Gardening Without Irrigation: or without much, anyway
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Steve Solomon >> Gardening Without Irrigation: or without much, anyway
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7 Created by: Steve Solomon ssolomon@soilandhealth.org
Cascadia Gardening Series
Gardening Without Irrigation: or without much, anyway
Steve Solomon
Introduction
Starting a New Gardening Era
First, you should know why a maritime Northwest raised-bed gardener
named Steve Solomon became worried about his dependence on
irrigation.
I'm from Michigan. I moved to Lorane, Oregon, in April 1978 and
homesteaded on 5 acres in what I thought at the time was a cool,
showery green valley of liquid sunshine and rainbows. I intended to
put in a big garden and grow as much of my own food as possible.
Two months later, in June, just as my garden began needing water, my
so-called 15-gallon-per-minute well began to falter, yielding less
and less with each passing week. By August it delivered about 3
gallons per minute. Fortunately, I wasn't faced with a completely
dry well or one that had shrunk to below 1 gallon per minute, as I
soon discovered many of my neighbors were cursed with. Three gallons
per minute won't supply a fan nozzle or even a common impulse
sprinkler, but I could still sustain my big raised-bed garden by
watering all night, five or six nights a week, with a single, 2-1/2
gallon-per-minute sprinkler that I moved from place to place.
I had repeatedly read that gardening in raised beds was the most
productive vegetable growing method, required the least work, and
was the most water-efficient system ever known. So, without adequate
irrigation, I would have concluded that food self-sufficiency on my
homestead was not possible. In late September of that first year, I
could still run that single sprinkler. What a relief not to have
invested every last cent in land that couldn't feed us.
For many succeeding years at Lorane, I raised lots of organically
grown food on densely planted raised beds, but the realities of
being a country gardener continued to remind me of how tenuous my
irrigation supply actually was. We country folks have to be
self-reliant: I am my own sanitation department, I maintain my own
800-foot-long driveway, the septic system puts me in the sewage
business. A long, long response time to my 911 call means I'm my own
self-defense force. And I'm my own water department.
Without regular and heavy watering during high summer, dense stands
of vegetables become stunted in a matter of days. Pump failure has
brought my raised-bed garden close to that several times. Before my
frantic efforts got the water flowing again, I could feel the
stressed-out garden screaming like a hungry baby.
As I came to understand our climate, I began to wonder about
_complete_ food self-sufficiency. How did the early pioneers
irrigate their vegetables? There probably aren't more than a
thousand homestead sites in the entire martitime Northwest with
gravity water. Hand pumping into hand-carried buckets is impractical
and extremely tedious. Wind-powered pumps are expensive and have
severe limits.
The combination of dependably rainless summers, the realities of
self-sufficient living, and my homestead's poor well turned out to
be an opportunity. For I continued wondering about gardens and
water, and discovered a method for growing a lush, productive
vegetable garden on deep soil with little or no irrigation, in a
climate that reliably provides 8 to 12 virtually dry weeks every
summer.
Gardening with Less Irrigation
Being a garden writer, I was on the receiving end of quite a bit of
local lore. I had heard of someone growing unirrigated carrots on
sandy soil in southern Oregon by sowing early and spacing the roots
1 foot apart in rows 4 feet apart. The carrots were reputed to grow
to enormous sizes, and the overall yield in pounds per square foot
occupied by the crop was not as low as one might think. I read that
Native Americans in the Southwest grew remarkable desert gardens
with little or no water. And that Native South Americans in the
highlands of Peru and Bolivia grow food crops in a land with 8 to 12
inches of rainfall. So I had to wonder what our own pioneers did.
In 1987, we moved 50 miles south, to a much better homestead with
more acreage and an abundant well. Ironically, only then did I grow
my first summertime vegetable without irrigation. Being a low-key
survivalist at heart, I was working at growing my own seeds. The
main danger to attaining good germination is in repeatedly
moistening developing seed. So, in early March 1988, I moved six
winter-surviving savoy cabbage plants far beyond the irrigated soil
of my raised-bed vegetable garden. I transplanted them 4 feet apart
because blooming brassicas make huge sprays of flower stalks. I did
not plan to water these plants at all, since cabbage seed forms
during May and dries down during June as the soil naturally dries
out.
That is just what happened. Except that one plant did something a
little unusual, though not unheard of. Instead of completely going
into bloom and then dying after setting a massive load of seed, this
plant also threw a vegetative bud that grew a whole new cabbage
among the seed stalks.
With increasing excitement I watched this head grow steadily larger
through the hottest and driest summer I had ever experienced.
Realizing I was witnessing revelation, I gave the plant absolutely
no water, though I did hoe out the weeds around it after I cut the
seed stalks. I harvested the unexpected lesson at the end of
September. The cabbage weighed in at 6 or 7 pounds and was sweet and
tender.
Up to that time, all my gardening had been on thoroughly and
uniformly watered raised beds. Now I saw that elbow room might be
the key to gardening with little or no irrigating, so I began
looking for more information about dry gardening and soil/water
physics. In spring 1989, I tilled four widely separated, unirrigated
experimental rows in which I tested an assortment of vegetable
species spaced far apart in the row. Out of curiosity I decided to
use absolutely no water at all, not even to sprinkle the seeds to
get them germinating.
I sowed a bit of kale, savoy cabbage, Purple Sprouting broccoli,
carrots, beets, parsnips, parsley, endive, dry beans, potatoes,
French sorrel, and a couple of field cornstalks. I also tested one
compactbush (determinate) and one sprawling (indeterminate) tomato
plant. Many of these vegetables grew surprisingly well. I ate
unwatered tomatoes July through September; kale, cabbages, parsley,
and root crops fed us during the winter. The Purple Sprouting
broccoli bloomed abundantly the next March.
In terms of quality, all the harvest was acceptable. The root
vegetables were far larger but only a little bit tougher and quite a
bit sweeter than usual. The potatoes yielded less than I'd been used
to and had thicker than usual skin, but also had a better flavor and
kept well through the winter.
The following year I grew two parallel gardens. One, my "insurance
garden," was thoroughly irrigated, guaranteeing we would have plenty
to eat. Another experimental garden of equal size was entirely
unirrigated. There I tested larger plots of species that I hoped
could grow through a rainless summer.
By July, growth on some species had slowed to a crawl and they
looked a little gnarly. Wondering if a hidden cause of what appeared
to be moisture stress might actually be nutrient deficiencies, I
tried spraying liquid fertilizer directly on these gnarly leaves, a
practice called foliar feeding. It helped greatly because, I
reasoned, most fertility is located in the topsoil, and when it gets
dry the plants draw on subsoil moisture, so surface nutrients,
though still present in the dry soil, become unobtainable. That
being so, I reasoned that some of these species might do even better
if they had just a little fertilized water. So I improvised a simple
drip system and metered out 4 or 5 gallons of liquid fertilizer to
some of the plants in late July and four gallons more in August. To
some species, extra fertilized water (what I call "fertigation")
hardly made any difference at all. But unirrigated winter squash
vines, which were small and scraggly and yielded about 15 pounds of
food, grew more lushly when given a few 5-gallon,
fertilizer-fortified assists and yielded 50 pounds. Thirty-five
pounds of squash for 25 extra gallons of water and a bit of extra
nutrition is a pretty good exchange in my book.
The next year I integrated all this new information into just one
garden. Water-loving species like lettuce and celery were grown
through the summer on a large, thoroughly irrigated raised bed. The
rest of the garden was given no irrigation at all or minimally
metered-out fertigations. Some unirrigated crops were foliar fed
weekly.
Everything worked in 1991! And I found still other species that I
could grow surprisingly well on surprisingly small amounts of
water[--]or none at all. So, the next year, 1992, I set up a
sprinkler system to water the intensive raised bed and used the
overspray to support species that grew better with some moisture
supplementation; I continued using my improvised drip system to help
still others, while keeping a large section of the garden entirely
unwatered. And at the end of that summer I wrote this book.
What follows is not mere theory, not something I read about or saw
others do. These techniques are tested and workable. The
next-to-last chapter of this book contains a complete plan of my
1992 garden with explanations and discussion of the reasoning behind
it.
In _Water-Wise Vegetables _I assume that my readers already are
growing food (probably on raised beds), already know how to adjust
their gardening to this region's climate, and know how to garden
with irrigation. If you don't have this background I suggest you
read my other garden book, _Growing Vegetables West of the
Cascades,_ (Sasquatch Books, 1989).
Steve Solomon
Chapter 1
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
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 particles in mm Number of particles per gm
Surface area 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
Suppose we tilled a plot about April 1 and then measured soil
moisture loss until October. Because plants growing around the edge
might extend roots into our test plot and extract moisture, we'll
make our tilled area 50 feet by 50 feet and make all our
measurements in the center. And let's locate this imaginary plot in
full sun on flat, uniform soil. And let's plant absolutely nothing
in this bare earth. And all season let's rigorously hoe out every
weed while it is still very tiny.
Let's also suppose it's been a typical maritime Northwest rainy
winter, so on April 1 the soil is at field capacity, holding all the
moisture it can. From early April until well into September the hot
sun will beat down on this bare plot. Our summer rains generally
come in insignificant installments and do not penetrate deeply; all
of the rain quickly evaporates from the surface few inches without
recharging deeper layers. Most readers would reason that a soil
moisture measurement taken 6 inches down on September 1, should show
very little water left. One foot down seems like it should be just
as dry, and in fact, most gardeners would expect that there would be
very little water found in the soil until we got down quite a few
feet if there were several feet of soil.
But that is not what happens! The hot sun does dry out the surface
inches, but if we dig down 6 inches or so there will be almost as
much water present in September as there was in April. Bare earth
does not lose much water at all. _Once a thin surface layer is
completely desiccated, be it loose or compacted, virtually no
further loss of moisture can occur._
The only soils that continue to dry out when bare are certain kinds
of very heavy clays that form deep cracks. These ever-deepening
openings allow atmospheric air to freely evaporate additional
moisture. But if the cracks are filled with dust by surface
cultivation, even this soil type ceases to lose water.
Soil functions as our bank account, holding available water in
storage. In our climate soil is inevitably charged to capacity by
winter rains, and then all summer growing plants make heavy
withdrawals. But hot sun and wind working directly on soil don't
remove much water; that is caused by hot sun and wind working on
plant leaves, making them transpire moisture drawn from the earth
through their root systems. Plants desiccate soil to the ultimate
depth and lateral extent of their rooting ability, and then some.
The size of vegetable root systems is greater than most gardeners
would think. The amount of moisture potentially available to sustain
vegetable growth is also greater than most gardeners think.
Rain and irrigation are not the only ways to replace soil moisture.
If the soil body is deep, water will gradually come up from below
the root zone by capillarity. Capillarity works by the very same
force of adhesion that makes moisture stick to a soil particle. A
column of water in a vertical tube (like a thin straw) adheres to
the tube's inner surfaces. This adhesion tends to lift the edges of
the column of water. As the tube's diameter becomes smaller the
amount of lift becomes greater. Soil particles form interconnected
pores that allow an inefficient capillary flow, recharging dry soil
above. However, the drier soil becomes, the less effective capillary
flow becomes. _That is why a thoroughly desiccated surface layer
only a few inches thick acts as a powerful mulch._
Industrial farming and modern gardening tend to discount the
replacement of surface moisture by capillarity, considering this
flow an insignificant factor compared with the moisture needs of
crops. But conventional agriculture focuses on maximized yields
through high plant densities. Capillarity is too slow to support
dense crop stands where numerous root systems are competing, but
when a single plant can, without any competition, occupy a large
enough area, moisture replacement by capillarity becomes
significant.
How Plants Obtain Water
Most gardeners know that plants acquire water and minerals through
their root systems, and leave it at that. But the process is not
quite that simple. The actively growing, tender root tips and almost
microscopic root hairs close to the tip absorb most of the plant's
moisture as they occupy new territory. As the root continues to
extend, parts behind the tip cease to be effective because, as soil
particles in direct contact with these tips and hairs dry out, the
older roots thicken and develop a bark, while most of the absorbent
hairs slough off. This rotation from being actively foraging tissue
to becoming more passive conductive and supportive tissue is
probably a survival adaptation, because the slow capillary movement
of soil moisture fails to replace what the plant used as fast as the
plant might like. The plant is far better off to aggressively seek
new water in unoccupied soil than to wait for the soil its roots
already occupy to be recharged.
A simple bit of old research magnificently illustrated the
significance of this. A scientist named Dittmer observed in 1937
that a single potted ryegrass plant allocated only 1 cubic foot of
soil to grow in made about 3 miles of new roots and root hairs every
day. (Ryegrasses are known to make more roots than most plants.) I
calculate that a cubic foot of silty soil offers about 30,000 square
feet of surface area to plant roots. If 3 miles of microscopic root
tips and hairs (roughly 16,000 lineal feet) draws water only from a
few millimeters of surrounding soil, then that single rye plant
should be able to continue ramifying into a cubic foot of silty soil
and find enough water for quite a few days before wilting. These
arithmetical estimates agree with my observations in the garden, and
with my experiences raising transplants in pots.
Lowered Plant Density: The Key to Water-Wise Gardening
I always think my latest try at writing a near-perfect garden book
is quite a bit better than the last. _Growing Vegetables West of the
Cascades_, recommended somewhat wider spacings on raised beds than I
did in 1980 because I'd repeatedly noticed that once a leaf canopy
forms, plant growth slows markedly. Adding a little more fertilizer
helps after plants "bump," but still the rate of growth never equals
that of younger plants. For years I assumed crowded plants stopped
producing as much because competition developed for light. But now I
see that unseen competition for root room also slows them down. Even
if moisture is regularly recharged by irrigation, and although
nutrients are replaced, once a bit of earth has been occupied by the
roots of one plant it is not so readily available to the roots of
another. So allocating more elbow room allows vegetables to get
larger and yield longer and allows the gardener to reduce the
frequency of irrigations.
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