Why are autoflowers so small?

People keep asking, James, why are autoflowers so small? Unable to directly answer the question I went to the great and powerful Google for some answers.

Here is what I found.

Autoflowering cannabis seeds have become an essential part of home growing.

  1. They are the fastest way to grow cannabis.
  2. Most autoflower strains grow from seed to harvest in around 75 days.
  3. They are also easy and convenient to grow.
  4. Indoors they grow under a fixed light cycle using 20 hours of daily light.

Autos are potent too, on the same quality levels as the best-feminized strains.

But occasionally autos will remain small and dwarf-like.

I am left with questions like is it always down to poor-quality seeds? Are there any professional tips and tricks to get the best from your autoflower seeds? Keep reading to find out what my search has revealed. 

What is the average autoflower size & how tall can they get?

Most reach an average autoflower height of around 20in – 40in (50cm – 100cm). The final autoflower size depends on the genetics and the growth conditions which you provide.

Sativa autoflower seeds can stretch a little more than indica autos. Sativa auto genetics may also take a couple of weeks longer to ripen than indica varieties, this can allow them to reach somewhat larger autoflower heights than the more compact indica-leaning autos. The largest autoflowers tend to be Sativa-leaning and can reach 50in – 69in (1.25m – 1.75m) tall.

Because the average autoflower height tends to be somewhat shorter than photoperiod strains, many growers find that they can manage with slightly smaller grow tents. This means that the recommended grow tent height for autoflowers can be shorter.

You may be able to grow your autoflower seeds inside tents that are perhaps just 59in (1.5m) tall.

Reasons why your autoflower stays small

You may have got unlucky and simply got a short, squat phenotype. Don’t despair, many of these will still offer potent, firm buds.

Or you may have had the misfortune to encounter an issue during growth that has permanently stunted your auto. There is a range of reasons which could explain why your autoflower has stayed small.

If environmental conditions (nutrients, pH, lighting, over-watering, etc) are the cause, it’s worth noting that autos only have limited time to recover from any problems. With a typical 75-day life cycle, there simply isn’t much time to recover and repair if things go wrong.

You may also have simply chosen the wrong autoflower seed choice. Most of the top-tier autoflower seed suppliers use solid genetics and proven breeding techniques, honed over many years. But there are still some notoriously unreliable seed companies that routinely produce substandard autoflower seeds. They use poor genetics and have a higher chance of producing dwarf plants.

The best advice is to select the best auto seeds from those with proven track records in autoflower seed development, perhaps with a cannabis cup or two.

Autoflowers have a shorter growth/veg period than photoperiod plants

Because autoflower seeds grow with a fixed lifetime, usually around 75 days, there is less time available to overcome any difficulties. Often an autoflower shows signs of bloom around a month, or so, after germination. Autoflower seed varieties such as Auto Blueberry and Auto Blackberry Kush can be ready to harvest in as little as 8-9 weeks after seed germination. With such a fast-track life, an autoflower has little spare time to recover from any growth problems or environmental issues during the first weeks of vegetative growth.

If you have low temperatures during the early growth of an auto you may permanently reduce the ability of your auto to reach its full potential. Likewise, if you have serious nutrient issues, overwatering, poor lighting, etc you will also likely see permanently stunted growth on your autoflower plants

Photoperiod plants on the other hand have genetics that allows them to bask in vegetative growth conditions for as long as you allow long (18 hour+) daylight hours. So if your photoperiod plants look a little small for your liking, you can simply allow them an extra few weeks of veg growth.

Not enough space and your autoflower is root-bound

If your autoflower is root bound it means that the roots have been kept in a very small space. When the roots are tangled in a small root ball it can cause serious restrictions to the eventual autoflower size and the final autoflower height. In the worst cases, the roots can’t be encouraged to grow correctly and the auto is permanently stunted.

What size container do autoflower plants need?

One common problem which restricts average autoflower height is the lack of root space. Incomplete root development restricts autoflower height as well as overall autoflower size. However, when the cannabis root system can grow easily in a light, aerated environment with access to nutrients and oxygen the autoflower size can benefit greatly.

Many auto growers like to use pot sizes of around 5 gallons (15-20 liters) indoors. Note that if you use a small container, such as a 3-liter pot, the average autoflower height may be significantly shorter, perhaps no more than 30cm tall.

As well as ensuring that you don’t suffer from an autoflower that is root bound, you also need to allow sufficient space around your plant.

How much space does an Autoflower need?

Try to allow enough room so that the auto side blooms are not touching/shading each other. As well as allowing the plants space to grow this also minimizes the effects of plants having to grow in the shadow of their neighbor.

How to start your autoflower growth properly

The key to success when growing autoflower seeds is good planning. Choosing the best autoflower seeds and combining them with optimum conditions is the most solid foundation to build on. If you haven’t grown autoflower seeds before, do your research carefully. Time spent planning your next autoflower grow is time well spent.

Experienced autoflower growers will use a proven grow medium which is usually light and aerated. They will often use plant containers such as airpots or felt sacks. These allow greater levels of root oxygenation. Remember, healthy cannabis roots mean a healthy cannabis plant.

The successful autoflower grower will also fully understand the importance of optimized grow room conditions. Avoiding wild fluctuations in temperatures/humidity and maintaining your autoflower plant in the ‘sweet spot’ of steady environmental conditions is key. Avoiding overfeeding and underfeeding is one of the more important rules.

If you can maintain stable, optimized grow room conditions and if you can combine this with good cannabis genetics then you should be set for a great harvest!

The importance of optimized autoflower seed germination conditions

The process of auto cultivation starts with autoflower seed germination. There are many different options for germinating cannabis seeds.

But some of the common weed seed germination methods may not necessarily have the greatest latitude. Jiffy pods, for example, can cause problems for some growers if they are soaked rather than moist. If your cannabis seed germination conditions are simply too wet then the cannabis root will not be able to grow quickly. In the worst cases, where seed germination conditions are too wet, the cannabis root will die or it will be severely compromised. Moist, not soaked, conditions are essential during cannabis seed germination and subsequent growth.

Once your cannabis seeds have germinated, you need to ensure that their container will offer the best growth conditions, nutrition, and aeration. If you can do this, your cannabis seedling will have the best chance of reaching its full genetic potential. That will allow your plant to reach a good autoflower height, instead of being small with a low yield.

For many soil/compost growers, a hygrometer (such as the Blumat hygrometer) is a great way of accurately measuring soil moisture content and knowing when the soil is too dry/too wet.

What pot size is best suited for an autoflower?

Autoflowers need plenty of grow medium early on in order to achieve their full potential. What size pot for autoflowers? For an outdoor grow we recommend a container size of minimum 20L (XXL varieties need ideally 30L). For indoor growers, containers around 15-20 liters are usually sufficient. Air pots and felt sacks (or similar) do a great job. Those that grow in hydroponic systems such as deep water culture will often achieve the best autoflower heights and the largest autoflower size.

It is recommended to either germinate the autoflower seeds in their final container or transfer them immediately after germination to their final container. Keeping autoflowers in small containers for too long will lead to the same outcome, tiny plants that haven’t been able to develop.

Your autoflower may have a nutrient deficiency or excess

Keeping your autoflower plant in the nutrient sweet spot from germination to harvest is the goal. However, if your plant gets insufficient nutrients it will be unable to reach the same autoflower height which it could have with good nutrition. With a reduced autoflower size, you can expect reduced yields too.

Over-feeding and over-watering cannabis plants are quite possibly the most common mistakes made by less experienced growers. The cannabis root system is easily burned by excessive nutrients. If your auto gets nutrients that are too strong it will permanently stunt future growth. This will result in reduced average autoflower heights.

Correct nutrient application, i.e. avoiding a nutrient excess/deficiency often comes with experience. If in doubt, consider using an organic approach with a good light soil mix augmented with slow-release organic nutrients from companies such as BioTabs. These make it easier to get the correct nutrition.

You may only need to provide a top dressing of general purpose bloom nutrients in the last few weeks of bloom when you use slow-release organic nutrients.

What type of soil is best suited for autoflowers?

The use of a light mix soil is recommended as seedlings and autoflowers in particular do not like nutrient rich soil early on, since this can easily burn the delicate cannabis roots. Some growers add some coco fiber to their soil mix to increase aeration. Other growers feel happy with a high quality ready-made light soil/compost mix bought from their local grow shop. Remember, if you damage or stress your autoflower plants it could have lasting effects on the eventual yield and the final autoflower height.

Your autoflower is getting too much light

It may sound counter-intuitive, but a small autoflower can also be the result of too much light. The same light that you use for your photoperiod feminized cannabis seeds (on a 12/12 light schedule) may simply deliver too much light for your autoflowers under 20 hours of daily light. If this happens, the plant receives more light energy than it can use and the result is a small, stunted plant that has struggled under non-optimized conditions.

It may sound counter-intuitive, but a small autoflower can also be the result of too much light. The same light that you use for your photoperiod feminized cannabis seeds (on a 12/12 light schedule) may simply deliver too much light for your autoflowers under 20 hours of daily light. If this happens, the plant receives more light energy than it can use and the result is a small, stunted plant that has struggled under non-optimized conditions.

This can be quite disconcerting for the grower that has just invested heavily in the most powerful grow room light possible. A new, high-powered grow light can be a great investment for the indoor grower (especially an LED light), but they sometimes come with a learning curve that may take a grow or two to master. The best grow light suppliers will be able to recommend hanging heights and intensities to suit different types of cannabis (auto vs photoperiod fems) at different stages of growth.

Because many indoor autoflowers are cultivated under 20 hours of daily light, they prefer slightly lower light intensity than a photoperiod in bloom. The logic is simple to grasp. Any light will produce a set amount of light, measured in micromoles (μmol) per square meter per second. When that light is on for 20 hours per day (autoflower seeds), it produces a greater number of daily micromoles than when it is on for just 12 hours per day (photoperiod feminized cannabis seeds).  Therefore, in order to retain the same amount of daily light (DLI, or Daily Light Integral) autoflowers can give better results with a slightly lower-powered light compared to the light requirements of a photoperiod strain in bloom.

Too much light for an autoflower, a practical example

A good practical example of this can be seen with the high-quality Fluence LED grow lights. Consider the Fluence SPYDR 2x (330w) and Fluence SPYDR 2p (645w) (note that there is also a Fluence SPYDR 2i at 630w). All the lights are made for a 1.2m x 1.2m grow room. The 330W SPYDR 2x is perfect for autoflowers under 20hours of daily light. However, in the same tent, both the SPYDR 2p (645W) and SPYDR 2i (630W) are simply way too strong to be used for autoflowers on a 20/4 schedule and you will risk light burn on both the leaves and flowers.

However, the SPYDR 2p (645W) and 2i (630W) will deliver superb results when blooming photoperiod feminized plants under 12 hours of daily light. Using the same principle, you can also appreciate how the SPYDR 2p (645W) and SPYDR 2i (630W) would also be too powerful for small seedlings under 18 hours of daily vegetative light.

Your autoflower is not getting enough light

Indoor, or outdoor, cannabis plants will struggle to reach their full potential without access to intense light with the right spectrum. If you don’t have sufficient light it will definitely compromise the final autoflower height.

Many indoor growers have a grow room of around 1.2m x 1.2m (4 feet x 4 feet). Serious growers would consider a 600W HPS about right for such a grow room. LED growers would probably consider a well-built LED light with a 400-600W power draw. However, if a 250W HPS (or a 200W power-draw LED) was used the yields would be considerably lower. You may also find that low light levels reduce the average autoflower height.

How to ensure proper light exposure to your autoflower?

One of the most important aspects of any indoor grow room is the power and quality of the grow light. Various types of light are possible, including fluorescent, ceramic metal halide, plasma, HPS and LED.

During early vegetative growth, lights rich in blue wavelengths are particularly effective. During bloom, lights rich in red wavelengths help the buds to pack on weight. If you have a light that is underpowered for the area it has to illuminate, you can expect to see reduced yields, restricted potency, and reduced average autoflower heights.

Most of the more professional grow light manufacturers accurately explain the precise area that their light can illuminate. They will also give recommended hanging heights for their LED at different stages of growth. This will vary according to the power of the LED light as well as the style/shape of the LED. Note that LED grow lights with several parallel light bars can be placed closer to the plant canopy than other types of LED where the light comes from a single source. Once you have bought a good quality LED light and set the LED light height for your autoflowers you can expect some high-quality harvests.

The professional cannabis grower may also invest in a light meter that will accurately measure both the power and the spectrum of the LED light at canopy level.

One way that many home growers try to maximize the effectiveness of their light levels is to maintain a flat canopy where all the blooms are equidistant from the light and can bathe in optimized PPFD (light intensity) levels. With an even, flat canopy of blooms, shadows can be minimized and all the blooms can take their share of light. Growing with the SOG method (Sea Of Green) or SCROG (Screen Of Green) method is popular with many growers and is a way of maximizing yields. 

Other key factors to look out for when growing autoflowers

Autoflower seeds are regarded by many as the fastest and easiest way to grow cannabis. Most growers get great results from autos, but you may wish to consider variables such as pH and correct outdoor planting times for autoflowers.

Growing autoflower seeds and pH control

When growing autoflower seeds in hydroponic systems or coco fiber, ensuring the correct pH (usually around pH 5.8) is vital. If you stray too far away from the target pH the cannabis roots may struggle to absorb key nutrients… even if they are present. This results in nutrient deficiencies and plant health issues. In the worst cases, the damage from using the wrong pH can be irreversible. This will reduce the average autoflower height as well as diminishing the overall autoflower size.

When growing in a decent amount of soil/compost, pH worries are less relevant since soil has a natural ability to buffer pH.

Growing autoflower seeds outdoors

When growing autoflower seeds outdoors it is important to select a warm and sunny 3 month growth period to get the largest autoflower size. If you put your seeds/seedlings outdoors too early the plants will struggle to grow due to the cold temperatures, and especially the cold nights. This can cause permanent stunting, reducing the autoflower height and the eventual autoflower size.

Always plant your seedlings outdoors after the last frost has passed. Note that the most successful outdoor auto growers often germinate their seeds indoors and grow the seedlings under artificial lights for a couple of weeks until the plant is strong enough to face outdoor conditions.

What size pot for an autoflower outdoors? You can plant the auto directly in the ground and give it unrestricted root space. Or you could use a 20-30 liter container which should allow ample root space to produce a large autoflower.

What are the best yielding autoflower seeds?

Not only are the best autoflower seeds capable of producing highly potent cannabis (on a par with the best photoperiod genetics) they can also deliver very high yields. 

Using the best yielding autoflower seeds, the most experienced hydroponic growers can routinely get autoflower harvests of 200g + per plant with techniques such as deep water culture. Skilled coco fiber growers with good quality LED grow lights and good nutrient management experience can expect to get 100g or more from the best yielding autoflower seeds. Experienced soil growers can expect yields of 75-100g, more in fully optimized conditions with high-yielding auto seeds.

The harvest quality and quantity are determined by several factors. Your autoflower seeds need the best possible start in life, which means good germination and care in the first few weeks. At all costs, avoid overwatering and overfeeding your delicate young plants since this may permanently restrict their potential. Trying to grow in waterlogged soil is one of the most frequent reasons that growers end up with small autos.

How to get the best results from your autoflower seeds

Use optimized grow room conditions. The best yielding autoflower seeds thrive under quality LED grow lights. If you can keep your plant under good environmental conditions, in the nutrient sweet spot, and with quality lighting, you too can expect seriously heavy harvests.

Some growers love nothing more than seeing a room full of heavy-yielding plants. Others prefer to grow the strongest possible genetics that they can get, even if that may mean sacrificing a little bit of yield. If you focus purely on the potency of your harvest, then the following article may provide useful reading. Enjoy growing your autoflower seeds!

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