Weed: definition: A plant where you don't want it.
Dandelion, prickly lettuce, crab grass, plantain, dock, and thistles: many people would be happy not to see these plants ever again, yet every spring they come up smiling in their gardens and yards. Every year they are sprayed, sworn at, pulled, tilled, and yet despite being more than officially un-invited, they keep coming back.
That is because the weed's motto is: If there is space, I will be there.
Do not give them space.
Research at the University of Purdue in the ways of weed science has found new ways of working with weeds. There have webinars on the topics located at this url: http://www.ydae.purdue.edu/oarei/webinars.html#soil. The new method of viewing weeds is as a component of an ecosystem rather than some obnoxious anomaly. Instead of repeatedly tearing out the weeds, new research indicates you will have better luck if you change the environment so they will not want to grow.
Not from Temperate forest to Amazonian jungle change; instead think what the weeds like, and do not make that available to them. Again, if there is space, they will grow. Is your lawn full and healthy? You probably have fewer weeds. If you do not fertilize according to the lawn needs, and you mow every weekend to 2,” then you are probably mowing weeds.
Rangeland sciences teaches us that frequent repeated grazing causes roots to be shallow and few, whereas longer rests lead to longer, more luxurious roots: allowing more water penetration when it rains (versus ponding or flowing to the river), more summer grass growth, and less space for your common lawn weeds.
If you zoom in on that dandelion nestled amongst your fescue or perennial rye, and happen to take a shovel by its side, digging to see where its root goes, you will notice that the root is totally different than lawn roots.
There are approximately three general categories of roots, the fibrous ones, the tap roots, and the nitrogen fixers. Plants can be a combination thereof, such as the soybean which has both a tap root and is a nitrogen fixer, or just of one type, like the fibrous rooted wheat or corn.
In your small hole in the lawn you'll probably notice most of the grass roots (fibrous rooters) stop about 3” down. However, that dandelion root pokes on. He is able to survive by scaling past those lawn roots and lapping up all the nutrients that escaped their reach. If we integrate rangeland science with the weed science of Purdue, what we find is that less mowing might just mean less dandelions. Letting the tops of the grass grow taller means their bottoms grow deeper, making it harder for tap-roots to establish and adding competition to those that are already there.
So re-consider your 2” setting, perhaps bump it up an inch. Let the lawn mower rest a few days and see what happens. There is more to weeds than just root depth, so do not expect a miracle. More will come in the next episode of Farm Science in Your Backyard. At the NRCS we consider this part of integrated pest management, a component of Conservation planning. If you are a farmer who likes this sort of thing, please feel free to call: 937-492-6520 and ask for Melissa. You might be interested in conservation planning or one of the other services our agency offers. The USDA-NRCS is an equal opportunity provider and employer.