I was watching TV and saw a comercial for something like Scotts Turff Builder. I got to thinking: What the hell kind of chemical lets grass grow but kills the weeds?
Is it something in the weeds? Did the grass anticipate man wanting a perfect gree lawn and developed some kind of natural defense to weed killer? More importantly, is it really that safe to be released into the ground soil, especially if millions of American men want perfect lawns (a la Hank Hill)?
Well, Turf-Builder® uses the chemical 2-4D (a close relative of Agent Orange, Agent Purple, and the other color I never remember). Basically, it is a growth hormone that acts on the variety of plants that include most broadleaf weeds but not grasses. The growth hormone causes the leaves to require more nutrients than the roots can provide and the plant starves itself to death.
Clover, chickweed, and similar plants are (like grass) not affected by 2-4D. They are generally attacked with 2-4-5T (or some related compound) that, I believe, is a metal, as is arsenic. I do not know whether the lethal agent works the same way for them, or not.
Grass used to be killed by arsenic (which also is lethal to trees and people), but I do not know if that agent is still used. Mature crabgrass is not susceptible to most weed-killers and I think arsenic is still used. However, crabgrass is an annual plant and its seed may be killed before it germinates. (Unfortunately, it germinates as soon as there have been three consecutive days reaching 80° F so there is no point in buying/spreading Crabgrass Pre-emergent if you have already had a hot spell.)
IIRC most weeds are dicots while grasses are generally monocots.
http://homechannel.iafrica.com/sagardening/yourgarden/10453.htm
http://www.enforcer.com/Oweed.html
http://www.monsanto.com/monsanto/about_us/product_overviews/roundup_links.html
"What is Roundup?
Monsanto introduced Roundup herbicide in several world markets in 1974. Glyphosate, its active ingredient, is a white, odorless solid that dissolves in water. Glyphosate is the common name for N-(phosphonomethyl)-glycine, and it kills the entire plant from the leaves to the roots. Care must be taken to protect the leaves of desirable plants from the herbicide.
How does it work?
When Roundup or another of Monsanto’s glyphosate herbicides is sprayed on plant foliage, it is absorbed and then moved —or translocated — throughout the plant’s tissues. Once inside the plant, glyphosate inhibits the production of an enzyme, called EPSP synthase, which in turn prevents the plant from manufacturing certain aromatic amino acids essential for plant growth and life. "
etc etc
http://www.britannica.com/eb/article?eu=40970
http://www.ag.uiuc.edu/~vista/abstracts/aHERBWORK.html
http://www.ag.uiuc.edu/~vista/pdf_pubs/HERBWORK.PDF
This .pdf file has pretty everything you need re herbicide categories and active killing mechanism(s)
You recall correctly (actually all grasses are monocots). Rather, most weeds that people think of when the word “weeds” pops up are dicots. Namely clower, chickweed, dandelions and the like. There’s all sorts of weedy grasses (crabgrass and its friends) out there that require a specialized weed control that’ll affect them and not turf grasses (bluegrass, bermuda…etc). It’s fairly easy to make a dicot only weed killer, a bit harder to make a monocot killer that won’t affect turf which is why they’re typically not near as effective.
For that matter, just about any weed killer will kill either type of plant if applied in sufficent quantites.
No doubt, the evil masterminds at Monsanto are hard at work genetically building grasses that are resistant to Monsanto herbicides so they can continue to control the market. They’ve already come up with Roundup resistant soybeans so you can just spray your fields at will and I can’t imagine turf is that far behind.
Well, there’s a pretty good bit of information here.
There also seems to be a general assumtion that everyone knows what the words dicots and monocots mean.
Just in case:
‘Cot’ refers to cotyledon, which is the first leaf (or two in dicots) produced by the embryo of flowering plants.
There are many other differences between monocots and dicots, such as parallel leaf veins (monocots) vs. nonparallel leaf veins (dicots), and flower configurations: dicots usually have flowers with sepals, petals, and anthers, in multiples of 5, and monocots usually have flowers with parts divided into 3s or multiples thereof. The seeds are different too. Imagine a kernel of corn (monocot) with it’s singular energy store, and a peanut (dicot) which is divided into two halves.
The 2 types have different reactions to certain chemicals.
That’s why it’s not uncommon to see a broadleaf killer combined with a lawn (monocot) food.
It may boil down to different levels of tolerance. You can kill broadleaf weeds without hurting your lawn, but I don’t know if the reverse is true. The only ‘grass’ killer I’ve ever seen is for spot application. That may be because (like Roundup™) it will kill pretty much everything.
This is about all I can retain from my highschool biology…it was a long time ago.
It has been many years since I took the Scott’s Lawn Pro certification courses, but I do recall some specific info from my years as a landscaper, lawn mower, and pesticide applicator.
Scott’s Turf Builder contains no pesticides of any sort. However, Scott’s Turf Builder Plus 2 contains herbicides, and Turf Builder Plus 4 contains herbicides and pesticides. I seen to recall they used to sell a Plus 6 (added fungicides) but I never saw it because fungus isn’t a problem around here.
IIRC, the active ingredient in Plus 2 is arsenic. The way it works, as Scott’s told me, is that broadleaf weeds catch more of the granules due to the larger surface area compared to grasses, where the particles tend to fall off. Plus 2 is best applied during a stretch with no rain, for two reasons. The granules must not get washed off within 48hours or the herbicide is useless. The pesticide acts by drying up the plant, it interferes with water circulation within the plant tissues, so it is most deadly in hot dry weather.
If you look really close at your yard after applying Plus 2, the grass is also adversely affected by the chemicals, but it recovers after a few days. The weeds do not.
It’s on its way: http://www.turfnet.com/news/scotts_alliances.asp