What You See is Not Always What You Get (October 1, 2014)

Preemergence herbicides do not always kill weeds before they emerge. In almost all cases, the weed seed must germinate for the herbicide to work. Some are absorbed only by the roots, some only by the shoots and some by both. Dacthal and Eptam,for instance, are not absorbed by roots. They are absorbed at the growing points between the radical and the cotyledon leaves. Weed seeds often will germinate, emerge and then die. Kerb is absorbed by the roots and shoots and unlike most of the other preemergent herbicides, it is translocated to the whole plant. Weeds will sometimes emerge and slowly die. It also has some activity on very small weeds when applied post emergence. It is not uncommon for fields to look clean early on and look weedy later. If there are 1000 weeds per acre for instance, 90% control still leaves 100 weeds behind to grow and become visible from a long way away. Often the only way to keep lettuce fields weed free is by using 2 or 3 herbicides, tillage and hand weeding.

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To contact Barry Tickes go to: btickes@ag.arizona.edu.