Exactly.
Here, if it rains in the summer (which it does, occasionally), the rain
only penetrates the first few millimeters of clay soil, and the water
evaporates rapidly. Mulch on top of this soil does not reach the soil
surface. It would have to rain for days on end for water to penetrate the
mulch and then get far enough down to plant roots.
Which is why the garden in the back yard here is a series of mounds, or
berms, mostly of clay soil, with a lot of gravel mixed in. The only time
the mounds get sufficient water to reach the roots of bulbs, etc., is from
melting snow in late winter.
I also, incidentally, have a couple of “rain gardens”, which are
constructed in exactly the opposite way such gardens are usually made:
raised beds of sand and gravel, nothing else. Highly-permeable “soil” which
allows rain from a brief thunderstorm to penetrate down to roots.
One of these beds, which has too much sand, has been a spectacular
failure, since I overlooked the possibility of a perched water table at the
interface between a pile of sand and gravel, and clay soil. The sand also
remains wet at a greater depth than anywhere else in the garden; bulbs
planted from the middle to lower end of the sand pile tend to rot.
Bob Nold
Denver, Colorado, USA
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