Ann wrote:
"David E. Ross" expounded:

What you should do is dig down
inside the bed to a depth of at least 4 feet from the top of the
expected planting level.

Four feet??!? That's a little over the top. 18" should do.

18" might be okay for annuals and perennials. Even then, tomato roots
might go 6 feet down in good soil.

For woody shrubs, root might easily grow to a depth equal to the height
of top growth.

Note also that I said "from the top of the expected planting level". If
the bed is 2 feet above the natural soil, then you have to dig down into
the natural soil only 2 feet.

If you have a good mix of compost and natural soil in the RAISED bed
that totals 4 feet, then roots are very likely to grow even deeper than
the soil was prepared. If you have no mix at all, the roots will be
reluctant to grow beyond the compost. Compost by itself is too friable
to provide a good anchoring of a shrub.

--

David E. Ross
http://www.rossde.com/.

Don't ask "Why is there road rage?" Instead, ask
"Why NOT Road Rage?" or "Why Is There No Such
Thing as Fast Enough?"
http://www.rossde.com/roadrage.html