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The Garden Plot for July 27, 2007
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Watering Vegetable Gardens”
How much water does your garden need? On heavy soils, too much water can waterlog soil and suffocate roots. Excessive irrigation can leach fertilizer, especially nitrogen, below plant roots, and possibly into our
groundwater. On the other hand, we all know what happens if we don’t water enough. So, how do we know when and how much water to apply? The following guidelines should help.
First, know your soil type. Clay loam soils hold approximately 2” of water per foot. Clay soils hold more water, while loam soil holds less. Sandy soils hold the least amount of water. So, if you put on an inch of water, it will wet a sandy soil to a rooting depth of 24 inches, a loam soil to 16 inches, and a clay soil to 11 inches.
Second, figure out how much water your plants need based on rooting depth and growth stage. Seedlings and new transplants need frequent, but short duration irrigations. When vegetable crops reach their mature size, they need longer duration irrigation based on their rooting depth. Deeper roots need longer irrigation times. Shallow rooted vegetable crops root to a depth of about 10 inches and include broccoli, cabbage, corn, lettuce, onion, and potato. Moderately deeply rooted vegetables can stretch their roots to 18 inches. They include bean, beet, carrot, cucumber, eggplant, pea, pepper, and summer squash. Deeply rooted vegetable crops can push roots to a depth of more than 20”. Deep rooters include pumpkin, winter squash, tomato, and asparagus.
Third, calculate the output of your irrigation system. What amount of water do your sprinklers put out per minute, or per hour? Put buckets out in various spots and irrigate as usual. Then, measure the inches of water collected.
Fourth, adjust irrigation frequency and amount to our natural precipitation. For example, April through early June, we normally expect a good amount of rainfall. So, we can irrigate less in the spring than in July and August. Precipitation can be measured with a rain gauge, or by checking the national weather service at noaa.gov.
Finally, adjust irrigation frequency to our average evapotranspiration rate each month.. Evapotranspiration is the amount of water plant leaves lose to the atmosphere on a hot, dry day. It would be a little like calculating how much we sweat. Contact your local Extension agent for this information or try usbr.gov
The amount of water a garden needs as irrigation equals the rate of natural precipitation subtracted from the rate of evapotranspiration. Our natural precipitation for the whole month of July is, on average, only 0.9 inches. The average evapotranspiration for Missoula in July is 5.2 inches. So, 5.2 inches minus 0.9 inches equals 4.3 inches of irrigation water needed for the month of July. That’s about one inch of water per week in July, needed just to match the water lost by plants to the atmosphere.
So, to simplify this a bit, here’s an example: My irrigation system puts out 25 gallons of water per minute. It takes 40 minutes to put one inch of water on 1000 square feet of garden. In my loam soil one inch of water should infiltrate to a depth of 16” in forty minutes. For a deep rooting crop, such as tomato, that would not be enough when tomatoes are mature and starting to produce fruit. Remember that, in July, we need to add one inch of water per week just to cover evapotranspiration. I would need almost twice that much irrigation time per week for deep rooted tomatoes. The best way to deliver the water would be 3 times per week for 25 - 30 minutes each time. If I had a sandy soil, I would still water 3 times per week, but I would shorten each irrigation time to 20 minutes.
Serious market gardeners and home gardeners can more carefully calculate the irrigation requirement for each vegetable crop using calculated crop coefficients. For more information, contact your local County Extension or Natural Resource Conservation Service.
Helen Atthowe's new short program of gardening tips
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