This is a ‘green and pleasant land’, due at least in part to our national obsession with lawns 🙂 Now that the cooler, damper weather is upon us, it’s time to stop bemoaning the brown and crispy look of our lawns (did that happen this year?) and look towards making the grass greener and glossier for next year. There’s a whole range of grass seed and feed products to help you cover bare patches, together with specialised lawn care plant food and compost to ensure lush growth.
We lavish so much care on our lawns that it’s easy to think of grass as a weak and needy plant, but in fact it’s quite the opposite. Hiding amongst those green leaves is one of the best defences in the plant kingdom, which has led to notable changes in herbivores as they evolve to cope. In effect, every time a herbivore sets out to have a little nibble of a grass, it runs the risk of being glassed.
Glassing is a physical attack using a glass as a weapon. (Wikipedia)
Grasses (plants in the Poaceae family of plants, also known as the Gramineae) suck up soluble silica from the soil, do some wonderful (and not entirely understood) chemistry on it and lay it down in their tissues as silicon dioxide, in structures known as phytoliths. Silica can make up 2-6% of the dry weight of grass leaves, which is 10-20 times higher than you’d find in dicotyledonous plants (grasses are monocotyledonous, meaning that they emerge with single seed leaves rather than a pair – very obvious when you’re doing the weeding!). And silica is the main ingredient in glass.
The shape of the phytoliths is different for different grass species, and since they are inorganic (in the chemistry sense) they don’t rot down with the rest of the plant tissues, they’re left behind and endure for quite some time. This means that they’re very useful in archaeology, where they can be used to help identify plants at dig sites. They can be found in prehistoric tooth plaque (ew!), grinding wheels, kitchen equipment and even in the remains of ancient gardens. Phytoliths are abundant in the fossil record, shedding light on the diets of extinct animals.
That same fossil record shows us that there’s a connection between teeth evolution and a grass-based diet. Whereas mammals like us have teeth that stay the same size once they emerge, herbivores with a steady diet of grass have evolved growing teeth, because the phytoliths – all that silica – makes grass abrasive, and wears down their teeth. There’s a reason why the slugs migrate off your lawn at night to attack your veggies, they’re much easier to eat! And recent research suggests that those pesky phytoliths make digesting grass harder too, making a grass diet less nutritious. Silica itself is indigestible, passing through animal guts and being returned to the soil in their poo; there’s a silica cycle.
This is important information, when you realise that grasslands are amongst the largest ecosystems in the world, contributing to the livelihood of over 800 million people by providing food and fodder, energy crops, wildlife value, carbon and water storage. They cover 20-40% of Earth’s surface, depending on how you define ‘grassland’. Whilst our lawns are grown for their amenity value, even they act as carbon sinks and help to prevent local flooding (when compared to hard landscaping). And grass clippings are the main fuel for plenty of compost heaps up and down the country….
Americans are even more lawn crazy than we are. In 2005 there were 40 million acres of turf in the US, 1.9% of the land, making lawn grass America’s largest ‘crop’. NASA measured it from space.
The Poaceae is the fifth largest plant family in terms of number of species, and the most economically important. Lawn grass aside, 7 out of 10 of the world’s most important food crops are silicon accumulators. Crop grasses include wheat, barley, rye, sorghum, rice, millet, oats, sugarcane and bamboo. And one you may have in your garden – maize (Zea mays, sweetcorn).
That’s OK, because we don’t eat sweetcorn leaves, so your teeth are safe! But if you take away one thing from this then it should be respect for grasses – if you run your hands idly down their stems you could end up with a sharp cut from their rows of tiny silica ‘teeth’. You’ve probably already felt the bite of another (non-grass) silica accumulator; nettles use it to make the nasty needles they use to inject their formic acid stings.
Anderson Lab (n.d.). Silica as a plant defense.
Diep, F. (2011). Lawns vs. crops in the continental U.S. ScienceLine.
Hunt, J. W., Dean, A. P., Webster, R. E., Johnson, G. N., & Ennos, A. R. (2008). A novel mechanism by which silica defends grasses against herbivory. Annals of botany, 102(4), 653-656.
Massey, F. P., Ennos, A. R., & Hartley, S. E. (2006). Silica in grasses as a defence against insect herbivores: contrasting effects on folivores and a phloem feeder. Journal of Animal Ecology, 75(2), 595-603.
Milesi, C., Elvidge, C. D., Dietz, J. B., Tuttle, B. T., Nemani, R. R., & Running, S. W. (2005). A strategy for mapping and modeling the ecological effects of US lawns. J. Turfgrass Manage, 1, 83-97.
Wile , R. (2015). The American Lawn Is Now The Largest Single ‘Crop’ In The U.S. Huffington Post.
Hartley S.E. (n.d.) Grasses bite back: silicon-based defences in crops and other grasses.