Irrigation – Measuring How Much

Q: I am a Master Gardener volunteer in our Extension office. We have had a number of people call in and ask about watering vegetables and flowers. I tell them that one inch per week is recommended. My problem is how to determine one inch of water when using watering cans or a wand nozzle with individual plants. I know how to measure irrigation using tuna cans but how does one do it otherwise?

A: The bottom line is to have moisture penetrate deep enough to be at or below the roots of the plants after watering. I guess the only way to really tell how deep the water has penetrated is to dig down with a trowel and see. A watering wand delivers approximately five gallons per minute. I estimate that a small tomato needs one gallon and a big one might need five gallons of water per week when it is really hot.

I use soaker hoses along my vegetable rows and they work remarkably well. A hose can irrigate a two foot wide swath. The manufacturer says that a soaker hose usually delivers one gallon per foot per hour when the flow-limiting washer at the inlet is left in place. That means that a fifty foot row of mature tomatoes needs to be soaked for ninety minutes, once per week.