This is the VOA Special English Agriculture Report.

Some trees can survive a long time without water. Think of trees that grow in the desert. But other trees may need more moisture than they can get from rainfall or from the air -- from, say, coastal or mountain fog.

The soil stores moisture during cooler weather. But most of that moisture is gone by the middle or end of the hottest months of the year.

Trees and other plants can look thirsty. Leaves can become droopy and hang downward. They can also turn yellow. Yellowing can be a sign of too much water. But it can also be a warning sign of too little water.

The Vacaville Tree Foundation is a volunteer community group in California. It has advice for watering newly planted trees and established trees.

With a newly planted tree, the roots have not yet spread out from the root ball. The root ball can become dry faster than the dirt around it. So put water on the area of the root ball and the surrounding soil until the roots become established.

Once a tree is well established, water deeply instead of often. The amount of water needed depends on the tree and the soil. Clay soils hold water for longer periods while sandy soils hold less water. During the hottest season, a deep watering may satisfy a tree for anywhere from ten days to four weeks.

Part of skilled watering is knowing to stop when the soil needs a little time to absorb the water. Otherwise the water runs off the surface and gets wasted.

Ted Swiecki is a plant scientist in California with the Phytosphere Research company and the Vacaville Tree Foundation. He says people should not water established trees at the base of the trunk. This can harm the tree.

Too much water in the soil at the base of a tree can lead to the growth of fungi. If the area is too wet, harmful organisms have a better chance to invade the tree and cause disease.

Mr. Swiecki says this is true especially in Mediterranean and semi-dry climates. Many trees in these climates have adapted to having a dry area near their base during the hottest season.

TED SWIECKI: "Water displaces air in the soil. And roots are aerobic; they require oxygen (for the soil) to function properly. So if you keep the soil saturated and there's no air there, and then you are basically starving the roots for oxygen."

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. For more growing advice, and to learn English, go to voaspecialenglish.com. You can also find us on Facebook, Twitter and YouTube at VOA Learning English. I'm Bob Doughty.

aerobic: living or occurring only in the presence of oxygen 需氧的

saturated: soaked with moisture; drenched 渗透的

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（来源：VOA 编辑：崔旭燕）