Q: I have several areas where I’m using a drip system and am wondering how much water I should be using and for how long. Also, how do I measure water? I have a Kousa dogwood, a raised bed with vegetables and herbs, an Asian pear and saffron bulbs. On another zone (with a different hose), I have potatoes, two blueberry bushes, a boxwood and assorted flowers.
A: Soaker hoses and drip systems are a great convenience, but you have encountered their primary difficulty — figuring out how long they should run.
The annoying answer to how much you should irrigate is: It depends.
It depends on how hot it is, the type of plant you’re watering and the type of soil. A year like 2015, which was hotter and drier than normal, would require more irrigation than 2016, which enjoyed periodic summer rain. When using timed irrigation, it is a good practice to adjust the length of the watering sessions several times during the season. It is also important to remember to turn a timed system off during rainy periods. The environment and your water bill will thank you for using no more water than necessary.
Monitor plants for signs of water stress. Symptoms of water stress are wilting or folding leaves, dull or gray-green leaves, leaf drop or new leaves maturing smaller than older leaves.
Drip irrigation wets a smaller area than sprinkling, so must be done more frequently. Daily or alternative day watering is necessary. But it is also important to monitor soil moisture and allow it to dry some between watering.
It is useful to determine how much water is being applied by the emitters. Some systems have emitters that allow a specific amount per hour. Others can be calculated by catching the output over a timed cycle. Then you’ll know how much water is applied per time period.
The goal is to apply enough water to wet the entire root zone. This will depend on the plant. Rhododendrons and azaleas have shallow root zones so would not require as deep watering. Trees will require much more water.
A rough rule of thumb is to run emitters 15 minutes in spring and several hours on hot, dry summer days. Use a hand trowel or auger to dig down and determine how well the emitters are accomplishing the task. There’s no better way to find out what’s going on than to look.
If you have different soil types within your yard, keep in mind that clay soil will hold water and sandier soil will not. Mulching plants is a great way to reduce water needs in spring and summer.
The age of your plants is also important. New plants should receive more water for the first two years, since most of the root mass is still in the nursery soil.
Washington State University has a great online calculator that can be found here.
Everyone seems to be gardening during the pandemic so if you’ve got questions, turn to Ask an Expert, an online question-and-answer tool from Oregon State University’s Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website and type in a question and the county where you live.