The thorniest question.
My short answer is: Start with something like 45 minutes, observe, and adjust. But there are a lot of variables, which is why it’s a thorny question, and which is why observing and adjusting is necessary. Read on for details and reasoning and experience.
I gave a presentation on drip irrigation at the Gardening with Class conference for school gardens on Saturday, put on by the Master Gardeners of San Diego County, and I had straightforward answers to most people’s questions such as, “When should I run the system?” (Whenever you can watch it. In other words, just not in the middle of the night. This is because it’s harder to know if an emitter is blocked or if a poly tube has been punctured if you aren’t able to watch the system run.)
But how long to run drip irrigation?
“Uh . . . it depends,” I said.
There are many factors to consider. Emitters range in volume from 0.5 gallons per hour to 4 gallons per hour, or even wider. So you can imagine that running a system for one hour will give plants a very different amount of water according to the type of emitters used.
Then there is the number of emitters. There are drip lines with inline emitters spaced every 6 inches up to every 18 inches, or even wider. So if a broccoli plant is drinking from multiple emitters under it on a drip line that has emitters spaced every 6 inches, you will need to run that system for a shorter duration than if the emitters were spaced every 18 inches.
On top of that — or under it, actually — is the soil type. In a raised box for vegetables that is filled with potting mix-type soil, a drip system should not be run as long as for vegetables planted in the ground in a clayey soil. A clayey soil can soak up a lot of water and hold onto it, but a loose and light potting mix-type soil lets water percolate down, down, down to below where a plant’s roots will reach.
And there are even more variables, but how could I answer to help people at least get started? I referred them to a handy reference table in a booklet published by the irrigation supplies manufacturer, DIG. It says that this time of year, October, a drip system on vegetables should be run for 30 minutes to one hour every three days.
I also mentioned that I’m running my own drip irrigation on my vegetables for 30 minutes every three days on average in October, and I have 0.5 gallon-per-hour emitters spaced every 12 inches, and my soil is a sandy loam (plants are in the ground). There’s your starting point.
I should have noted that in the height of summer, July, I was running the same drip system for 30 minutes every two days (every other day). My summer vegetables did well on this regimen.
I finished my answer by saying that a good way to get started is by shooting in the middle, say 45 minutes every three days, and then watching how the plants respond. Is there any wilting, especially after the sun cools in late afternoon? Then raise the time to an hour or decrease the interval to two days and see if it makes enough of a difference. Or does the surface of the soil still appear wet after the three-day interval (when it’s time to run it again)? Then decrease the time to 30 minutes or lengthen the interval to four days.
If the surface of the soil still appears wet (dark in color) after your scheduled interval, then you shouldn’t water again yet because you’ll be watering too frequently and either wasting water or drowning your plants, or both. Dig down and you’ll find that most vegetables in most soils don’t have roots within an inch of the soil surface. Moisture in that upper inch is therefore nearly useless to the plants. That upper inch of soil is also quite vulnerable to evaporation from the sun. Wait to water again until the soil is no longer wet where the vegetable roots are — that’s an inch deep and deeper.
The short of it: Start with something like 45 minutes every three days, observe, and adjust.
This is for now, October. In winter, I’d start from 45 minutes every five days and be at the ready to stop the system from running if there’s rain. In spring, I’d start from 45 minutes every three days (just like fall). In summer, I’d start from 45 minutes every two days.
There. The thorns have been pruned off. Naw, just dulled a bit. But it’s a start.
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