A freshly transplanted Black pine. From our Masters' Series Pine book.

When it comes to transplanting, timing depends on your climate. Here in northern Vermont seasons can change rapidly and winters are very cold, so fall transplanting season starts and ends early (more on this below) If you live in a warmer climate fall comes later for you and there is more margin for error.

One advantage to transplanting in the early fall is when the spring season starts, little or no growing time is lost. Roots grow in the fall when the soil is still warm and after diminished light and cool nights have shut top growth down. By the time the soil becomes too cold, the roots have recovered. When spring finally arrives, top growth should be vigorous.

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There are a couple caveats however: First, it’s usually best to avoid transplanting deciduous and tropical trees in the fall. Unless you live in a very forgiving climate or you’ve got a magic touch.
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This brilliant Ginkgo has nothing to do with fall transplanting, but it does show some great fall color. I borrowed it several years ago from our Bill Valavanis.

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Second, heavy root pruning is best saved for the spring. If cold weather sets in too soon before the roots have recovered, winter survival can be threatened. If you live where the winters are very cold, better stick to light root pruning in the fall.

Don’t wait too long, especially if you live where seasonal changes can happens quickly. Here in northern Vermont, we’ve had the best results from the last few days of August to the around the 10th of September (our first hard frost can happen any time from mid-September to early October, though there’s almost always several weeks of mild weather after that, which helps keep the soil warm enough for roots to recover).

The photos and some of the text used here are from a post we originally did in August, 2015 that was titled Fall Transplanting: Timing and a Light Touch.
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Pieces of the pie. From Bonsai Today issue 39.

The technique shown here is particularly good if you want to replenish the soil while leaving some of the roots undisturbed. Doing this lessens stress and hastens recovery.

This technique is also useful if you want to move a tree from a larger to a smaller pot, or into a pot that has a different shape. It also works when you want to replenish some of the soil and then put the tree back into the same pot and is particularly useful for repotting forest plantings.

However, this only works with pot-bound trees. The roots need to be well enough established to hold the soil together when you take the tree out of the pot.

Before. A Satsuki azalea in the wrong pot.

Two useful transplanting aids. Myconox helps replace the all-important mycorrhizal fungi, and Dyna-Gro K-L-N is a rooting compound that is formulated to reduce stress and encourage root growth.

Cutting corners. Because you’re going from a rectangular pot to a round pot, you start by cutting off the corners.

Removing the bottom roots. With pot-bound trees you want to remove some of the lower roots. This encourages roots to spread out rather than grow down; because the tree mirrors the roots, it encourage the above ground growth to spread, rather than shoot straight up. Another reason to remove the bottom roots is to make more space to replenish the soil.

After. The tree looks much better in this pot and healthy new roots can grow into the fresh soil. All you need now is water and a light application of gentle, slow-release Green Dream fertilizer to enrich the soil for the new root growth (many people discourage fertilizing right after transplanting, but with a light application of a mild slow-release fertilizer, there's no risk).