JoAnne Skelly: Occultation for weed control |

JoAnne Skelly: Occultation for weed control

JoAnne Skelly

I learned a new word this week, occultation. Researching it, I found it was an astronomical term describing one object blocking out another from an observer.

However, I heard it in reference to the tarping of soil with black plastic to reduce weeds by blocking out the sun.

Soil solarization with clear plastic is a common organic practice for weed and pest management. Under clear plastic, soil temperatures can rise high enough to kill weed seeds and seedlings, disease organisms and insects. However, research indicates that temperatures under black plastic stay lower than under clear plastic, so pathogens or fungi are not killed. But, black plastic can be more effective at killing weeds. It blocks the sunlight so that plants cannot produce the sugars they need for growth and development through photosynthesis. An added benefit of black plastic is that if fewer organisms are killed because of lower soil temperatures, fewer beneficial soil organisms die as well.

My friend Will is trying to kill a small plot of Russian knapweed. These invasive weeds have an extensive and hardy perennial root system that is difficult to kill. The black plastic may kill them or weaken them enough for him to dig most of them out once he removes the plastic. Hopefully, the tough plants won’t thrust their way through the plastic. Or, they may only go dormant until the plastic is removed.

For a small area, black plastic is worth the experiment. Site preparation and the timing of the plastic application are important to reducing Russian knapweed. Mowing the area first will be necessary if weeds or old stems are taller than four inches. The soil must be moist, and then the area covered. The ideal is to cover it with at least four mil plastic before plants emerge in late winter and keep it covered until following winter. It is best to use a plastic that resists breaking down in the sun. It must be held tightly against the soil, which may involve digging a trench four to six inches deep around the area and burying the edge of the plastic in the trench with soil.

Tamping down the soil around the covered plot will make it more secure. Being covered in full sun for a minimum of 10 hours per day will increase chances of reducing these noxious weeds.

Although I had heard of using black plastic many times in my career, the term occultation was new to me.

JoAnne Skelly is associate professor and extension educator, Emerita at University of Nevada Cooperative Extension. She can be reached at