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

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 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"