There are many ways to start a bonsai. Growing bonsai in the ground is just one of them, and in some ways, one of the best. When you see photos of the famous classic bonsai, they all seem to have large developed trunks. In addition to buying one of these masterpieces, you may be able to create one of your own.
Planting your starter bonsai in the ground will help your plant develop a bigger trunk and more developed roots much quicker than in a container.
Actually, growing bonsai in a container slows down the thickening process. Save your bonsai pots for when the tree is more developed.
My first experience with bonsai in the ground was with Fukien Tea in South Florida.
I had the distinct advantage of growing tropical plants in a sub-tropical zone. Almost everything grows quickly under those circumstances. From small starter plant to a large trunk, this Fukien Tea took about five years.
As our business grew, we also planted varieties of Ficus. We found the Green Island variety to be especially fast to develop a fat trunk.
We also discovered a number of temperate trees that did well in our sub-tropical environment. Elms and several types of juniper, including shimpaku, were also very successful subjects.
This juniper was originally a young staked nursery juniper stock plant. Glenn Hilton and the late Joe Samuels planted it in the ground and let it grow. As it got taller, the trunk was still pliable, they wrapped it around a piece of rebar (steel rod) and secured it with ties.
As the trunk aged, curves developed, the ties were loosened and re-wrapped. Glenn continued training the tree, carved out the jin and shari (dead wood) and treated it with lime sulfur. It wasn't long, before it was on display.
There are also many advantages of starting bonsai this way in many other climates. Brent and Susie Walston do their own bonsai growing in northern California and many are in the ground.
For an excellent article on developing trunks read Brent Walston's page. Brent also gives very explicit details on how to cut trees during training to create great new shapes.