Urban farm: Irrigation and saving water

 It’s very difficult for the beginner to conceive of the relatively large amount of water it takes to soak the soil.
It’s very difficult for the beginner to conceive of the relatively large amount of water it takes to soak the soil.

Too little water is the most common cause of crop failure we have seen at new urban farms. It’s very difficult for the beginner to conceive of the relatively large amount of water it takes to soak the soil.

It is very common for novices to take the watering wand, move it over a bed once or twice, and think the job is done. A mere quarter inch of the soil is moistened by this method—the water doesn’t make it to the plant’s roots.

On the other side, the results of too much water include air-starved roots and a higher incidence of fungal diseases, which are brought on by too much green, watery plant growth. This can also be a problem in extremely damp and humid climates. In this case, in addition to providing the right amount of water, ensuring sufficient airflow around plants by pruning, trellising, and staking will help.

Plant roots need air to perform their functions. Alternating soaking the entire root zone with a few days to a week for soil water reserves to evaporate and drain away, bringing in air from above, is required for plants to thrive. If the root zone is constantly sopping wet, your plants will appear sickly and weak—again, a condition easy to confuse with a pest or disease problem. An exception to the rule of allowing the soil to dry out for a few days is newly seeded beds. Unless it’s raining, direct-seeded crops must be watered daily until the first true set of leaves emerges.

With seedlings, gently water them immediately after planting to minimize the shock of moving to a new home. Using a cup to pour water around the root zone instead of overhead spraying minimizes damage to tender leaves.

As a general rule of thumb, after plants have a few sets of true leaves you can give your garden between ¾ inch and 1½ inch of water per week split into multiple, more frequent doses in hot or windy weather. To measure the amount of water given, either through hand or sprinkler watering, using a ruler, mark a shot glass or jar at half-inch intervals and set it in your bed. Use your watch to time how long it takes to fill the glass to the desired level.

This is the amount of time you will need to hold your wand over each little section of bed. If you are hand watering, ignore the cup and arc the watering wand back and forth as you normally would to soak the bed. Obviously, if you hold the wand directly over the cup it will fill in no time, but receiving droplets as you pass by will take much longer and give an accurate idea of the length of time you should water each bed. Chances are it’s going to be much, much longer than you ever imagined.

So, do you give ¾ inch or 1½ inch of water? Over the growing season, the amount of water to give your plants depends on the weather, your soil structure, and the crop type.

Source: Novella Carpenter and Willow Rosenthal