Gardening Without Irrigation: or without much, anyway | Page 9

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.
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