Weed Wisdom November 2018

The annual Kochia migration

by Dr. Charles Geddes

As I write, wind gusts hit up to 100 km per hour in Lethbridge and the annual migration of feral Kochia is in full swing. Kochia is a tumbleweed. This weed grows into a large, bushy, oval, top-heavy plant that senesces in October. Plants with this structure are most common in areas where kochia is not competing with other plants, like low-lying saline areas, where this species often thrives. As kochia plants dry down, the stem weakens, making it easy for the stem to break off on a windy day.

On days like today, kochia tumbleweeds blow across fields in southern Alberta and elsewhere. While tumbling, the plant losses its seed, resulting in dispersal of the weed population to new areas and new fields. This is of particular concern for spread of herbicide-resistant kochia populations, like those resistant to glyphosate or synthetic auxin herbicides.

Kochia scoparia, (kochia for short), is among the most problematic weeds in the southern Canadian prairies and often robs many growers of crop yield. Difficulty managing this weed is, in part, due to the evolution of herbicide resistance (in western Canada) to groups 2, 9, and now 4. While many growers implement targeted integrated weed management programs, it is difficult to reduce the spread of this herbicide-resistant weed because it is efficient at both pollen- and seed-mediated gene transfer.

Recently, Agriculture & Agri-Food Canada (AAFC) researchers studied how much drops during kochia migration and over what distance. To do this, they attached tracking collars to kochia plants and set them free in the wind (the experiment was limited to a single field to limit the spread the populations). In this study, kochia plants traveled at speeds of up to 11 km/hr and lost about 90% of their seed in the first km. However, it is likely that the other about 10% of seed would remain attached for much longer distances.

The next step is to determine options to limit the distance travelled by kochia plants. Fence lines at tree rows work well to stop kochia in its tracks, but what about fields that are no longer fenced and where trees have been removed? Does crop stubble height matter? Or narrow wind-rows of crop left standing in in fields? These are all questions that we cannot answer, yet…

For the time being, we know that when kochia competes with other plants, like a dense crop stand, it loses its ability to form a tumble weed. This tool could be used in field areas with standing crop by using high seeding rates, narrow row spacing, etc. In more-ruderal areas outside of fields, mowing can inhibit the tumbleweed growth form, meaning that the plants wont blow away as easy. Ongoing research at AAFC is looking at both of these tools, and how they can be optimized to manage kochia more effectively.

Reference:

Beckie, H.J., R.E. Blackshaw, L.M. Hall, and E.N. Johnson 2016. Pollen- and seed-mediated gene flow in kochia (Kochia scoparia). Weed Science 64:624-633