Gardening Without Irrigation: or without much, anyway | Page 6

gardens
without irrigation but is just irregular enough to be worrisome. West of
the Cascades we go into the summer growing season certain we must
water regularly.
My own many-times-revised book Growing Vegetables West of the
Cascades correctly emphasized that moisture-stressed vegetables suffer
greatly. Because I had not yet noticed how plant spacing affects soil
moisture loss, in that book I stated a half-truth as law: Soil moisture
loss averages 1-1/2 inches per week during summer.
This figure is generally true for raised-bed gardens west of the
Cascades, so I recommended adding 1 1/2 inches of water each week
and even more during really hot weather.
Summertime Rainfall West of the Cascades (in inches)*
Location April May June July Aug. Sept. Oct. Eureka, CA 3.0 2.1 0.7
0.1 0.3 0.7 3.2 Medford, OR 1.0 1.4 0.98 0.3 0.3 0.6 2.1 Eugene, OR
2.3 2.1 1.3 0.3 0.6 1.3 4.0 Portland, OR 2.2 2.1 1.6 0.5 0.8 1.6 3.6
Astoria, OR 4.6 2.7 2.5 1.0 1.5 2.8 6.8 Olympia, WA 3.1 1.9 1.6 0.7 1.2
2.1 5.3 Seattle, WA 2.4 1.7 1.6 0.8 1.0 2.1 4.0 Bellingham, WA 2.3 1.8
1.9 1.0 1.1 2.0 3.7 Vancouver, BC 3.3 2.8 2.5 1.2 1.7 3.6 5.8 Victoria,
BC 1.2 1.0 0.9 0.4 0.6 1.5 2.8
*Source: Van der Leeden et al., _The Water Encyclopedia,_ 2nd ed.,
(Chelsea, Mich.:Lewis Publishers, 1990).
Defined scientifically, drought is not lack of rain. It is a dry soil
condition in which plant growth slows or stops and plant survival may
be threatened. The earth loses water when wind blows, when sun shines,
when air temperature is high, and when humidity is low. Of all these
factors, air temperature most affects soil moisture loss.
Daily Maximum Temperature (F)*
July/August Average
Eureka, CA 61 Medford, OR 89 Eugene, OR 82 Astoria, OR 68
Olympia, WA 78 Seattle, WA 75 Bellingham, WA 74 Vancouver, BC
73 Victoria, BC 68
*Source: The Water Encyclopedia.
The kind of vegetation growing on a particular plot and its density have
even more to do with soil moisture loss than temperature or humidity or
wind speed. And, surprising as it might seem, bare soil may not lose

much moisture at all. I now know it is next to impossible to anticipate
moisture loss from soil without first specifying the vegetation there.
Evaporation from a large body of water, however, is mainly determined
by weather, so reservoir evaporation measurements serve as a rough
gauge of anticipated soil moisture loss.
Evaporation from Reservoirs (inches per month)*
Location April May June July Aug. Sept. Oct Seattle, WA 2.1 2.7 3.4
3.9 3.4 2.6 1.6 Baker, OR 2.5 3.4 4.4 6.9 7.3 4.9 2.9 Sacramento, CA
3.6 5.0 7.1 8.9 8.6 7.1 4.8
*Source: The Water Encyclopedia From May through September
during a normal year, a reservoir near Seattle loses about 16 inches of
water by evaporation. The next chart shows how much water farmers
expect to use to support conventional agriculture in various parts of the
West. Comparing this data for Seattle with the estimates based on
reservoir evaporation shows pretty good agreement. I include data for
Umatilla and Yakima to show that much larger quantities of irrigation
water are needed in really hot, arid places like Baker or Sacramento.
Estimated Irrigation Requirements:
During Entire Growing Season (in inches)*
Location Duration Amount Umatilla/Yakama Valley April-October 30
Willamette Valley May-September 16 Puget Sound May-September 14
Upper Rogue/Upper Umpqua Valley March-September 18 Lower
Rogue/Lower Coquille Valley May-September 11 NW California
April-October 17
*Source: The Water Encyclopedia In our region, gardens lose far more
water than they get from rainfall during the summer growing season. At
first glance, it seems impossible to garden without irrigation west of the
Cascades. But there is water already present in the soil when the
gardening season begins. By creatively using and conserving this
moisture, some maritime Northwest gardeners can go through an entire
summer without irrigating very much, and with some crops, irrigating
not at all.

Chapter 2
Water-Wise Gardening Science

Plants Are Water

Like all other carbon-based life forms on earth, plants conduct their
chemical processes in a water solution. Every substance that plants
transport is dissolved in water. When insoluble starches and oils are
required for plant energy, enzymes change them back into
water-soluble sugars for movement to other locations. Even cellulose
and lignin, insoluble structural materials that plants cannot convert
back into soluble materials, are made from molecules that once were in
solution.
Water is so essential that when a plant can no longer absorb as much
water as it is losing, it wilts in self-defense. The drooping leaves
transpire (evaporate) less moisture because the sun glances off them.
Some weeds can wilt temporarily and resume vigorous growth as soon
as their water balance is restored. But most vegetable species aren't as
tough-moisture