A weed is sometimes defined as a plant we haven't learned to use yet. Sharon Astyk redefines weeds by seeing them as indicators of changes to the land, and eating them:

http://scienceblogs.com/casaubonsbook/2010/05/getting_intimate_with_my_weeds.php

This theory begs a practical test. I can send you all the "Canada" thistle (Cirsium arvense) you want. I recommend you plant it in every corner of your property. Enjoy!

Some weeds are extremely invasive and impossible to kill without herbicides. They will choke out crops and gardens quite happily. I really don't like using toxic goop, but this is war.

The thistle colony in my potato patch thinks of 2,4,D as "gin and tonic" and glyphosate (Roundup) as "a slice of lime in a nice refreshing Corona."

http://www.agriculture.gov.sk.ca/Canada_Thistle_Control

http://en.wikipedia.org/wiki/Cirsium_arvense

One thing about seeing fields of known invasive weeds that amuses me is that the farmers or landowners complain about them taking over their property, yet they have steadfastly refused to do anything to control them before they got really bad. They've let them get so bad their acreage looks like the weed is their main crop.

Many weeds can be controlled by mowing or chopping before they form seed heads. Many perennial weeds like this thistle can be worn down if mowed repeatedly, removing the source of energy (leaf growth + photosynthesis), and starving the roots into oblivion.

I guess wishful thinking just doesn't cut it.

Sue

One word - Kudzu

It eats houses. You can hear it growing. It likes waterways, dry land, wet land, roads, houses, power poles, power lines, fences, and less than mobile people.

You can't kill it, you can barely control it, it won't go away. Only cold stops it.

I just spent quite a bit of money getting rid of a crop of weeds in my backyard. I made the mistake of not putting down mulch before the rainy season. I will never make that mistake again. Those weeds took over my backyard and killed some plants I paid good money for. There is nothing positive about the weeds around me.

Kudzu

For northerners and westerners unfamiliar with kudzu the slide show on this page will give you some idea of what it is and how it blankets and smothers everything in its path. How it can be a bit scary how it often seems to be a monstrous green invader:

http://www.fsu.edu/~imsp/silent_invaders/new_weeds/guide/plants/kudzu/kudzuindex.html

The rest of the information pages have good information, you might want to come back to it, the slides are from Florida and give you an idea of conditions here, but what you need to know is:

It is a problem. Especially in the south where it doesn't freeze. It's early summer growth rate threatens to chase down and engulf young children and domestic animals. One fellow I know used to walk out to his car in the morning swinging a machete. He claimed it was light exercise and was good for his nerves. He would store this tool on the floorboard of the car and hack his way back to the house after work. Even with twice daily dedication he never succeeded in doing more than keeping one side of his house clear of it.

But, on the other hand, both humans and animals can eat kudzu. Cattle, sheep and goats love it. The Japanese use kudzu in herbal medicine. Look around. You simply don't see overgrowth of kudzu in cattle grazing areas.

Reference:
http://grandpacliff.com/InvSp/Kudzu.htm

Given the huge quantities of it available it would seem to be a potential emergency resource. I like the idea of feeding the hungry masses with resources you have on hand.

Unfortunately much of the kudzu is near roadsides where chemical fallout from vehicles and the activities of DOT and others who have attempted to control the plant by chemical means makes consumption unpalatable, if not unwise.