Autoflower seeds

Autoflower seeds (also known as “automatic seeds” or “auto seeds”) produce cannabis plants that flower independently of the light hours they receive, unaffected by photoperiod changes.

Most cannabis strains are photoperiod-dependent, meaning their flowering stage begins when daylight hours shorten and nights grow longer. In these strains, the photoperiod regulates not only the flowering but also the vegetative stage, allowing the option to keep plants in an indefinite growth stage under artificial lights that simulate long days and short nights.

In contrast, autoflowering strains flower based on age. The autoflowering strains offered by Sweet Seeds® typically start flowering around three weeks after the appearance of the cotyledons, usually when the plants have developed their third or fourth set of true leaves. This ability allows them to grow and flower year-round, unaffected by seasonal light variations.

These modern autoflowering marijuana strains are some of the fastest-flowering strains available today, able to complete their flowering stage in just five weeks. Combined with the three-week vegetative phase, they reach harvest approximately two months after seed germination.

At Sweet Seeds®, we focus on developing easy-to-grow plants that are highly aromatic and tasty, while also offering a vigorous structure with a taller profile compared to the original autoflowering genetics from which they proceed. Most importantly, these strains provide higher yields and enhanced stability.

Typically, autoflower seeds produce plants that don’t exceed one meter in height. This limitation arises because many breeding improvements in autoflowering strains have been made using autoflowering genetics and commercial indica strains selected for fast flowering. However, this compact size didn't fully meet the needs and expectations of certain cannabis growers seeking taller-stemmed autoflowering plants. Responding to this demand, Sweet Seeds® concentrated on creating tall-stemmed autoflowering strains, leading to the development of the Big Devil® family and our "XL" autoflowering strains, crafted specifically for clients who desired extra-large autos. With optimal growing conditions, these strains can easily surpass a meter in height, significantly boosting production and making them some of the most productive autoflower seeds available.

This tall-stemmed characteristic was achieved by incorporating new genes with a sativa predominance into the genetic pool of autoflowering strains.

In 2013, we introduced a line of exotic and visually striking purple-flowering autoflowering strains, which we named “The Red Family.” The most captivating feature of these strains is their stunning appearance at full maturity, as the buds take on a rich, dark purple hue.

Strains in The Red Family are the result of hybridizing and stabilizing Sweet Seeds® genetics with a unique, exotic purple-flowering autoflowering strain. The purple-flower trait is inherited from an autoflowering genetic line with roots in the Chitral region of the Pakistani Hindu Kush, near Afghanistan.

Approximately 90% of the specimens in The Red Family develop purple or reddish tones in their flowers, with some plants also showing purple hues in their leaves during flowering. Once harvested and dried, the buds turn an even deeper purple, almost black in color.

Although the autoflowering trait in most modern autoflowering strains is inherited from cannabis Ruderalis lineages native to cold climates, this doesn’t mean that these improved strains can thrive or fully develop during the European winter.

In winter, with limited daylight and temperatures dropping below 10ºC by night, these strains slow down both their growth and flower production. Under such conditions, they may even stop growing altogether. When temperatures reach around 0ºC, the meristems and leaves can suffer damage, potentially leading to the death of the small autoflowering plants.

The best season to germinate autoflower seeds for maximum potential is from spring to autumn, when temperatures are milder. The more hours of light they receive, the better the final results will be.

Another important aspect to consider for optimal growth is pot size. Autoflowering plants do not develop extensive root systems, so they don’t require large containers at first. The ideal approach is to plant them in a small pot (around 1 liter) immediately after the autoflower seeds are germinated between paper towels (or with your favourite method). This ensures that the soil doesn’t remain overly moist for long periods, as excess moisture and lack of oxygen in the soil during the first few weeks can hinder root growth, lead to deficiencies, and even cause fungal infections in the roots or the base of the main stem. Since autoflowering plants need to grow quickly, any early stress or problems will have a negative impact on their final height and production.

When the plants reach about their 17th day, they can be transplanted into larger pots, ideally ranging from 5 to 10 liters, or even up to 20 liters. Transplanting should be done with great care to avoid breaking or damaging the roots, and it’s crucial that the new soil matches the original. This is also an excellent opportunity to introduce beneficial fungi or bacteria (like Mycorrhiza or Trichoderma) into the soil. You can also cover any part of the stem that has stretched too much, as buried stems will generate new roots. At this stage, the plants will begin their flowering phase.

For tall-stemmed autoflowering plants, such as those from the Big Devil® family, it’s possible to transplant them into a 10-liter or larger pot at the start of flowering.

If you’re using high-quality soil, there’s generally no need to apply liquid fertilizers during the first 15 to 20 days. However, using a root stimulator during the initial waterings can support the rapid development of a robust root system. From this point, nutrients and stimulators can be introduced, as with any other cannabis plant. A flush around the 5th or 6th week will help keep salt levels in the soil low. For taller autoflowering strains, such as Big Devil Auto® (SWS15) or “XL” genetics, you can continue using a grow fertilizer during the first week of flowering to ensure the plant has sufficient nitrogen for the vigorous growth that typically occurs during the initial flowering weeks, where many plants can triple in size.

Advantages:

• Mold Prevention: Many outdoor growers in humid areas, where cannabis plants are highly susceptible to fungal attacks due to environmental conditions, have started growing these strains in the spring to avoid the high-risk conditions of September-October. This timing helps in keeping the plants safe from fungus that can cause rot in large buds. Additionally, growers in high mountain and cold regions are also turning to these strains for spring-summer growing, achieving impressive results.
• Quick Growth and Flowering: Autoflowering strains complete their life cycle rapidly, allowing growers to harvest aromatic and resinous buds as soon as two months after seed germination.
• Discretion: Thanks to their fast development and moderate height, the plants are very discreet and well-suited for growers seeking a low-profile option. This is particularly valuable for cannabis users in locations where laws are restrictive or prohibitive regarding personal cannabis growing.
• Theft Prevention: Being discreet and grown outside the usual season, these plants are less likely to attract the attention of thieves or prying eyes.
• Resistance to Light Pollution: Since they flower independently of the photoperiod, autoflowering plants are unaffected by light pollution, such as streetlights, which would otherwise inhibit flowering in photoperiod-dependent strains. This feature, combined with their compact stature, makes them ideal for urban cannabis growing on balconies, in small gardens, and other limited spaces.

When grown indoors with artificial light, the best results for autoflower seeds can be achieved with photoperiods of 18 hours or more. Many cannabis growers maintain a schedule of 20 hours of light and 4 hours of darkness throughout the entire life cycle of the plants, leading to excellent outcomes.

The size of the pots and the use of liquid fertilizers should follow the guidelines outlined in the previous chapter on outdoor growing.

Advantages:

• Fast Development and Flowering: Autoflowering plants are selected for their rapid growth cycle, typically being ready for harvest 60-70 days after germination.
• Energy and Labor Savings: While autoflowering strains require an indoor photoperiod of 18-20 hours of light and 4-6 hours of darkness throughout their life cycle, this additional light expense is more than compensated for by the shorter overall growth period—saving approximately one month compared to photoperiod-dependent plants. This results in significant energy cost savings and reduces the labor and fertilizer needed during the growing process.
• Utilizing Empty Spaces: Growers who maintain dedicated areas for mother plants and cuttings can make effective use of empty spaces by flowering autoflowering plants under an 18/6 photoperiod.
• High-Quality Autoflowering Varieties: Modern autoflowering strains rival photoperiod-dependent polyhybrids in vigor, resin production, cannabinoid levels, terpene profiles, and flower density.

To understand the origin of most autoflowering genetics, it’s important to consider the cold regions of Eastern Europe (Hungary, Southern Siberia, Russia, and others) and Central Asia, where the first wild Cannabis Ruderalis strains emerged. In these areas, harsh climates—especially the cold temperatures and long winters—offer only three or four months each year with adequate conditions for cannabis growth and reproduction. It’s likely that these strains developed the unique autoflowering trait as an adaptive advantage, allowing them to flower and produce autoflower seeds within a brief period, thus ensuring the survival of the species in such inhospitable environments.

Cannabis Ruderalis is a subspecies of Cannabis Sativa. Soviet botanist Dmitri Janischewsky first described and catalogued it in 1924. At the time, it was considered a "bad weed," due to its low THC content and lack of appeal for medicinal or recreational use. It was also deemed unsuitable for industrial purposes, like fiber or paper production, because of the small size of the plants.

Today, wild autoflowering plants similar to Ruderalis can still be found in some regions with a history of cannabis cultivation, especially central North America and Canada, though smaller populations can occasionally be found across the country. These plants have adapted to their environment over time, and without human intervention, have lost many of the traits that are typically selected by growers.

It’s possible that autoflowering genes are present in the gene pool of most cannabis strains. Ruderalis strains, along with other wild autoflowering plants, may have developed from naturally selected populations of Cannabis Indica with short flowering times. Current consensus suggests that both "domesticated" and wild cannabis strains share a common genetic origin, making it likely that many cannabis strains still carry autoflowering genes in their DNA.

At Sweet Seeds®, we’ve observed that the autoflowering trait behaves as though it may result from "damaged" genes—genes that no longer respond to photoperiod changes and thus cannot flower based on decreasing light hours.

After the 1970s, a few pioneers at cannabis breeding began to recognize the hidden potential of autoflowering strains and started crossbreeding them with high-THC strains. Their goal was to harness autoflowering traits like fast flowering, short stature, cold resistance, and resilience against local pests and diseases in plants with higher THC levels and more desirable aromas. This marked the beginning of breeding programs aimed at incorporating these autoflowering traits into high-THC strains.

The first documented experiments to cross Ruderalis with high-THC strains were conducted by Ernest Small from Agriculture Canada in Ontario during the 1970s.

In the 1980s, famed breeder Neville, owner of the pioneering Seed Bank, experimented with Ruderalis hybrids, including crosses with Mexican strains, Skunk #1, and various Indicas. Although these hybrids matured earlier than typical Mexican strains, they had lower THC content and were quite inconsistent in flowering time and calyx-to-leaf ratios.

Meanwhile, on British Columbia’s Gulf Islands in Canada, an anonymous outdoor grower noticed that a photoperiod-dependent strain he had cultivated for several years—usually harvested in October—contained a few plants that matured much earlier, around late July or early August. Through years of selection, he developed a seed line that retained the autoflowering trait along with the effects and aromas of his cherished strain. This work led to the creation of Mighty Mite.

Mighty Mite quickly gained popularity, especially among Canadian growers who valued its ability to be harvested before summer’s end and before the arrival of mold. In northern regions, it often replaced fast Indicas that had been adapted to cold climates. Later, it was also cultivated by indoor growers and hybridized with more potent strains.

One of the first autoflowering strains released by Sweet Seeds®, Speed Devil Auto® (SWS11), was developed in early 2009 from a selection of specimens from a Canadian autoflower seed line, acquired through a seed exchange. Through multiple generations of selection, we believe this strain may be closely related to the famous and primitive Mighty Mite.

Above all, we, the Sweet Seeds® breeders, consider ourselves passionate users and collectors of genetics. We actively search for and select the finest strains to preserve them indefinitely, allowing us to enjoy them today, tomorrow, and for the rest of our lives. We believe that future generations will also benefit from our work, which is both our vocation and our passion. Our selection criteria for the best mothers focus primarily on their desirable qualities rather than their origin, lineage, or pedigree.

Using these exceptional plants, we employ traditional breeding techniques as well as modern sex-reversal methods in feminized plants to create 100% autoflowering and 99.9% feminized populations.

As a pioneering seed bank, Sweet Seeds® has been at the forefront of developing and improving modern feminized autoflowering strains. We embraced these new genetics from the moment we first received autoflower seeds.

Since 2007, when we began our initial experiments with autoflower seeds, we have recognized the immense potential of the autoflowering trait in these fast-growing cannabis plants. This realization prompted us to experiment and cross primitive autoflowering strains with our collection of selected clones, with the ultimate goal of producing 100% autoflower seeds that embody the cannabinoid levels and aromas of our cherished elite clones.

The first generation of Sweet Seeds® autoflowering strains was introduced during the 2009 and 2010 seasons, featuring Speed Devil Auto®, Fast Bud Auto® (SWS16), and Big Devil Auto®. These three strains achieved tremendous success for the seed bank.

Speed Devil Auto® made a significant impact on the cannabis community by becoming the first autoflowering plant to win an award at a competition. In a blind evaluation at the I Copa de la Marina Baixa (Alicante 2010), Speed Devil Auto® triumphed over several photoperiod-dependent strains, including Jack Herer, White Widow, and Diesel, among other feminized modern polyhybrids.

In 2011, Fast Bud Auto® claimed the first prize at the 1st Cannabis Cup Nature Grow, becoming the second autoflowering strain to win an award, this time in a special category for autoflowering plants.

The renowned magazine Soft Secrets awarded the title “Auto del Año” (Auto of the Year) to the Big Devil® family in 2014, marking the first time this specialized international cannabis publication recognized an autoflowering genetic. That same year, our most modern Big Devil® strain, Big Devil XL Auto® (SWS28), won 1st Prize at the III Copa Cannábica Expogrow Irún 2014.

These initial autoflowering strains served as the genetic foundation for introducing autoflowering traits into the next generation of Sweet Seeds® autoflower seeds. We hybridized them with elite photoperiod-dependent clones to create the second generation of autoflowering strains in our catalog. Subsequent generations resulted from breeding programs aimed at stabilizing the autoflowering trait within some of the best and most celebrated Sweet Seeds® genetics.

Autoflowering strains have arrived on the cannabis scene, and they are here to stay, securing their rightful place in the Hall of Fame of Cannabis Varieties.

When we label our autoflowering strains with different genetic generations (3G, 4G, 5G, etc.), we refer to generational steps rather than strict generational classifications. A new generational step is established each time the presence of Cannabis Ruderalis genetics in our autoflowering strains is reduced, typically as a result of crossing with photoperiod-dependent genetics.

In every cross between an autoflowering genetic and a photoperiod-dependent genetic, the resulting hybrid inherits 50% of the genetic from each parent. This means that with each crossing, we effectively reduce the primitive genetic contribution of Cannabis Ruderalis by 50%, since the autoflowering parent only passes on half of its genetic content to the offspring.

Regarding the first autoflowering generation used by Sweet Seeds® as the starting material to introduce autoflowering genes into our subsequent strains, we estimate that it contained approximately 25% genetics from Cannabis Ruderalis. Although this is a suspicion and cannot be determined with certainty, it provides a useful reference point.

When we utilized these genetic lines to breed with elite clones from our mother plant bank, the resulting hybrids contained only 12.5% Cannabis Ruderalis genetics, which we designate as 1G (first generation). This designation reflects the substantial genetic evolution that occurred compared to the previous generations, which featured 25% Cannabis Ruderalis.

Following this pattern, our 2G strains contain 6.25% Cannabis Ruderalis, 3G strains have 3.125%, 4G strains have 1.562%, and so on. Each time we cross a last-generation autoflowering genetic line with a photoperiod-dependent elite clone, we reduce the amount of Cannabis Ruderalis genetics in the resulting hybrid by 50%.

Currently, the percentage of Cannabis Ruderalis genetics in modern Sweet Seeds® autoflowering strains is minimal. However, within that small segment, we find the genes responsible for the autoflowering trait and the fast life cycle characteristics.

With the arrival of autoflowering strains, as cannabis breeders we were forced to return to breeding from seed lines or populations as these strains don’t allow selection and keeping clones due to their autoflowering characteristic.

As Sweet Seeds® breeders, we keep working with all our autoflowering strains, keeping them in constant evolution. Mainly due to the need to select the best specimens in each generation to use as parentals in the following generation. Given the wide genetic variations presented by cannabis in general and the modern autoflowering strains in particular, it is essential to select generation by generation to avoid undesirable genetic drifts.

Photoperiod-dependent genetics remain much more stable as years go by because we use selected elite clones to produce the commercial seed. In the production of photoperiod-dependent feminized seeds about 99% of the breeders use elite clones and not seed populations. We always use the same elite clones to produce each strain. Always using the same parentals has its benefits because the results are easily predictable, they are always the same. Only when a breeder finds an elite clone that contributes better characteristics to the offspring, while keeping the strain’s characteristic traits, a parental can be substituted, but this is not a common practice at all. The drawback of always using the same parentals is that this way the commercial seed populations never evolve, and the genetic frequencies remain constant by using the same source of genetics.

On the other hand, we, the breeders who work with autoflower seeds, cannot keep in clone form the best specimens that we find, so we must work with seed populations. This forces us to a permanent work of parental selection, not only during the development of breeding and improvement, but also every time that the commercial seeds are reproduced. Depending on the selected parentals, these populations can undergo genetic drifts in one or other direction, without keeping the genetic frequencies present in the commercial seed.

This way we keep these autoflower seed populations in constant evolution. This turns the breeding work with these genetics into a much more exciting and personal work for the breeder. In this case the breeder has the option of performing selective pressure to lead the seed population to the ideal and more desirable traits.

From our point of view there are two main reasons that led to the quality improvement in these autoflowering genetics, from the first wild primitive Cannabis Ruderalis genetic lines, to the arrival of last generation autoflowering strains. First, the previously mentioned condition of working from seed populations and selecting the parentals in each generation, as this requires that the population is under constant evolution to the extreme of desirable traits as long as there is selective pressure from the breeder and variability in the gene pool. And secondly, the hybridization between autoflowering genetic lines with the best photoperiod-dependent clones resulting from decades of selection.

Since we, the Sweet Seeds®’ breeders, started to understand the immense potential of the autoflowering trait confined in these small varieties, we started crossing them with our best photoperiod-dependent elite clones, searching for automatic flowering plants but with a quality at least similar to the one found in our beloved photoperiod-dependent mothers. The creation of new autoflower seeds was performed by hybridizing autoflowering genetics with selected photoperiod-dependent elite clones from our bank of mothers.

When we hybridize an autoflowering genetic with a photoperiod-dependent clone, the first goal is to fix the autoflowering trait in 100% of the hybrid population. This is relatively simple because this trait behaves as if it only depended on a single gene and shows typical mendelian inheritance. The autoflowering trait is recessive, so in the first filial generation of the hybrid no autoflowering specimens appear. It is necessary to reproduce this generation with itself to find 25% of autoflowering specimens in its offspring. Crossing these specimens found in the second filial generation with each other, we obtain the third filial generation featuring 100% of autoflowering specimens.

Parental selection in autoflower seeds takes place in every generation and starts very early, when seeds germinate. When they are seedlings, we eliminate all the specimens that show malformations or any kind of deficiency. During growth we continue with the parental selection, eliminating all the specimens that show undesirable traits. During flowering we keep eliminating the parentals where we observe undesired traits and, in the end, we will only have the ones that turn out to be the most productive, aromatic, resinous and, in general, the group of plants that feature the agronomical, organoleptic and chemotypic characteristics, bringing us closer to our ideal genetic goals.

When we perform these hybridizations to introduce the autoflowering trait in a new photoperiod-dependent genetic, at Sweet Seeds® we often use a selection of the best specimens from a feminized autoflowering population of plants as pollen donors. These specimens are subjected to sex reversal procedures that allow us to obtain the male flowers. And as pollen receptors we use the elite clone in which we want to introduce the autoflowering genes.

In the first programmes of selection and improvement of our first autoflowering strains at Sweet Seeds® we exceptionally used regular seeds (producing males and females). But from then on, all our breeding work with automatic strains have been performed in absence of males. By other words, the pollen receptors have always been females and the pollen donors have always been females to which the sex has been reversed (only to obtain the pollen). These types of crosses bring about populations of feminized seeds, also known in genetic terms as ginoics. This technique ensures a very high femininity with very residual hermaphroditism occurrences detected in the lines developed using this method.

On the other hand, not all cannabis varieties and not all the specimens within a variety respond well to sex reversal techniques. Performing the whole procedure with females and reverse the sex from the pollen donors in each generation, allows for a big advantage to the breeder who seeks an autoflowering and feminized commercial seed. The advantage lays on the fact that with this we ensure that in the end of the breeding programme the plants originated by the resulting seeds will have a good response to the same sex reversal technique, needed to reproduce the feminized autoflower seeds.

If we were to perform the whole procedure with females and males (regular seeds), we could come to the conclusion that when the strain development is finished and we want to cross some females with each other to obtain feminized autoflower seeds, these wouldn’t have a good response to the sex reversal technique, producing very few or nonexistent viable pollen and consequently very few seeds, if any at all. Working throughout the whole programme with reversed females as pollen donors, we perform selective pressure in each generation over the parentals that feature good response to the sex reversal technique and that can produce viable and abundant pollen, carrying on these traits to the next generation. With this, at the end of the breeding programme, the new strain will feature a good response to the sex reversal technique.

If you reached to this point of the text, we would like to wish you sweet and happy harvests. But we would also like to remind you that cannabis is a magical plant, shaped by human hands since ancient times, and it generously reciprocates the care and attention given to it.