant HOWEVER it’s discussion of the complex interplay of environment and genetics in the development of female flowering is field populations is easily interpolated to intersexing stressors in indoor growing conditions and helps us relate to the behavior of Cannabis in nature.
“Hemp is a facultative short-day plant, characterized by a strong adaptation to photoperiod and a great influence of environmental factors on important agronomic traits such as “flowering-time” and “sex determination.”
“Herein, we review the current knowledge of phenotypic and genetic data available for “flowering-time,” “sex determination,” and “fiber quality” in short-day and dioecious crops, respectively.”“Photons from Near Infrared LEDs can delay flowering in short-day soybean and Cannabis: Implications for phytochrome activity”
“There are concerns in the Cannabis industry that photons from NIR LEDs cause monecious flowering. Cannabis is naturally dioecious; only female plants are desired for medical Cannabis cultivation.
Monoecious flowering is often confused with hermaphroditism. Botanically, these terms are distinct: monoecious refers to the presence of separate male and female flowers on the same plant, while hermaphrodite refers to the presence of both male and female reproductive organs within an individual flower.
In practice, the distinction is not important because both monoecious and hermaphroditic Cannabis produce pollen and potentially reduce product quality and value. The tendency of Cannabis to form monoecious or hermaphroditic plants is under genetic and environmental influence.”“When the going gets tough, the tough turn female: injury and sex expression in a sex-changing tree”
This research isn’t specifically about Cannabis, but we think it’s a great look at how some plants respond to stress by becoming stronger and female. We know that early pruning produces more vigorously branching Cannabis plants and flower production in females, but we don’t know whether pruning before sex differentiation increases the likelihood of going female.
“We found that severe damage such as full defoliation or severe pruning increased odds of changing sex to female and decreased odds of changing to male. In fact, no pruned male trees flowered male 2 years later, while all males in the control group flowered partially or fully male. After full defoliation, trees had 4.5 times higher odds of flowering female.”
“The influence of physical damage on sex expression varies across species. In general, poor plant health seems to correlate with maleness, but the experience of trauma is more complex. In some species with generally constant sex expression, the removal of branches, stems, flowers, or storage tissues promotes maleness; in other species similar actions promote femaleness, at least temporarily within a flowering season.”
NOW TO WIND THINGS UP, HOW ABOUT A LITTLE SPECULATION?
If you’re up for it here are some of our other more speculative thoughts on herming in Cannabis.
Let’s begin by framing sexual differentiation in Cannabis from a perspective we think everyone can relate to from our shared human experience. Let’s imagine a typical family with a Mother, Father and three kids. Not unusually, three very different kids. One gay, one straight, and one bi. Same mother, same father - right? Same home, same family influences, same environment, but each taking a different sexual path in life. Lots of physical features in common - you can see they’re brothers and sisters - but manifesting as very different people. It happens all the time in every family - right?
We believe, along with most science, that this happens because diversity is simply nature’s way. With diversity, nature guarantees strength and survival of the species. We always look for nature’s preference for diversity behind every difference we see in our plants.
Animal life of all kinds needs males and females to reproduce little birds, bees and babies, but most plants don’t need gender differentiation - each individual plant can reproduce itself. My pollen, my flower, my seed - and look! More me!
But nature's unbreakable rule is diversity, so of course Nature sets it up so that not all plants can reproduce all by themselves. And so it’s in the name of diversity, nature’s dominant principle, that we have been given Cannabis, one of nature’s true marvels.
Of all plants, only 6% are dioecious and Cannabis is one of the rare plants that need gender differentiation to reproduce, but unlike animal life the little Cannabis plant isn’t ‘born’ with identifying male or female physical features. Almost all (but not all) animal life develops into either physical male or female before it is born, with physical sex differentiation having taken place long before “emergence”, or birth.
But Cannabis emerges from its seed casing as a physical being that is prepared to be either, or both, because Cannabis carries both male and female genetic ‘packages’ well into their development waiting and staying alert to see what circumstances tell it to be. In other words, maleness and femaleness in Cannabis is genetic, but the manifestation of that trait is determined largely (but not always) by environment. Natural diversity at work again.
Under non-stressed, natural conditions Cannabis responds to a set of environmental cues, as well as to its built-in genetics, by ‘deciding’ at the appropriate point to go on to be either a male or female. The science tells us that while Cannabis seed can withstand prolonged very cold temperatures in dormancy, once germination begins, cold temperatures at any point from then on can push the seedling further toward any tendencies it already has for manifesting either maleness or hermaphroditism. If you want nice healthy 100% (or close) females, give your plants ambient temps from the beginning.
Based on what we are learning about communications between plants, it may even be that in a natural environment, untended by humans, Cannabis makes this a community-level decision.
“OK - Listen up sprouts. We’re only gonna need 20 guys in this patch. Who are my volunteers?”
But what happens when the environment, especially the man-made environment, is sending the Cannabis plant confusing signals by creating one or more kinds of stress just at the time the plant is trying to decide what to become? The plant is being pushed and pulled - “What’s going on and which way do I go? Am I in danger? Am I going to be able to reproduce?”
Well, it seems to us that what we see as hermaphroditism may be a plant that couldn’t, or wasn’t able to decide on gender and so, because it had to do something, and was so damned confused and under pressure, that it just went ahead and became a little of both. The plant falls back on necessity - “Well if I have to, I’ll just do it all myself.”
We hope that doing this kind of personification of plants, a scientific no-no called anthropomorphism, doesn’t offend anyone’s sense of scientific propriety, since we find it a great way to try to understand what our plants are going through and why they are behaving the way they are. We try to put ourselves in the plant’s place and look at the world from that point of view.
To wrap this discussion up, here’s one of the clearest statements that we’ve encountered on sex and Cannabis (even though the English is a little off):
“The diversity of theories, and of proofs found in their reasoning, result from the fact that genotype is not only the information placed on chromosomes, but also the information from various cell organelles which contain nucleic acids.
Consequently, to affirm that the sex is determined only by chromosomes or only by environmental conditions constitutes, if not a total mistake, then at least a narrow, limited approach of an extremely important biological phenomenon, with special implications for live world evolution.
Therefore, we consider that the sex, like to any other phenotype character, is a resultant of the flow of hereditary information within genetic channels, but in concrete environment conditions, and the hemp is a good example in this direction.”