Grasslands and mowing seem to go together like maple trees and maple syrup. However, mowing the perennial grasses of our high desert can be a real threat to the long-term sustainability of the grasslands here, and mowing can reduce habitat for many species that depend on grasslands.
Much of our Sonoita plain and the foothills of the Santa Rita, Huachuca, and Patagonia Mountains are dominated by native, perennial grasses. There is no good way to tell the age of a grass plant, but long-term studies of native perennial grasses have found that they live at least 200 years and some maybe as long as 1,000 years.
Unlike annual agricultural grasses that live only one year, grama grasses and other native perennial grasses are well adapted to live in a climate where their physiology limits them to growing only in the summer. Unlike other grasses, including annuals used in agriculture for silage, the native perennial grasses require warm soils and do not green up until the monsoon rains arrive.
These native perennials have a structure and growth pattern that allow them to survive even in years of insufficient rainfall. Although those curly leaves of blue grama look dead in the winter and spring—dry and yellow—they are conserved by the plant. As little as 15 minutes after a rainfall event, blue grama starts moving sugars, nutrients, and the compounds needed for photosynthesis to those leaves from roots. They slowly green and over a few days start to manufacture sugar and capture carbon from the air to build seeds, roots, and new leaves. Much of the production of the green growth is then shunted back into the roots, a few inches below the soil, to be stored in case the rains stop.
Haying off the native grasses at the peak of their green growth, often when the seeds are not even mature, has become a growing practice in the Sonoita plains. If you are a landowner, you might be tempted to do this, but haying the native perennial grasses can break their growth cycle and lead to the loss of their vigor, promoting invasion by shrubs and thorny vegetation, along with loss of the shallow topsoil during heavy rains. If haying is done year after year, our grama grasses will be unable to store enough below-ground carbon, nutrients, and sugars and slowly will dwindle away, especially in dry years, as we have recently seen.
Cattle, if managed appropriately, are probably the best way to mow a native perennial grass pasture. In a well-managed grazing system, cattle are allowed to ingest only a small fraction of the carbon and nutrients, and they cycle most of it back as droppings and urine. Native herbivores (grass-eating animals) seem to have little effect on the grasses. Indeed, many of the native herbivores are not four-legged mammals—they are grasshoppers, butterflies, ants, lizards, snakes, birds, and other members of the abundant animal community found in native grasslands.
Mowing for fire protection should be done well after the grass seeds have filled out and hardened. Leaving the mowed annual growth on the soil means that most of the carbon and nutrients can then be cycled back into the soil. What doesn’t blow away will be broken down by soil microbes to be incorporated into the grass next year.
The one place you should mow is an area 100 feet out around your house in order to be protected from fast-moving grass fires. Landowners should follow the guidelines available at the Sonoita-Elgin Fire station for fire-safe landscapes.