Gardening Without Irrigation: or without much, anyway

S >> Steve Solomon >> Gardening Without Irrigation: or without much, anyway

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Though hot, baking sun and wind can desiccate the few inches of
surface soil, withdrawals of moisture from greater depths are made
by growing plants transpiring moisture through their leaf surfaces.
The amount of water a growing crop will transpire is determined
first by the nature of the species itself, then by the amount of
leaf exposed to sun, air temperature, humidity, and wind. In these
respects, the crop is like an automobile radiator. With cars, the
more metal surfaces, the colder the ambient air, and the higher the
wind speed, the better the radiator can cool; in the garden, the
more leaf surfaces, the faster, warmer, and drier the wind, and the
brighter the sunlight, the more water is lost through transpiration.

Dealing with a Surprise Water Shortage

Suppose you are growing a conventional, irrigated garden and
something unanticipated interrupts your ability to water. Perhaps
you are homesteading and your well begins to dry up. Perhaps you're
a backyard gardener and the municipality temporarily restricts
usage. What to do?

First, if at all possible before the restrictions take effect, water
very heavily and long to ensure there is maximum subsoil moisture.
Then eliminate all newly started interplantings and ruthlessly hoe
out at least 75 percent of the remaining immature plants and about
half of those about two weeks away from harvest.

For example, suppose you've got a a 4-foot-wide intensive bed
holding seven rows of broccoli on 12 inch centers, or about 21
plants. Remove at least every other row and every other plant in the
three or four remaining rows. Try to bring plant density down to
those described in Chapter 5, "How to Grow It: A-Z"

Then shallowly hoe the soil every day or two to encourage the
surface inches to dry out and form a dust mulch. You water-wise
person--you're already dry gardening--now start fertigating.

How long available soil water will sustain a crop is determined by
how many plants are drawing on the reserve, how extensively their
root systems develop, and how many leaves are transpiring the
moisture. If there are no plants, most of the water will stay unused
in the barren soil through the entire growing season. If a crop
canopy is established midway through the growing season, the rate of
water loss will approximate that listed in the table in Chapter 1
"Estimated Irrigation Requirement." If by very close planting the
crop canopy is established as early as possible and maintained by
successive interplantings, as is recommended by most advocates of
raised-bed gardening, water losses will greatly exceed this rate.

Many vegetable species become mildly stressed when soil moisture has
dropped about half the way from capacity to the wilting point. On
very closely planted beds a crop can get in serious trouble without
irrigation in a matter of days. But if that same crop were planted
less densely, it might grow a few weeks without irrigation. And if
that crop were planted even farther apart so that no crop canopy
ever developed and a considerable amount of bare, dry earth were
showing, this apparent waste of growing space would result in an
even slower rate of soil moisture depletion. On deep, open soil the
crop might yield a respectable amount without needing any irrigation
at all.

West of the Cascades we expect a rainless summer; the surprise comes
that rare rainy year when the soil stays moist and we gather
bucketfuls of chanterelle mushrooms in early October. Though the
majority of maritime Northwest gardeners do not enjoy deep, open,
moisture-retentive soils, all except those with the shallowest soil
can increase their use of the free moisture nature provides and
lengthen the time between irrigations. The next chapter discusses
making the most of whatever soil depth you have. Most of our
region's gardens can yield abundantly without any rain at all if
only we reduce competition for available soil moisture, judiciously
fertigate some vegetable species, and practice a few other
water-wise tricks.

_Would lowering plant density as much as this book suggests equally
lower the yield of the plot? Surprisingly, the amount harvested does
not drop proportionately. In most cases having a plant density
one-eighth of that recommended by intensive gardening advocates will
result in a yield about half as great as on closely planted raised
beds._

Internet Readers: In the print copy of this book are color pictures
of my own "irrigationless" garden. Looking at them about here in the
book would add reality to these ideas.

Chapter 3

Helping Plants to Need Less Irrigation

Dry though the maritime Northwest summer is, we enter the growing
season with our full depth of soil at field capacity. Except on
clayey soils in extraordinarily frosty, high-elevation locations, we
usually can till and plant before the soil has had a chance to lose
much moisture.

There are a number of things we can do to make soil moisture more
available to our summer vegetables. The most obvious step is
thorough weeding. Next, we can keep the surface fluffed up with a
rotary tiller or hoe during April and May, to break its capillary
connection with deeper soil and accelerate the formation of a dry
dust mulch. Usually, weeding forces us to do this anyway. Also, if
it should rain during summer, we can hoe or rotary till a day or two
later and again help a new dust mulch to develop.

Building Bigger Root Systems

Without irrigation, most of the plant's water supply is obtained by
expansion into new earth that hasn't been desiccated by other
competing roots. Eliminating any obstacles to rapid growth of root
systems is the key to success. So, keep in mind a few facts about
how roots grow and prosper.

The air supply in soil limits or allows root growth. Unlike the
leaves, roots do not perform photosynthesis, breaking down carbon
dioxide gas into atmospheric oxygen and carbon. Yet root cells must
breathe oxygen. This is obtained from the air held in spaces between
soil particles. Many other soil-dwelling life forms from bacteria to
moles compete for this same oxygen. Consequently, soil oxygen levels
are lower than in the atmosphere. A slow exchange of gases does
occur between soil air and free atmosphere, but deeper in the soil
there will inevitably be less oxygen. Different plant species have
varying degrees of root tolerance for lack of oxygen, but they all
stop growing at some depth. Moisture reserves below the roots'
maximum depth beecome relatively inaccessible.

Soil compaction reduces the overall supply and exchange of soil air.
Compacted soil also acts as a mechanical barrier to root system
expansion. When gardening with unlimited irrigation or where rain
falls frequently, it is quite possible to have satisfactory growth
when only the surface 6 or 7 inches of soil facilitates root
development. When gardening with limited water, China's the limit,
because if soil conditions permit, many vegetable species are
capable of reaching 4, 5, and 8 eight feet down to find moisture and
nutrition.

Evaluating Potential Rooting Ability

One of the most instructive things a water-wise gardener can do is
to rent or borrow a hand-operated fence post auger and bore a
3-foot-deep hole. It can be even more educational to buy a short
section of ordinary water pipe to extend the auger's reach another 2
or 3 feet down. In soil free of stones, using an auger is more
instructive than using a conventional posthole digger or shoveling
out a small pit, because where soil is loose, the hole deepens
rapidly. Where any layer is even slightly compacted, one turns and
turns the bit without much effect. Augers also lift the materials
more or less as they are stratified. If your soil is somewhat stony
(like much upland soil north of Centralia left by the Vashon
Glacier), the more usual fence-post digger or common shovel works
better.

If you find more than 4 feet of soil, the site holds a dry-gardening
potential that increases with the additional depth. Some soils along
the floodplains of rivers or in broad valleys like the Willamette or
Skagit can be over 20 feet deep, and hold far more water than the
deepest roots could draw or capillary flow could raise during an
entire growing season. Gently sloping land can often carry 5 to 7
feet of open, usable soil. However, soils on steep hillsides become
increasingly thin and fragile with increasing slope.

Whether an urban, suburban, or rural gardener, you should make no
assumptions about the depth and openness of the soil at your
disposal. Dig a test hole. If you find less than 2 unfortunate feet
of open earth before hitting an impermeable obstacle such as rock or
gravel, not much water storage can occur and the only use this book
will hold for you is to guide your move to a more likely gardening
location or encourage the house hunter to seek further. Of course,
you can still garden quite successfully on thin soil in the
conventional, irrigated manner. _Growing Vegetables West of the
Cascades_ will be an excellent guide for this type of situation.

Eliminating Plowpan

Deep though the soil may be, any restriction of root expansion
greatly limits the ability of plants to aggressively find water. A
compacted subsoil or even a thin compressed layer such as plowpan
may function as such a barrier. Though moisture will still rise
slowly by capillarity and recharge soil above plowpan, plants obtain
much more water by rooting into unoccupied, damp soil. Soils close
to rivers or on floodplains may appear loose and infinitely deep but
may hide subsoil streaks of droughty gravel that effectively stops
root growth. Some of these conditions are correctable and some are
not.

Plowpan is very commonly encountered by homesteaders on farm soils
and may be found in suburbia too, but fortunately it is the easiest
obstacle to remedy. Traditionally, American croplands have been
tilled with the moldboard plow. As this implement first cuts and
then flips a 6-or 7-inch-deep slice of soil over, the sole--the part
supporting the plow's weight--presses heavily on the earth about 7
inches below the surface. With each subsequent plowing the plow sole
rides at the same 7-inch depth and an even more compacted layer
develops. Once formed plowpan prevents the crop from rooting into
the subsoil. Since winter rains leach nutrients from the topsoil and
deposit them in the subsoil, plowpan prevents access to these
nutrients and effectively impoverishes the field. So wise farmers
periodically use a subsoil plow to fracture the pan.

Plowpan can seem as firm as a rammed-earth house; once established,
it can last a long, long time. My own garden land is part of what
was once an old wheat farm, one of the first homesteads of the
Oregon Territory. From about 1860 through the 1930s, the field
produced small grains. After wheat became unprofitable, probably
because of changing market conditions and soil exhaustion, the field
became an unplowed pasture. Then in the 1970s it grew daffodil
bulbs, occasioning more plowing. All through the '80s my soil again
rested under grass. In 1987, when I began using the land, there was
still a 2-inch-thick, very hard layer starting about 7 inches down.
Below 9 inches the open earth is soft as butter as far as I've ever
dug.

On a garden-sized plot, plowpan or compacted subsoil is easily
opened with a spading fork or a very sharp common shovel. After
normal rotary tilling, either tool can fairly easily be wiggled 12
inches into the earth and small bites of plowpan loosened. Once this
laborious chore is accomplished the first time, deep tillage will be
far easier. In fact, it becomes so easy that I've been looking for a
custom-made fork with longer tines.

Curing Clayey Soils

In humid climates like ours, sandy soils may seem very open and
friable on the surface but frequently hold some unpleasant subsoil
surprises. Over geologic time spans, mineral grains are slowly
destroyed by weak soil acids and clay is formed from the breakdown
products. Then heavy winter rainfall transports these minuscule clay
particles deeper into the earth, where they concentrate. It is not
unusual to find a sandy topsoil underlaid with a dense, cement-like,
clayey sand subsoil extending down several feet. If very impervious,
a thick, dense deposition like this may be called hardpan.

The spading fork cannot cure this condition as simply as it can
eliminate thin plowpan. Here is one situation where, if I had a
neighbor with a large tractor and subsoil plow, I'd hire him to
fracture my land 3 or 4 feet deep. Painstakingly double or even
triple digging will also loosen this layer. Another possible
strategy for a smaller garden would be to rent a gasoline-powered
posthole auger, spread manure or compost an inch or two thick, and
then bore numerous, almost adjoining holes 4 feet deep all over the
garden.

Clayey subsoil can supply surprisingly larger amounts of moisture
than the granular sandy surface might imply, but only if the earth
is opened deeply and becomes more accessible to root growth.
Fortunately, once root development increases at greater depths, the
organic matter content and accessibility of this clayey layer can be
maintained through intelligent green manuring, postponing for years
the need to subsoil again. Green manuring is discussed in detail
shortly.

Other sites may have gooey, very fine clay topsoils, almost
inevitably with gooey, very fine clay subsoils as well. Though
incorporation of extraordinarily large quantities of organic matter
can turn the top few inches into something that behaves a little
like loam, it is quite impractical to work in humus to a depth of 4
or 5 feet. Root development will still be limited to the surface
layer. Very fine clays don't make likely dry gardens.

Not all clay soils are "fine clay soils," totally compacted and
airless. For example, on the gentler slopes of the geologic old
Cascades, those 50-million-year-old black basalts that form the
Cascades foothills and appear in other places throughout the
maritime Northwest, a deep, friable, red clay soil called (in
Oregon) Jori often forms. Jori clays can be 6 to 8 feet deep and are
sufficiently porous and well drained to have been used for highly
productive orchard crops. Water-wise gardeners can do wonders with
Joris and other similar soils, though clays never grow the best root
crops.

Spotting a Likely Site

Observing the condition of wild plants can reveal a good site to
garden without much irrigation. Where Himalaya or Evergreen
blackberries grow 2 feet tall and produce small, dull-tasting fruit,
there is not much available soil moisture. Where they grow 6 feet
tall and the berries are sweet and good sized, there is deep, open
soil. When the berry vines are 8 or more feet tall and the fruits
are especially huge, usually there is both deep, loose soil and a
higher than usual amount of fertility.

Other native vegetation can also reveal a lot about soil moisture
reserves. For years I wondered at the short leaders and sad
appearance of Douglas fir in the vicinity of Yelm, Washington. Were
they due to extreme soil infertility? Then I learned that conifer
trees respond more to summertime soil moisture than to fertility. I
obtained a soil survey of Thurston County and discovered that much
of that area was very sandy with gravelly subsoil. Eureka!

The Soil Conservation Service (SCS), a U.S. Government agency, has
probably put a soil auger into your very land or a plot close by.
Its tests have been correlated and mapped; the soils underlying the
maritime Northwest have been named and categorized by texture,
depth, and ability to provide available moisture. The maps are
precise and detailed enough to approximately locate a city or
suburban lot. In 1987, when I was in the market for a new homestead,
I first went to my county SCS office, mapped out locations where the
soil was suitable, and then went hunting. Most counties have their
own office.

Using Humus to Increase Soil Moisture

Maintaining topsoil humus content in the 4 to 5 percent range is
vital to plant health, vital to growing more nutritious food, and
essential to bringing the soil into that state of easy workability
and cooperation known as good tilth. Humus is a spongy substance
capable of holding several times more available moisture than clay.
There are also new synthetic, long-lasting soil amendments that hold
and release even more moisture than humus. Garden books frequently
recommend tilling in extraordinarily large amounts of organic matter
to increase a soil's water-holding capacity in the top few inches.

Humus can improve many aspects of soil but will not reduce a
garden's overall need for irrigation, because it is simply not
practical to maintain sufficient humus deeply enough. Rotary tilling
only blends amendments into the top 6 or 7 inches of soil. Rigorous
double digging by actually trenching out 12 inches and then spading
up the next foot theoretically allows one to mix in significant
amounts of organic matter to nearly 24 inches. But plants can use
water from far deeper than that. Let's realistically consider how
much soil moisture reserves might be increased by double digging and
incorporating large quantities of organic matter.

A healthy topsoil organic matter level in our climate is about 4
percent. This rapidly declines to less than 0.5 percent in the
subsoil. Suppose inches-thick layers of compost were spread and, by
double digging, the organic matter content of a very sandy soil were
amended to 10 percent down to 2 feet. If that soil contained little
clay, its water-holding ability in the top 2 feet could be doubled.
Referring to the chart "Available Moisture" in Chapter 2, we see
that sandy soil can release up to 1 inch of water per foot. By dint
of massive amendment we might add 1 inch of available moisture per
foot of soil to the reserve. That's 2 extra inches of water, enough
to increase the time an ordinary garden can last between heavy
irrigations by a week or 10 days.

If the soil in question were a silty clay, it would naturally make 2
1/2 inches available per foot. A massive humus amendment would
increase that to 3 1/2 inches in the top foot or two, relatively not
as much benefit as in sandy soil. And I seriously doubt that many
gardeners would be willing to thoroughly double dig to an honest 24
inches.

Trying to maintain organic matter levels above 10 percent is an
almost self-defeating process. The higher the humus level gets, the
more rapidly organic matter tends to decay. Finding or making enough
well-finished compost to cover the garden several inches deep (what
it takes to lift humus levels to 10 percent) is enough of a job.
Double digging just as much more into the second foot is even more
effort. But having to repeat that chore every year or two becomes
downright discouraging. No, either your soil naturally holds enough
moisture to permit dry gardening, or it doesn't.

Keeping the Subsoil Open with Green Manuring

When roots decay, fresh organic matter and large, long-lasting
passageways can be left deep in the soil, allowing easier air
movement and facilitating entry of other roots. But no cover crop
that I am aware of will effectively penetrate firm plowpan or other
resistant physical obstacles. Such a barrier forces all plants to
root almost exclusively in the topsoil. However, once the subsoil
has been mechanically fractured the first time, and if recompaction
is avoided by shunning heavy tractors and other machinery, green
manure crops can maintain the openness of the subsoil.

To accomplish this, correct green manure species selection is
essential. Lawn grasses tend to be shallow rooting, while most
regionally adapted pasture grasses can reach down about 3 feet at
best. However, orchard grass (called coltsfoot in English farming
books) will grow down 4 or more feet while leaving a massive amount
of decaying organic matter in the subsoil after the sod is tilled
in. Sweet clover, a biennial legume that sprouts one spring then
winters over to bloom the next summer, may go down 8 feet. Red
clover, a perennial species, may thickly invade the top 5 feet.
Other useful subsoil busters include densely sown Umbelliferae such
as carrots, parsley, and parsnip. The chicory family also makes very
large and penetrating taproots.

Though seed for wild chicory is hard to obtain, cheap varieties of
endive (a semicivilized relative) are easily available. And several
pounds of your own excellent parsley or parsnip seed can be easily
produced by letting about 10 row feet of overwintering roots form
seed. Orchard grass and red clover can be had quite inexpensively at
many farm supply stores. Sweet clover is not currently grown by our
region's farmers and so can only be found by mail from Johnny's
Selected Seeds (see Chapter 5 for their address). Poppy seed used
for cooking will often sprout. Sown densely in October, it forms a
thick carpet of frilly spring greens underlaid with countless
massive taproots that decompose very rapidly if the plants are
tilled in in April before flower stalks begin to appear. Beware if
using poppies as a green manure crop: be sure to till them in early
to avoid trouble with the DEA or other authorities.

For country gardeners, the best rotations include several years of
perennial grass-legume-herb mixtures to maintain the openness of the
subsoil followed by a few years of vegetables and then back (see
Newman Turner's book in more reading). I plan my own garden this
way. In October, after a few inches of rain has softened the earth,
I spread 50 pounds of agricultural lime per 1,000 square feet and
break the thick pasture sod covering next year's garden plot by
shallow rotary tilling. Early the next spring I broadcast a
concoction I call "complete organic fertilizer" (see _Growing
Vegetables West of the Cascades_ or the _Territorial Seed Company
Catalog_), till again after the soil dries down a bit, and then use
a spading fork to open the subsoil before making a seedbed. The
first time around, I had to break the century-old plowpan--forking
compacted earth a foot deep is a lot of work. In subsequent
rotations it is much much easier.

For a couple of years, vegetables will grow vigorously on this new
ground supported only with a complete organic fertilizer. But
vegetable gardening makes humus levels decline rapidly. So every few
years I start a new garden on another plot and replant the old
garden to green manures. I never remove vegetation during the long
rebuilding under green manures, but merely mow it once or twice a
year and allow the organic matter content of the soil to redevelop.
If there ever were a place where chemical fertilizers might be
appropriate around a garden, it would be to affordably enhance the
growth of biomass during green manuring.

Were I a serious city vegetable gardener, I'd consider growing
vegetables in the front yard for a few years and then switching to
the back yard. Having lots of space, as I do now, I keep three or
four garden plots available, one in vegetables and the others
restoring their organic matter content under grass.

Mulching

Gardening under a permanent thick mulch of crude organic matter is
recommended by Ruth Stout (see the listing for her book in More
Reading) and her disciples as a surefire way to drought-proof
gardens while eliminating virtually any need for tillage, weeding,
and fertilizing. I have attempted the method in both Southern
California and western Oregon--with disastrous results in both
locations. What follows in this section is addressed to gardeners
who have already read glowing reports about mulching.

Permanent mulching with vegetation actually does not reduce
summertime moisture loss any better than mulching with dry soil,
sometimes called "dust mulching." True, while the surface layer
stays moist, water will steadily be wicked up by capillarity and be
evaporated from the soil's surface. If frequent light sprinkling
keeps the surface perpetually moist, subsoil moisture loss can occur
all summer, so unmulched soil could eventually become desiccated
many feet deep. However, capillary movement only happens when soil
is damp. Once even a thin layer of soil has become quite dry it
almost completely prevents any further movement. West of the
Cascades, this happens all by itself in late spring. One hot, sunny
day follows another, and soon the earth's surface seems parched.

Unfortunately, by the time a dusty layer forms, quite a bit of soil
water may have risen from the depths and been lost. The gardener can
significantly reduce spring moisture loss by frequently hoeing weeds
until the top inch or two of earth is dry and powdery. This effort
will probably be necessary in any case, because weeds will germinate
prolifically until the surface layer is sufficiently desiccated. On
the off chance it should rain hard during summer, it is very wise to
again hoe a few times to rapidly restore the dust mulch. If hand
cultivation seems very hard work, I suggest you learn to sharpen
your hoe.

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