The effect of summer humidity, or lack thereof, on night cooling is
often discussed by gardeners in the western United States. One person
suggested that plants that had evolved in regions with high summer
humidity and heat might be adversely affected by the night cooling
that occurs in dry summers. In my previous garden at 1600 feet
elevation east of Portland, Oregon, the night temperature drop in
midsummer could be as much as 40 degrees Fahrenheit. It will probably
be less in my new garden because it's closer to the city and lower
elevation (about 500 feet), so it may be affected by the urban heat
island factor. It was speculated in one conversation that plants that
have evolved with dry, cool nights simply "metabolize themselves to
death" in humid regions, while those from summer-humid regions are
"shocked" into near-dormancy by cold summer nights.
Obviously, not all plants from Japan or South Carolina will die in
the Pacific Northwest; we can grow perfectly decent hostas if we want
to. But there are a number of plants that simply fade away over the
course of two or three years here, while on the Atlantic Coast they
make huge, floriferous specimens.
Jane McGary
Portland, Oregon, USA
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