News for those who live, work and play in the Santiam Canyon

Steps determine how much water is right amount for garden

Linn County Master Gardener

OSU Linn County Master Gardener.

July rain is welcome for many reasons…reducing fire danger, reducing temperatures, keeping the grass green and plants healthy. The recent rain was really not enough to make a difference though. How much water will make a difference? 

For the big picture, the US West is in a long-term drought so fire danger is still high, but we can use the water resources we have to sustain green growth on our personal properties. Sprinklers and soakers come in a wide variety of designs that can be adapted to the spaces we want to water.

Oregon State University does a lot of research on irrigation, including dry farming (or lawns) without irrigation. Publications from OSU Extension provide summaries of the research for us to use at home. (catalog.extension.oregonstate.edu). 

One useful source of irrigation information is EC9311 Irrigation Rates and Frequencies for Western and Eastern Oregon Turfgrass. It explains how plant/grass species selection, soil characteristics, temperature, wind affect how long and how often to water a lawn or garden.

Evapotranspiration is the word that describes soil water lost to the atmosphere from surface evaporation and also leaf transpiration (sweating) of plants. Applying water in the morning, at dawn, is best to provide water to plants that can be taken up and transpired during the heat of day, except roses that are susceptible to black spot…they should be watered between 10 a.m. and 2 p.m. to keep the leaves dry and starve the thirsty little fungus! 

Studies updated in 2020 (https://www.usbr.gov/pn/agrimet/), show that average monthly evapotranspiration rates for Corvallis to be 6.9 inches of water in June, 8.6 inches in July and 7.7 inches in August. That means we have to replace that much water to the soil to maintain a healthy green environment. The little bit of recent rain wasn’t nearly that much.

Whatever way water is applied, it can be measured by placing a straight-sided container where the water falls and keeping track of the time it is on. The inches of water indicates how much is applied in that time with that particular sprinkler at that place. For an average, add up the inches and divide by the number of containers. Containers placed at different places near and far from the water sources, in different “zones”, on different hoses and emitters, or wherever the water application changes will show that the water is reaching the plants at different rates. Some plants will need more water than others so a one-sprinkler-serves-all approach may not be effective.

Once we determine how much water is applied to the yard or garden at one time, we can use the evapotranspiration rate data to figure how often to water. If we need to replace 8.6 or 9 inches in July, and the sprinkler at that spot applies 1 inch in 1 hour, we need to sprinkle or drip there a total of 9 times during the month, or twice a week. If the rate of irrigation is less, it should be run more often. If an area is only watered once a week, it should be run longer.

Soil type makes a difference on how deep the water will penetrate and roots will develop. Longer, stronger roots that reach deeper into the soil will result in healthier plants that can better adapt to occasional dry conditions. Sandy soil sucks up the water to greater depths but does not spread the water laterally. Clay spreads shallow water that does not penetrate into the root zone. Loose loamy soil with organic matter will absorb and hold moisture more effectively in the root zone. More water will have to be applied to sandy soil; clay soil will have to be watered for shorter periods of time more often. Besides average evapotranspiration rates we need to observe how plants and lawn respond to the water we give them where they live. 

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