his is the VOA Special English Agriculture Report.
这里是美国之音慢速英语农业报道。

When is a plant considered a weed? Experts at Penn State University have a simple answer: When the undesirable qualities outweigh the good qualities.
什么时候植物会被当成杂草？宾夕法尼亚州立大学的专家们给出了一个简单的答案：当其不良品质超过优良品质时。

Consider that crops generally produce several hundred seeds from each plant. By comparison, 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.
考虑到每株农作物通常产生几百颗种子。而相比之下，每株杂草能产生数万甚至数十万颗种子。而且，一些被埋在地里的种子可以存活40年乃至更久。

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.
要根除杂草意味着你必须除掉所有的种子和根系，这样杂草才不会再次长出来。但是，鸟类或风会将它们重新带到地里。

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.
对杂草更为常见的处理方式是适度控制，这样土地就能用于种植。专家们建议使用两种或两种以上的方法控制杂草。

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. 
化学除草剂或类似玉米麸等自然手段可以抑制杂草的生长。作物密集种植也能作为一种自然控制手段。

Bill Curran at Penn State in University Park, Pennsylvania, is a professor of weed science. He says one of the most common methods for suppressing weeds is dense planting.
宾夕法尼亚州立大学大学城分校的比尔·柯伦（Bill Curran）是一位杂草学教授。他说，密植是抑制杂草最常用的方法之一。

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 reduce weed growth.
他说，经过密植的具有竞争力的作物会很快遮蔽土地，这将有助于抑制杂草生长。杂草生长需要阳光，所以遮蔽阳光能够减少杂草生长。

Other controls include turning over the soil, pulling the weeds or covering them with mulch made of wood, garden waste or other material.
其他手段还包括翻土，拔除杂草或用地膜覆盖它们。地膜一般由木屑、花园废弃物或其他材料制成。
 
But even mulch has its limits. Natural resource specialists point out that weeds can be transported in mulch. This is also true of soil, grain, hay and animals. 
但是，地膜也有其局限性。自然资源专家指出，杂草可以在地膜里传播，还可以通过土壤，谷物，干草和动物传播。

Yet animals like sheep or goats eat weeds, so they can provide a biological control. Insects and other organisms can also act as biological controls.
然而，像绵羊或山羊之类的动物会吃杂草，因此它们可以起到生物控制作用。昆虫和其他生物也可以起到生物控制作用。

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.
控制杂草传播是杂草管理中的一个重要环节。农用机械应该远离杂草，如果无法做到，在离开杂草区域时清理干净农用机械和你的鞋子。

Some people burn weeds or bury them deeply or make them into mulch.
有些人会将杂草烧掉或深埋，或将它们变为地膜。

Professor Curran says another way to make use of weeds is to compost 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 animals that graze on weeds. 
柯伦教授说，另一种方式是将杂草制成堆肥。制造有机堆肥改善土质的过程会产生热量，这些热量会杀死绝大部分杂草种子。将杂草喂食动物通过动物消化的也能达到同等效果。