In my experience, gardeners often overestimate rainfall. A downpour appears to have thoroughly saturated the ground, but if you scrape down an inch you may find bone-dry earth.
Since watering – not too much and not too little – is one of the keys to a successful garden, it pays to be more analytical with this downpour.
The amount of rain that has fallen, how long to water your plants, or how much water plants need is often given in inches. As a general rule, plants need about an inch of water per week to really thrive. It’s easy to measure with a rain gauge, which you can either buy or make at home from nothing but a coffee or other can and ruler.
Because I’m the one with the rain gauge, my neighbor usually yells over the fence in the morning after a night shower: “How many centimeters did we get?”
A rain gauge can tell you how long to use your sprinkler to get that inch of water into the garden. Since the water distribution may not be even, place a few doses randomly over the area to be watered. Then turn the tap on and leave it on until the sprinklers fill the cans 1 inch deep with water.
It takes about 27,000 gallons to cover an acre of water 1 inch deep. One inch of water is 90 gallons in a 150 square meter yard.
If you’re watering with a bucket or watering can, use the 1 inch measure to determine the amount needed for a single plant. (It is especially important that newly planted trees and shrubs are watered for the first season.)
First estimate the area – from a bird’s eye view – over which the roots will spread. It is usually assumed that this corresponds to the horizontal spread of the branches. The roots of a rose bush that I planted last spring, for example, are now probably spread over a few square meters. Two square feet, or 288 square inches by 1 inch, is 288 cubic inches, or about 5 liters of water.
The rule could also be translated as follows: apply 2 1/2 liters of water per square foot every week. With a watering can, you can easily see how much water you are using. If you’re using a hose, calculate its performance over time, and then calculate how long you need to stand there to get enough water for the area to be irrigated. It’s probably longer than expected.
This irrigation guideline applies to soil that has not dried out too much. If the soil is bone dry because you haven’t watered or it hasn’t rained in three weeks, you’ll need to wet it thoroughly beforehand. Then, in a week, give that inch, or 2 1/2 quarts per square foot.
This “inch” rule also applies to plants whose roots are mainly in the upper toe of the soil, which includes most annual vegetable and flower plants. Plants deeper than a foot need more water; Corn roots, for example, run 3 feet deep.
Every gardener should have a rain gauge. Mechanical or computer-controlled gauges show you the amount of precipitation (depth in inches) to hundredths of an inch. Even a straight-rimmed coffee pot is likely to be accurate to within an eighth of an inch.
A rain gauge will tell you if a summer shower spilled enough water you can forget about watering for a week, or if what you thought was a heavy rain was just a tricky quarter-inch rain. If you only get a quarter of an inch of rain, the remaining three-quarters of an inch, or a few gallons of water per square foot, will need to come from your hose or watering can.
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