Watering Guide: How Much is an Inch of Water for plants

Thinking about water in inches can feel very confusing because an inch will look different for every container. An inch of water in a glass is quite different from an inch of water in a flowerpot or on a field. But for gardeners and farmers, inches of water is a standard industry measurement that you need to understand.

How much is an inch of water? The volume of an inch of water depends on the area it’s applied to. Multiplying the length and the width of the area you want to water and dividing the result by 231 will give you the number of gallons needed.

How does this conversion work, and why is water measured in inches anyway? Let’s talk about all the important features around this topic to understand watering in inches fully.

How much is an inch of water

The measurement unit that the inches relate to is square feet, so plants in the ground want one inch of water per square foot. There is a simple equation that will help you figure out how much water will measure to one inch across your garden:

Length of your garden in inches x width of your garden in inches

Since you are only using one inch of water, there is no need to multiply the equation by one. The answer to that equation gives you the volume of water that your garden needs every week, and you can easily convert that number into gallons to give you a liquid measurement. 231 Cubic Inches amounts to 1 gallon of water; all you need to do now is divide the resulting number by this amount.


Why are water requirements measured in inches?

Common advice for gardeners is to water your garden 1 inch per week. However, it is impossible to give a specific recommendation to gardeners because of factors like climate, specific plant, and soil type. One inch per week is a great rule of thumb that works across almost all of these variables.

This simple equation can save you from underwatering, and keep your plants in good health, no matter the climate. It is only a rule of thumb, and you may find that certain types of plants respond differently. Tomato plants, for example, are big drinkers and may require a little more water than this equation suggests. Several other variables, such as the weather and your watering style, may require you to adjust how much water you give your plants.


How much water is an inch per week?

The answer to this question will depend entirely on the size of the space you are watering, but we can use one square foot as a great example of how this would work:

1 square foot is 144 square inches. Assuming we want to add one inch of water, we end up with 144 cubic inches, which converts to 0.62 gallons. The conversion from cubic inches to gallons is complex, and it is much easier to use an online converting tool than to try and do the maths.

If you need to measure your water in liters, you will need to take one extra step and convert gallons to liters. Once again, an online tool is going to accomplish this much faster than trying to do the maths. In our example, 0.62 gallons converts to 2.36 liters.

If your space is larger than one square foot, you can change the numbers at the beginning of our example to get the answer you need. In short 1 square foot = 144 square inches = 0.62 gallons (for one inch of water) = 2.36 litres.


How to measure an inch while watering? 

You can use four empty cans to ensure that you are watering your garden evenly and that you are watering up to one inch. You will need to measure a half-inch of water in all four cans and mark the spot with a marker. Then when you water your garden, place the four cans in a grid across your garden.

After you have placed the cans, start timing and water your garden evenly, but pay attention to how they are filling up, and stop watering and your timer when they have filled up to the mark. The timer will show you how long it takes for you to water half an inch. Depending on water availability and your preference, you can water for this amount of time twice a week, or half of this time four times a week.

You can also use this method to determine if you are watering your garden evenly if you are using a sprinkler or other system. Just water for the time that you measured half an inch of water takes, and then pour out each can check that each has half an inch of water in it. You can place the cans in different parts of your garden if you are particularly worried about certain spots. They do not have to be exactly the same, but they should be roughly equal.


How timing affects water requirements 

When you chose to water your garden can make a small difference in the health of your plants. For example, if you water in the middle of a hot day, some water will be evaporated into the air and not get to your plants.

Similarly, if you are watering at night, your plants will not dry quickly, and you may be leaving your garden open to fungal spores which like moisture. If you water your plants in the morning, the leaves will dry quickly, and the roots will still get a good drink.

Your plants will need extra water during intense heat, particularly vegetable plants like cauliflower, celery, and squash. Plants like these also need consistent moisture, which can be hard to achieve during heatwaves. The best solution is to use drip irrigation, which can be achieved by placing a hose at your plants’ base and keeping it dripping all day.


Shallow vs. deep watering

Shallow watering and deep watering refer to how far down the water can soak into the soil. The roots of most plants are not sitting at the surface of the soil, and you want to encourage the growth of deeper roots so that your plants aren’t vulnerable to drying out when the soil surface is dryer in warmer weather.

Deep watering is difficult to achieve with one inch of water. For deep watering, you will want to soak your plants so that the water can soak to eight inches below the surface. You can do this by watering your plants less frequently but with a higher quantity of water.

Shallow watering would be when you use less water more regularly. This is a great way to grow your plants if you are using an irrigation system or a sprinkler, and you know that your plants are getting the right amount of water each day. If, however, you are using a hose or watering can, you can’t reasonably achieve this, in which case it’s better to grow heartier plants with deeper roots.

If you are not sure if the water is penetrating deeply enough into the soil, there is an easy test to find out. Water your plants with whatever method is normal, and then wait half an hour. After that, dig down into your soil with a trowel and see how wet the soil is beneath the surface. If it is not wet eight inches down, you may need to give your plants more water or try a different watering system to make sure the water is soaking all the way down.

Using one inch of water rule of thumb is a great way to make sure that your plants are getting enough water, but as we have discussed, things can be a little more complicated than that. Your plants will respond differently depending on the time of day, the heat, the type of plant, and a range of other variables. If you water your plants with one inch of water and find them still looking a little dry, you can always increase the quantity of water the plants get. But one inch of water is a great baseline at which to start.


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