In the equipment section we discussed the different kinds of irrigation systems, but now we need to talk about their use. As with seedlings, water in the garden can be either a blessing or a curse, depending on how much there is and how it is used; too much is as bad as too little. While it is absolutely necessary for good growth, there are times when it should be withheld if the crop is to live up to expectations. When he wrote the first version of his book, Big Sam felt irrigation was unnecessary except during severe drought; but by 1971 he had decided, on the basis of his own observations, that just about any garden, in any climate, could be improved by making sure the plants never went without.
Most garden crops, be they leafy, or roots, or fruits, consist primarily of water. And if a cucumber is, we will say, ninety- percent water, how can one expect to obtain perfect fruit if at any time there is a lack of moisture? In fact cucumber fruits, in their shape and size, offer a convincing record of the amounts of water which have been available to it (them). Cucumber growers will all have notices a fruit small at the stem end which suddenly swells out to normal size, or one which starts out nobly only to be squeezed into a miserable pointed end, or even those which swell and shrink and then swell again, thus giving visible evidence of the dry and rainy spells. Good fruits and leaves and roots will result if the plants are never allowed to suffer from lack of water, and the measure of this is the condition of the soil.
How often and how much to water, though, seems to be a matter on which it is impossible to get agreement. Some gardeners insist that frequent light waterings are best; others are convinced that light waterings cause the plants to develop root systems which are shallower than normal—making them less tolerant of drought—and that plants should only be watered when they are on the verge of wilting, and then deeply and thoroughly. Big Sam, as I mentioned above, has held both positions himself, and I must admit that, to a certain extent, I do as well.
When the plants are young, I water frequently; their roots are limited to the surface regions of the soil, and without water they will never develop. I water very shallow-rooted crops and fast-growing leafy greens frequently, right up until a week before harvest. But with most plants, once they have reached their full size, I water only if absolutely necessary. I feel that pampering them after that point lowers the quality of the harvest: whether it be corn, herbs, or carrots, tomatoes, melons or beets, I want the flavor of that vegetable to be expressed fully, to be concentrated. Since vegetables are largely water, it stands to reason that to give them extra as they approach maturity will only dilute their essence, that is “water down” their flavor.
So, there are certain critical times when particular plants either should or should not be watered if you want the best results. For example, peas and beans need plenty of water during pod development, or they will not fill our properly. Tomatoes must have a consistent water supply during fruit formation, or they may crack; the same is true of carrots and radishes as the roots enlarge. All these details will be found in the vegetable section under their respective headings, but there are a few rules-of-thumb that the beginning organic gardener would do well to know.
The general rule is that growing gardens need an inch of rain a week, and if this is not forthcoming, you should make up the difference. In order to track the rainfall you can purchase an inexpensive rain gauge, preferably one of the kind that you simply stock in to the ground (away from obstructions that might distort the rainfall pattern). Then when you water you can place this gauge within the spray pattern of the sprinkler, to measure what you are adding to the equation. Or you can simply use a container from around the house.
To choose an example: we use agricultural impact sprinklers in our gardens. These are the rotating brass kind you see on a golf course, and with sufficient water pressure they will cover an eighty-foot circle. We have found that it takes four and a half hours to apply an inch of water on the garden with a hundred-foot run of ¾-inch hose at normal household pressure. If you have a small garden and water by hand, you can time your watering to determine the appropriate times and amounts: figure the gallons per minute of your own hose by timing how long it takes to fill a gallon pail, and then multiply by the area you want to cover. One inch of water o n100 square feet comes to about 65 gallons.
When you should water involves two separate questions: how often, and what time of day. I decide when to water the same way I decide when to plant, by taking a handful of soil from the bed and squeezing it in my hand; if it will not hold together in a lump, it’s time to water. Some times are special, though: unless water squeezes out between my fingers, I always water immediately after transplanting or seeding; and unless the plants are physically wilting, I never water just before harvest. To check the depth of dryness dig a small hole and look at the cross-section view of the top six to eight inches of soil.
The best time of day to water depends on the situation. If you are in a hot, dry climate and water is precious, water in the evening (or the very early morning) so losses to evaporation are kept to a minimum. In humid areas it is a good idea to water only on sunny days, and always by lunchtime so that the plant leaves have a chance to dry off before evening; otherwise you are creating a good environment for bacterial and fungal infections to develop. If you can make sure the foliage dries completely at least once every 18 hours they will have a hard time getting established. This is especially true in greenhouses and cold frames, which are shut tight overnight.
In cold climates like ours it makes sense to water early in the day for another reason: the soil is cooled down by the irrigation water—especially in early spring, when soil and water are at their coldest—and if the sun has a chance to heat the soil right back up, the plants will suffer that much less. This is even more important when watering seedlings: professional nurserymen often heat their greenhouse irrigation water to avoid temperature shock on their seedlings.