I do not understand why "monte" can be "grass". As you can see in http://www.thefreedictionary.com/grass, this translation does not exist.
Actually, monte does mean grass, it's just one of many of the definitions. It is especially used to refer to general scrubby grass/plant stuff that cows or other grazing animals eat. I was studying abroad and my host family used this word-- they told me it meant grass/what grazing animals eat, and as a joke some people refer to salad as 'monte.' I think it generally refers to the grassy type plants in a grassland.
"Monte" is a hard word to translate into English. One of its definitions means "mountain" or "hill", one means "countryside" (like "campo"), and the last can be "woods", "forest", or "grassland". It would, then, mean "grass" in the context of a field and not the individual blades of grass.
This reference will help. http://www.spanishdict.com/translate/el%20monte
By they way, most grasslands exist in relatively dry areas. Some are desert, or near-desert, lands. Prime examples are the American west, beginning in about central Kansas, Nebraska and westward. These sorts of lands are also often called scrub-lands, or scrub.
In addition, there are many kinds of grasses. At least 70 species of tall-grass prairie grasses alone. Then their are short-grass prairies. And then grasslands that are not prairie grasses (such as the Everglades). ant then grasslands in other continents, (Africa, South America, etc.)
Here you have the translation : http://www.wordreference.com/es/en/translation.asp?spen=monte It is used talking about marihuana.
I think part of the problem with this word may be differences in vegetation. In Spain and parts of Mexico I have seen a lot of "general scrubby grass/plant stuff ". In more northerly areas this type of vegetation does not exist to a great extent. Natural vegetation tends to woodlands, farms have green fields and orderly crops. Weeds, now they are universal!
Still, seems very strange, but as others have said, it is one of the definitions. Seems strange because a hill can be covered by vegetation (any kind), or not. A search in the RAE (www.rae.es) brings many definitions, being "grán elevación natural del terreno" the primary meaning and "tierra inculta cubierta de árboles, arbustos, matas o hierba" the second meaning; there are more meanings. Of course the author can use the word this way, but I think it is not intuitive.