BT, I have found that small seedlings confined to a large pot from the get-go are stunted or at least don't have near the vigor starting off in a small pot and upcanning as needed. A container grown plant experiences totally different dynamics than one in the ground. I think it is an issue of air exchange at the root zone. Do you not use pure pine bark mulch as potting medium?
I use something similar to pine bark mulch, but not the typical mulch you find in bags to spread in gardens. It is more like a mix of small chips nickle sized down to tiny bits and some has been composted to the point of being dirt. It is a light mix with tremendous drainage and roots cut through it like a hot knife through warm butter on a hot Texas afternoon. Since finding out about it everything we grow has grown better and faster.
Where I came to try starting plants in larger pots was when we began planting small trees at our nursery in large pots, 25-gallon to 50-gallon pots, and were going to grow them to a large caliper size before selling them. They grew more rapidly and evenly and were healthier than the same types of trees that we started out in 3-gallon pots and then up-canned repeatedly until they reached the size we sold them at.
I tried the same thing with cannabis plants and saw the very same response. My plants grew faster, fuller and were healthier.
While it is only an assumption I half believe that what makes plants started in small containers appear more vigorous is they quickly fill the area they have for roots and then they get something of a stretch, like can happen to root-bound plants. To me it is an artificial growth spurt that the roots will later have to catch up with after re-potting, and I believe that is what some people call the slight stunting of growth that can at times be seen after re-potting. The plants are trying to regain that rough 50/50 ratio of above ground and below ground growth.
What is Root-Bound?
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Root-bound is where the roots of your plant outgrow the container they are contained in.
The following symptoms may be observed if you allow your plants to become root-bound:
1. Stunted Growth.
2.
Stretching.
3. Smaller and slower bud production.
4. Needs watering too often.
5. Easy to burn with low % nutrient solution mixtures.
6. Wilting.
A small plant uses up it's root space and it stretches. Ever notice how tall and gangly and straggly the last plants in trays are like at places like Lowes etc? The roots ran out of space and the plants shot up.
Before becoming very tall and gangly it would just appear to be a vigorous growth spurt. So, it appears there is a vigorous growth spurt, but it is not really caused by vigor, it's artificial due to growing environment/conditions forcing it.
If not re-potted soon after that is seen the plants would grow tall and gangly, stretched.
I never see that. I see a consistent rate of growth from seedling until flowering, and then the natural growth spurt occurs.