OK, so how does this stuff work? I put it on my lawn, and it kills weeds while feeding grass. How does it kill some plants and nourish others, with no effort on my part? Pretty cool, if you ask me–that’s some pretty smart stuff that can identify weeds (as opposed to grass) and kill it.
The “weed” portion is (or used to be) 2,4-D, a (not very distant) cousin to Agent Orange. It’s special property is to kill broadleaf weeds.
The most common broadleaf weeds that bother lawn enthusiasts are dandelions, hawkweed, and English Plantain. 2,4-D works on them quite well.
(Actually, the “feed” helps kill the broadleaf weeds, as well. 2,4-D is actually a growth hormone that causes the leaves to outstrip their root system. Since the “feed” is high nitrogen (which feeds the leaves) it adds a little push to the process.)
It won’t kill grasses (which is why your crabgrass thrives along with your merion blue) and it won’t kill smallleaf weeds such as chickweed (and clover?). Basically, it has been tailored to affect specific varieties of plants. There are also general poisons that will kill anything that is growing in the soil, but you would not usually put those on your lawn–particularly if you’ve paid for fertilizer.
For small-leaf weeds, you need a distant cousin called 2,4,5-T or some approximate variant.
For crabgrass, you used to need arsenic, although they may have some new stuff out. Since most grass killers will kill grass (duh!), your best defense against crabgrass is to use a seed killer (called a pre-emergent) prior to its germination in the Spring. (Three days of 80° weather and crabgrass sprouts.) Why does not the pre-emergent hurt the lawn? Because crabgrass is an annual that must re-seed itself each year, while bluegrasses, fescues, etc. are perennials that simply need to warm up to begin growing again.
Meanwhile, the “feed” portion helps all the stuff to grow.
Thank you very much for that informative and clear explanation. This thread can be closed as far as I’m concerned. Thanks again.
Your weed ‘n’ feed probably has some smaller powdery looking pellets that stick to the foilage. The nitrogen pelletes are the bigger ones that find their way to the soil. I would bet the instructions are to apply it to a damp lawn so that the smallish granuals with the selective herbicide will stick better.
BTW, many are of the opinion that these high nitrogen weed ‘n’ feed products are more bad than good.
You are better off using organic fertilizers to build a strong lawn that crowds out weeds.
Weed n Feed products are short term solutuions to long term problems. In addition, they contribute to all sorts of soil problems that will have you back buying more and more products…effectively, you’ll be chasing your tail while your tail grows shorter.
Good Luck. You’ll need it. I’m sure you’ll be buying disease controls, insect controls, extra seed, fighting thatch, curious brown spots…etc…etc.
Oh God, no. I’m not a person who will put that much time, effort, and money into a lawn. It stays relatively green and healthy without doing anything. I just wanted to kill a few weeds, and I did it. (My dad, on the other hand, does put enormous amounts of money into his lawn. It is greener than any golf course, and quite nice. It isn’t as big a deal to me, though. If I want to see green, I’ll just go to his house. 
)
Ummm, Agent Orange was a blend of 2,4,5-T and 2,4-D. And 2,4,5-T was responsible for the dioxins, which were the nasties in Agent Orange.