I have 6 bonsai seedlings: Delonix Regia, Jacaranda, Mimosifolia, Locust Albizza, and Spruce. They are only a month old. The Delonix Regia and Locust both have roots poking through the bottom of the pots. They are in 6 inch training pots. If I move them to 9 inch training pots, what do I do when their roots fill up the new pots? Will they be old enough for me to prune the roots? I don't have enough room for pots bigger then 9 inch.

You do not have any bonsai seedlings, you have tree seedlings. You will know when they are big enough to handle by the size, once you can hold each one easily,

It will be at least one possibly two or more years before they will be big enough to prune any roots, just move from one size pot to the next as the pots fill with roots.

The pots are huge for such small seedlings

Most of my trees is from nurcery stock 5-10 years old. None of them show any signes of beeing root pruned prior to me purshasing them. I started from seeds and cuttings 4 years ago and has not done any root pruning yet.

In my opinion trees that are to become bonsai in the future should be kept in as big containers as possible or in ground in the beginning and focus should be on developing thickness and tapering. First root pruning is done when they are lifted into a training pot.

There are people that prefere to prune tap roots at an early stage as well. But I have not seen much issues with doing that much later.

The biggest pots I have room for are 9"L, 6"W, 3"H. Is it possible to grow them into bonsai trees without a big pot?

Photosynthesis wrote: The biggest pots I have room for are 9"L, 6"W, 3"H. Is it possible to grow them into bonsai trees without a big pot?

 The short answer is yes.

But you need to concider the species, the goal for your tree and your current age.

You can do a really nice shohin chinese elm in less than 10 years in a small pot. Not in the final pot, but in a relativelly small pot. But an imperial pine takes 100 years even in a relativelly big pot. Try to grow it in the final pot from seeds and it is your great-great-great grandson that can enjoy the final product if the tree survive that long.

So the question is similar to how long is a hose.

Btw: if you do not have an outdoor space you do not need to worry much about the spruce. It will die within 3 years if kept on a windowsill in livingroom condition.

Thank you for the the information. So, if I try to grow them in small containers their whole life, how do I know when to prune the roots. If the roots completely fill up the pot and I don't prune them what would happen? 

Here is a picture of the Delonix Regia:

Root pruning is first and foremost done for aestetic reasons and it is done while repotting. Repotting is done when soil no longer drains well.

Your delonix can stay in that pot for quite a few years.

Thank you for the help!😊