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.

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?