He also pointed us to a great article on the importance of drainage when planting trees. This is a big issue here in the Central Valley becase most areas tend to have a fairly impervious hardpan around 18" under the topsoil. This is an issue even if laying sod, but is really a problem with large trees. If the water doesn't penetrate the hardpan, the roots won't follow the water down, and you end up with surface roots, split concrete, broken pipes, and general unpleasantness.

So make sure to break through that hardpan! And pay attention to surface drainage!

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