Gardening Without Irrigation

Chapter 1. Predictably Rainless Summers

Predictably Rainless Summers

In the eastern United States, summertime rainfall can support 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

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 stressed vegetables may survive, but once stressed, the quality of their yield usually drops markedly.

Yet in deep, open soil west of the Cascades, most vegetable species may be grown quite successfully with very little or no supplementary irrigation and without mulching, because they're capable of being supplied entirely by water already stored in the soil.

Soil's Water-Holding Capacity

Soil is capable of holding on to quite a bit of water, mostly by adhesion. For example, I'm sure that at one time or another you have picked up a wet stone from a river or by the sea. A thin film of water clings to its surface. This is adhesion. The more surface area there is, the greater the amount of moisture that can be held by adhesion. If we crushed that stone into dust, we would greatly increase the amount of water that could adhere to the original material. Clay particles, it should be noted, are so small that clay's ability to hold water is not as great as its mathematically computed surface area would indicate.

Surface Area of One Gram of Soil Particles

Particle type Diameter of Number of particles particles Surface area in mm per gm in sq. cm.

Very coarse sand 2.00-1.00 90 11 Coarse sand 1.00-0.50 720 23 Medium sand 0.50-0.25 5,700 45 Fine sand 0.25-0.10 46,000 91 Very fine sand 0.10-0.05 772,000 227 Silt 0.05-0.002 5,776,000 454 Clay Below 0.002 90,260,853,000 8,000,000

Source: Foth, Henry D., Fundamentals of Soil Science, 8th ed. (New York: John Wylie & Sons, 1990).

This direct relationship between particle size, surface area, and water-holding capacity is so essential to understanding plant growth that the surface areas presented by various sizes of soil particles have been calculated. Soils are not composed of a single size of particle. If the mix is primarily sand, we call it a sandy soil. If the mix is primarily clay, we call it a clay soil. If the soil is a relatively equal mix of all three, containing no more than 35 percent clay, we call it a loam.

Available Moisture (inches of water per foot of soil)

Soil Texture Average Amount

Very coarse sand 0.5 Coarse sand 0.7 Sandy 1.0 Sandy loam 1.4 Loam 2.0 Clay loam 2.3 Silty clay 2.5 Clay 2.7

Source: Fundamentals of Soil Science.

Adhering water films can vary greatly in thickness. But if the water molecules adhering to a soil particle become too thick, the force of adhesion becomes too weak to resist the force of gravity, and some water flows deeper into the soil. When water films are relatively thick the soil feels wet and plant roots can easily absorb moisture. "Field capacity" is the term describing soil particles holding all the water they can against the force of gravity.

At the other extreme, the thinner the water films become, the more tightly they adhere and the drier the earth feels. At some degree of desiccation, roots are no longer forceful enough to draw on soil moisture as fast as the plants are transpiring. This condition is called the "wilting point." The term "available moisture" refers to the difference between field capacity and the amount of moisture left after the plants have died.

Clayey soil can provide plants with three times as much available water as sand, six times as much as a very coarse sandy soil. It might seem logical to conclude that a clayey garden would be the most drought resistant. But there's more to it. For some crops, deep sandy loams can provide just about as much usable moisture as clays. Sandy soils usually allow more extensive root development, so a plant with a naturally aggressive and deep root system may be able to occupy a much larger volume of sandy loam, ultimately coming up with more moisture than it could obtain from a heavy, airless clay. And sandy loams often have a clayey, moisture-rich subsoil.

Because of this interplay of factors, how much available water your own unique garden soil is actually capable of providing and how much you will have to supplement it with irrigation can only be discovered by trial.

How Soil Loses Water

Continue reading Gardening Without Irrigation free in the Wunder reader →