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.