On Pulling Weeds

“A weed is a plant whose virtues have not yet been discovered,” said Ralph Waldo Emerson. This is a profound observation about judgements and the role of words in conveying judgements.

Nowadays, with our quite extensive knowledge of plants and their benefits, we persist in calling quite useful plants weeds just because we don’t want to see them on our lawns.

Well, some of us don’t want them on our lawns, but I don’t mind in many cases. For me a homogeneous lawn is unnatural, and I am quite happy with the mint, clover, and other unidentified species on our lawn.

Which brings us to dandelions. They are valuable plants; the roots can be roasted to make ersatz coffee, the flowers can be used to make dandelion wine, and the leaves can be put in salads (much like chicory) and are loaded with vitamins and minerals. Out of respect for my neighbours (yes, I have had some unhappy episodes with past neighbours), however, I feel compelled to get rid of ours.

However, I would never use herbicides on my lawn, as they poison our children and pets. I had an interesting conversation with my neighbour, Pat, about this one day. I was out pulling weeds with a mechanical contraption (that you can buy at your favourite hardware store) and he told me that he used herbicides on his lawn only once. The runoff killed many of the koi in his pond, and he never used herbicides on his lawn again. If they killed his fish, he went on, imagine what they do to us.

Besides the free physical exercise that it provides, pulling weeds does give one’s mind the opportunity to fruitfully wander. It’s been an historically wet May in these parts, and pulling weeds in moist earth is fairly easy, but nevertheless one does noticeably better if one pulls slowly and steadily. In this way, one has a better chance of removing the entire root; if you break the root (which is likely if you pull sharply), then there is a much better chance that the plant will grow back, and sooner.

So that’s the key point: a gradual increase in the strength of the pull is more likely to pull the entire root out, but a jerk is more likely to break the root. Why?

The same phenomenon manifests when one pulls a plastic produce bag from a roll in a grocery store. A sharp snap breaks off the first bag, but a gradually increasing pull just unrolls the roll. The same occurs with a toilet paper roll, which is especially maddening with those cheap one-ply rolls that you invariably find in public toilets.

The standard textbook example of this phenomenon is a block suspended by a string, with a second string hanging from the bottom of the block. If the bottom string is given a sharp pull, it breaks. If the bottom string is given a slow, gradually increasing pull, the upper string breaks. Check this link for a demonstration and explanation.

It takes time for a force to be transmitted. It’s much like a train engine connected to a long line of train cars. As the engine begins to move, you can hear the series of clunks as each car begins to move in turn. It takes time for the motion of the engine to be transmitted to each car in turn.

Although it’s not the same phenomenon, because it’s not forces being transmitted, you can see a similar sequence of time delays when a line of automobiles begins to move when a traffic light turns green. In this case it is driver reaction time that causes the time delays, but the effect (staggered starts for the cars) is similar to the staggered starts for the train cars.

In the case of the string, or the dandelion root, one can think of each as a line of atoms, all connected by “chemical bonds” (electrostatic forces, really) which are analogous to the linkages that connect train cars. Pulling on one end of a string (or pulling on the dandelion root) is just like the train engine starting to move.

(This post first appeared at my other (now deleted) blog, and was transferred to this blog on 22 January 2021.)