Recently I was asked to help identify prostrate knotweed (Polygonum aviculare) that was collected from an alfalfa field. Admittedly, the tough and wire-like specimen that was submitted to me at the end of autumn didn't much resemble the succulent seedlings that I have often observed emerging in tree and vine systems in early spring. Consequently, I dug out my 'Weeds of the West' and 'Weeds of California and Other Western States' books and double-checked with colleagues to ensure that my ID was accurate. This post is meant to build on that effort and describe the morphological traits that are characteristic of prostrate knotweed and how to differentiate the species from a similar-looking weed, spotted spurge (Euphorbia...

Why is weed identification important? Simply stated, because not all management strategies are equally effective against all weed species. The weed community present in a given field, orchard, or vineyard may be comprised of species that are differentially sensitive to different herbicide modes of action or physical control practices. Even closely related species may respond differently to different herbicide active ingredients. For example, according to UC IPM crop management guidelines, metribuzin and S-metolachlor can provide partial control of yellow nutsedge (Cyperus esculentus) but no control of purple nutsedge (Cyperus rotundus). Consequently, to develop effective weed control programs, managers should be aware...

According to the CalFlora website (http://www.calflora.org/), 21 species of amaranths occur (to some extent) in California. While many are non-native, a few, including prostrate pigweed (Amaranthus blitoides) and Palmer amaranth (Amaranthus palmeri), are indigenous. While it may be convenient to lump all of the genus Amaranthus together when considering weed management options, proper identification is important for understanding the potential for crop yield loss (not all amaranths were created equal with respect to competitiveness) and the possibility of herbicide resistance (populations of Palmer amaranth with resistance to glyphosate have been confirmed in the...

Kassim Al-Khatib, Director of the UC-Statewide Integrated Pest Management (UC-IPM) Program, sent a couple links about the identification of Palmer amaranth and comparison to other amaranth (pigweed) species.  I thought they were useful and potentially of interest to California weed managers so I thought I'd share:

In this first link, Dr Bill Johnson and Travis Legleiter, weed scientists from Purdue University have a video explanation for helping growers identify Palmer amaranth and explain differences between other related species in Indiana (click here for...