Is Your Lawn Lucky Enough to Have Clover?

The lawns of Montclair are filled with clover! How do I know? Well, I saw a post about it on Secret Montclair (what more evidence do you need) and I've seen it with my own two eyes on my almost daily covid constitutional with my husband, John, and our dog Dobbie.

Here's Dobbie in disbelief about being taken on another walk. Note some clover in the background.

Spring 2020 was a boom for chipmunks and clover in our fair town. If you are anti-clover or just ambivalent about this lovely plant, I hope my words will change your perspective.

Many people consider clover to be a weed. There is no true scientific definition of what a weed is--it's truly subjective because a plant one person considers a weed may be coveted by another. But I think we can all agree that a weed is generally a plant that people do not want in their yard and for many, clover fits the bill. Americans love their lawns and there are many that believe that we should move away from them. I can't argue against getting rid of the lawn but I don't see that happening en masse anytime soon. However, the idea of what constitutes the ideal lawn has changed over the years. Clover used to be a staple in grass seed but the public's perception of clover changed. During WWII, an herbicide commonly known as 2,4-D (2,4-dichlorophenoxyacetic acid) was developed with the purpose of starving German and Japanese citizens by killing their potatoes and rice crops. It didn't work. So the herbicide was repurposed in American agriculture. In 1945, the American Chemical Paint Company bought and marketed 2,4-D as "Weedone," which was quite effective at killing dicotyledons (broadleaf plants) while leaving monocotyledons such as rice, corn and wheat untouched. The chemical companies then saw an opportunity to sell their herbicide that extended beyond agriculture: American homeowners and their love of the iconic American lawn. And there begins the downfall of clover in the lawn because 2,4-D killed clover but did not harm grass, a monocotyledon. Successful marketing had changed America's perspective of a plant that was once desired as part of a healthy lawn to an undesirable lawn plant--clover was now considered to be a weed and was removed from grass seed.

An illustration of Weedone with 2.4-D. Source

Clover, however, is making a comeback. So let's dive into why that might be. Because this is a science blog and I'm a science teacher, here comes the science. The type of clover most people have in their lawn is commonly known as white clover or Dutch clover and its scientific name is Trifolium repens. White clover is not native to our region but it has become naturalized and is commonly used as a foraging food and crop cover when mixed with grass seed. Clover is a member of the legume family, just like peas and peanuts. Many legumes are special because they have a symbiotic relationship with a species of bacteria called rhizobia. Rhizobia are special because they are nitrogen-fixing bacteria. If it's been a while since you've taken a biology class, let's do a quick recap of why nitrogen is an element that is essential for life. Nitrogen is present in all amino acids, which are the monomers, or building blocks, of proteins. Proteins are the workhorses of the cells and you would not be alive without proteins catalyzing our reactions (enzymes), protecting against pathogens (antibodies), protecting us from the damage of UV rays (melanin) or allowing our muscles to contract (actin and myosin). Nitrogen is also an element in nucleic acids (DNA and RNA) and ATP, which is the energy currency molecule of the cell. Nitrogen is the most abundant gas in the atmosphere but we cannot breathe in nitrogen and use it for all of our cells' needs. Nitrogen is diatomic and the two N molecules are held together by a triple bond--in other words, it's incredibly stable and we don't have any enzymes that can break it down. But do you know what does have the enzyme, called nitrogenase, that can convert atmospheric nitrogen into a usable form for all living things on this Earth? Rhizobacteria! There are other types of bacteria than can fix nitrogen but they don't have a special relationship with legumes, such as clover. Nitrogen is often a limiting nutrient in agriculture so growing a plant that can add nitrogen to the soil will promote plant growth. As a bio teacher, I knew that rhizobacteria live in nodules on legumes' roots but I did a deep dive into the symbiotic relationship between the bacteria and the plant and let's just say mind blown.

The green arrows are pointing to clover nodules. Source

Rhizobacteria live in the soil and are attracted to clover's roots by molecules called flavenoids, which are a type of phytochemical. The bacteria then attach themselves to root hairs in a complicated process that involves lectins and cellulose fibers from clover and bacteria fimbriae, which are protein hairlike-structures sticking out of the cells. The clover then senses chemicals released by the bacteria, which causes the root hairs to curl around the bacteria. The bacteria infect the root hairs which will eventually result in root cell division to create a nodule that surrounds the bacteria. Subsequently, the bacteria change dramatically by losing their cell walls and forming large branching cells called bacteroids. They are now entirely dependent on the clover for survival. The clover is quite good at making its own food through photosynthesis and the plant sends some sugar down to the nodules. The clover benefits because the rhizobacteria fix atmospheric nitrogen, which clover uses for many things including chlorophyll synthesis. More chlorophyll means more photosynthesis which means more food for the rhizobacteria which means more cell division which means more nitrogen for the clover--this is a classic mutualistic relationship. It's so intricate that the two organisms, so drastically different, work together to make a protein called leghemoglobin. Leghemoglobin is similar to hemoglobin, the molecule that carries and transports oxygen in our red blood cells. The leghemoglobin ensures that there is just enough oxygen for the bacteria to efficiently carry out nitrogen fixation--too much oxygen inhibits nitrogenase and too little inhibits cellular respiration. 

I like to anthropomorphize in biology, which I think is amusing and fun as long as one understands that molecules and cells have no wants or desires. And when I anthropomorphize the relationship between rhizobacteria and clover, I think something like this: "Hey bacteria, come over here, I'll build a nice little house for you. Sure clover, I'll send some biologically useful nitrogen your way. Thanks, man, here's some food, make yourself at home!"

A model depicting the mutualistic relationship between rhizobacteria and legumes. Source

Honestly, I could write even more about this mutualistic relationship between the bacteria and clover because it's even more bizarre than what I presented with "cheater" rhizobacteria showing up that don't fix nitrogen but take the carbohydrates from the plant and the plant responding by reducing the amount of food it sends into its nodules. The topic even made into an AP Biology Exam Free Response Question. Like Richard Feynman said, "I think nature's imagination is so much greater than man's, she's never going to let us relax."

Besides clover's amazing ability to fix nitrogen thanks to rhizobia, I think it's a pretty little plant. The leaves have a pleasing shape, the white flowers add some interest to an otherwise monochrome lawn and its sweet fragrance fills the late spring/early summer air. Clover has the tendency to stay green and it easily forms mats due to its stolons, or horizontal plant stems. As far as the advantages of a clover lawn, it requires less water, no herbicides, no fertilizer (hello nitrogen fixation), and importantly, it provides nectar for bees. Bees need our help these days so either plant some clover or leave the clover in your yard be for the dear bees. 

A delicate clover flower stands tall in my front lawn

About six decades ago, agricultural chemical companies were effective at changing the public's perception of clover. When my husband and I first bought our house in Montclair almost 17 years ago, I desired a clover-free lawn. But after learning about clover, my perception changed and I appreciate this remarkable plant and think lawns without it look sterile. I much prefer the public's perceptions on the plant be based on knowledge, not on an agricultural chemical company's marketing efforts. So consider giving clover a chance to thrive in your lawn and be an ally in clover's comeback!

There are many online sources to buy clover. See here for one.