so we all know that autos grows however they want and bloom whenever they want
question is, what is the smallest pot you can use for optimum growth?
i mean they dont get that big you know
1 gallon minimum?
IMO you don't even need to go as big as a gallon, especially with an Auto. I'm not saying using a 1 gal or larger is a bad thing, it'll work just fine, just that it isn't necessary as long as the correct density grow medium is used. I've found the using the correct density grow medium makes all the difference the world when trying to factor pot size. Obviously there will be exceptions, pro's and con's, limitations, etc. but so far I've found the end results to be very acceptable. I am by no means an expert or scientist but as I understand it the logic is as follows:
Roots themselves don't take up much room.
They require oxygen, water, and nutrients.
More energy is put towards expanding the root system in time of need, and less energy when satisfied.
When satisfied roots will grow in what ever shape, size, or medium you want, when not satisfied they will never give up trying to grow until they either succeed or die.
Root systems do not require nutrient filled media, so long as all deficiencies are compensated for in the water.
Putting the above items together and for the sake of the OP, factoring out all types and styles of growing except for a medium filled container, the result is the use of an extremely light, loose, and airy medium with excellent drainage yet must still have the ability to retain a decent amount of water. Mixing and fine tuning such a mixture will allow the use of 16oz "Red Party Cups" for clones, veg, sexing, and even flowering if you so choose. This is also not limited to Autoflowers. The same mixture will allow regular strains to grow out wonderfully in considerably smaller pots as well.
I recently finished up 3 Afghan Kush Ryder (see avatar pic) grown one each in a 16oz party cup, a 1/2 gal container, and a 1 gal container. All 3 were germinated together, and always received the same amount of light. After the first week it appeared the plants in the two larger containers were growing great, while the small one wasn't doing well at all. I soon realized I hadn't factored in additional watering due to less medium. I upped the water for the small one and with in a week it had caught up to the others. End result was a nice little harvest, all three being nearly identical in height at about 2 ft.
Having an extremely light and airy medium will allow the roots to have little issue filling nearly all the space while using considerably less energy.
Having good drainage will allow as many watering periods as you want.
Using a soil-less medium allows You to control exactly how much nutrients it gets and when it gets them.
Decent water retention allows longer periods to be taken between watering periods.
I currently have a sativa dominant hybrid in it's 7th week in flower requiring a screen for main colas and supports for side colas. It is currently just over 3 feet tall, has no signs of being root bound, and is only in a 10.5 " pot. I also use the same mix for seedlings, clones, veg, and sexing. The largest plant I've grown from seed to harvest in a 16oz red party cup came in at 2' tall.
As for the medium, I've tried many different grow mediums and ultimately decided to mix my own. It's ultimately the growers decision and personal preference, but I have found my mix works great for my needs.
It consists of:
Coco coir
Perlite
Vermiculite
Grow Stones
Small amount of Happy Frog
The mix has a small amount of nutes from the happy frog which, added to the water retention properties of the vermiculite, one could give a good watering and leave for a few days.
The coco coir is not only an acceptable ph level, but contains no nutrients providing you greater feeding control. It also has the benefits of holding water and being lightweight when dried out.
The perlite keeps everything from compacting and aids in medium airation.
Lastly the grow stones provide fantastic air and liquid retention, while also helping resist medium compaction.
Facing little resistance the roots will begin to take over the entire container. Every time you water a small amount of medium with drain out the pot, over time leading to more root room. Utilizing as much of the space inside the pot as possible not only allows larger plants in smaller containers but provides many of the benefits from regular hydroponics grow systems as well as many of the benefits of growing in a soil less mix.
So, enjoying my rambling novel?!? lol If I'm lucky there might even be an answer or some information you might find useful in there somewhere!
Hope this helps!