Special English 又叫“慢速英语”，是VOA 专为全世界非英语国家初学英语的听众安排的一种简易、规范的英语广播节目。该节目创始于50年代末期，是VOA 的专家们研究如何与世界各地的英语学习者进行交际的产物。它正式开播于1959年10月。当时只面向欧洲和中东，但由于这个节目适合许多国家英语学习者的需要，所以它的广播对象不久就扩大到世界其他地区，并很快在全世界范围内产生了广泛的影响。现在这个节目对欧洲、非洲和拉丁美洲每晚广播一次，对加勒比地区每晚广播一次（星期天除外），对东南亚广播次数最多，每天上午两次，晚上三次。
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2009.05.26
The Seeds of Weed Control
This is the VOA Special English Agriculture Report.
When is a plant considered a weed? Experts at Penn State University have a simple answer: When its undesirable qualities outweigh its good qualities.
Consider this: Crops generally produce several hundred seeds per plant. But each weed plant can produce tens or even hundreds of thousands of seeds. And some buried seeds can survive up to forty years, or even longer.
Horseweed has become
resistant to chemical
weed killers Eradicating weeds means that you have to remove all the seeds and roots so the plants will not grow back. But birds or the wind can reintroduce them to the land.
A more common way to deal with weeds is to control them enough so that the land can be used for planting. Experts advise using two or more control methods to deal with weeds.
Chemical weed killers or natural treatments like corn gluten can suppress weed growth. Dense planting of a crop can also act as a natural control.
Bill Curran is a professor of weed science at Penn State, in University Park, Pennsylvania. He says one of the most common methods for suppressing weeds is dense planting.
He says a dense, competitive crop that quickly shades the soil will help suppress many weeds. The seeds need light to grow, so blocking the sun will r
educe weed growth.
Other controls include turning over the soil, pulling the weeds by hand or covering them with mulch made from wood, garden waste or other material. Mulch is widely used, but even mulch has its limits.
Natural resource specialists in the Queensland government in Australia note that weeds can be transported in mulch. This is also true of soil, grain, hay and animals.
Yet animals like sheep or goats can provide a biological control —— they eat weeds. Insects and other organisms can also act as biological controls.
Preventing the spread of weeds is an important part of weed management. Farm vehicles should be kept out of areas with weeds. If that is not possible, then clean off the equipment and your shoes when leaving.
People in Queensland are advised to take weeds and garden waste to a waste center or burn them, bury them deeply or make them into mulch.
Professor Curran says composting weeds is another way to make use of them. The process of making organically rich compost produces heat. The heat will kill many, though not all, weed seeds. The same is true of seeds that pass through farm animals that graze on weeds.
And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. Archives are at www.voaspecialenglish.com. I'm Steve Ember.
