Calculating water requirements

Asked September 29, 2016, 10:19 AM EDT

I was reading through the Efficient Use of Water in the Garden and Landscape post and am a bit confused.

In regards for drip irrigating a vegetable garden, they recommend 0.5-1 GPH emitters spaced 16-24 inches. Let's use a 4'x4' raised garden bed with 1 GPH emitters as an example. We'd require about 6 emitters.

Then it says for a closely spaced vegetable garden would need 6 hours of watering a 3 2-hour waterings a week.

If we do the math, that comes to 36 gallons a week for a 4x4 bed. 6 emitters x 1 GPH x 2 hours x 3 waterings a week = 36 gallons.

I've also read that a vegetable garden needs about 1 inch of water per week. According to a rainfall calculator I found a 4x4(16 sq ft) area would only need 9.97403 gallons to reach 1 inch of water.

This seems like a HUGE difference, 36 gallons vs ~10 gallons.

I realize that watering very much depends on your climate but I'm just trying to get some sort of baseline to start from and will adjust from there. I'm in Bedford, TX and have filled my raised beds with Mel's mix(1/3 compost, 1/3 peat moss, 1/3 vermiculite). It's pretty fluffy. I'm worried it's too fluffy.

I know i'm way overthinking all of this but it's my first gardening experience and i'd like to start off on the right foot! Any help regarding watering would be greatly appreciated.

Tarrant County Texas

1 Response

You are no more confused than any other new gardener. The variables to determine when to water and how long to run irrigation has to do with sunlight, temperature, relative humidity, wind speed, irrigation devices, soil water holding capacity, soil infiltration rate, plant water requirements, new or established plants, and time of year. Give me a few minutes and I will think of other variables…

I’m not writing this not to be more confusing but just to let you know why everyone is confused not just you.

Unfortunately, the soil mix you have started with is not the best for this area because there is so much calcium in the municipal water and both calcium and sodium in well water. Vermiculite holds salts.

I have a list of web sites from Texas A&M Extension. Each of these web sites has good advice.

Vegetable gardening is dynamic because you raise one crop and then plant another so there are different water requirements. I just planted spinach (October) from seeds because spinach starts easy from seed. I am going to water that row daily until the seedling are up and grow several leaves which will mean the plants have good roots. If I plants transplants, I would not water daily unless the sun, wind and type of soil dried out the soil that quickly.

Next to the spinach row I have already planted rows of kale, broccoli, cabbage and lettuce transplants. These transplants are already growing, so they do not require daily watering. More confused yet?

I am using ½ sand ½ compost soil mix because it drains well. I add compost every time I plant. I use ½ inch inline drip tubing with 12” spacing. I place a row of the drip tubing 12 inches apart. I have attached pictures.

I suggestion keeping the soil evenly moist, not wet. The roots need air. Wet soil will not have enough air. Check the moisture with a 6 to 8 inch long screw driver, like sticking a toothpick in a cake. So the when and how long will have to be determined on site keeping in mind all the variables. Run the drip tubing for one hours and see how moist and how deep the water is. If too dry, water another 30 minutes until you figure out timing. Of course that timing will change as the variables change

Tarrant County Master Gardener Help Desk 817-884-1944

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Vegetable Planting dates for Tarrant County

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