How big is "big enough" for a grow out pot?

Bonsai Nut

Nuttier than your average Nut
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Charlotte area, North Carolina
I am curious about opinions on this subject.

When growing out immature material to bring it up to "pre-bonsai" status, the commonly accepted procedure for fastest results involves planting the tree in the ground. For some of us with small yards or unsuitable locations (low sun, poor drainage, steep hills, etc), the only alternative appears to be nursery pots. When I think of trees been grown in the ground, and the frequent root trimming, etc, I am thinking that all of this growing could easily be accomplished in a larger (25 gallon) nursery pot or less. There are numerous disadvantages to growing out in a pot, including the cost and weight of all of the soil, the lack of the insulating benefits of being in the ground, etc. However when I plant a tree in a larger pot I can often return in 2-3 years and find that the roots still have plenty of room to expand before I would consider the tree being root compromised.

I'd like to hear from people with experience with grow-out fields or with lots of grow-out occuring in large pots.

P.S. I know a lot depends on the size of the stock, for example my smaller JM grafts can grow for a couple of years easily in a two-gallon pot wthout being crowded. So let's assume we're talking about stumps of 2" or greater diameter at the soil.
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Michigan, USA
Brent's article, "Why the earth is not like a pot" covers this in detail. A growing pot, box, training pot, etc should never be much larger than 2" wider than the root mass. The soil outside of the root mass may well stay wetter longer and cause additional problems. This is why nurserymen "pot up" meaning that once a plants roots colonize a container, the plant is "slipped" into the next size up pot, and so on. They don't just plant a seedling into a 5 gallon pot....which tells us a lot, if faster growth was obtained by doing so, they would.


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