How much water to put on a new rain garden

KPUD's new Rain Garden must be kept watered until the plants are established. This could take a couple of years. This posed a question, how much water is enough? As a utility water conservation manager who is advising people to use water wisely, I needed to find out more. Marty, our hydrogeologist and I decided to approach finding out exactly how much to water scientifically and do the math.

First I had to find out how much water I was putting on the garden. I am watering with an ordinary fan type sprinkler that I got at the big box store down the road. It so happened that it's spray covers my approximately 23 square foot garden.

$9.00 sprinkler in the rain garden

 The rule of thumb is use one inch of water per week. Using a fancy sprinkler calibration system, I measured how long it took my sprinkler to water the garden this much. Homeowners can do this by doing the tuna can test.

accurate sprinkler gauge

I placed several  calibration cups through the garden. It took me a minute to figure out how this container measured only an inch, but it was pointed out by someone a little more savvy than me (Marty) that it was configured to offer a larger opening to allow the water to easily be collected.

 

calibration cups placed throughout the garden

I then turned on the sprinkler and let it run until .1 of an inch was collected. I determined that it took 20 minutes.I also determined that the sprinkler distributed pretty evenly throughout the garden. 

Marty looking at the cup measurement of .1 inch.

 Doing the math I calculated that it takes my sprinkler 3 hours and 10 minutes to get one inch to the garden!! Marty figured that multiplying the square footage of the garden by .62 gallons, It would take 328 gallons to arrive at my one inch per week.

(.62 gallons is the amount of water it takes to cover one foot of area with one inch of water.)
Alright, I decided, although letting the sprinkler run for three hours is counter-intuitive for a water conservation professional, I'll do it. Then  . . . .wait, it rained the other night. After running the sprinkler for half that time, I stopped it and got out my garden trowel.

I use a 4" garden towel to test if the soil is wet before I water

 The blade of my garden trowel is four inches. I gathered some soil from that depth.It was wet!! So, this is what I decided to do. Only run the sprinkler for 1 1/2 hours twice a week and test the soil for dampness before I water to see if it is necessary at all. Of course all bets are off if the plants are showing stress. This watering regime should keep the plants going until the winter rains take over in a month or so. I may have to continue watering another summer, but after that they will do just fine on their own.