Brush Management: Past, Present, Future

8. Timing

8 Timing The Key To Successful Brush And Weed Control With Herbicides Mesquite (Prosopis glandulosa) has been the nemesis of southwestern rangelands for many years, seemingly always. We have been attempting to control mesquite at least since the 1940s. And yet, we still have the brush problem with us. It is often said that we are no nearer today than we were 40 or 50 years ago in our understanding of how to control noxious plants, regardless of the species. However, if one is cognizant of the phenological stage and the physiological status of the plant as well as the environmental conditions at the time of applying herbicide, the target species can be effectively controlled . This chapter will primarily address aerial spraying of foliar-applied herbicides, since satisfactory control is most difficult to achieve. Some attention will also be given to both individual plant treatment (basal and spotfoliar application of herbicides) and soil application of dry herbicides. One of the primary tenets of controlling noxious species is recognizing the time at which herbicides are translocated to the perennating organs (i.e., buds) and tissues. Since foliar-applied herbicides can be translocated via the carbohydrate (total nonstructural carbohydrates; TNCs) stream in the plant, it is important to know the TNC trends and the energy allocation patterns within plants (source:sink relationships, Cobb and Kirkwood 2000). Several years ago, Pitelka (1977) identified energy allocation patterns in three different lupines; an annual, an herb, and a nonsprouting shrub (Table 8.1). It is interesting to note in plants from all three growth habits the amount of energy that is allocated to the flowering/reproductive process. As is the case, the flowering process requires a substantial amount of energy, which signifiRonald E. Sosebee 85 Table 8.1. Energy allocation (%) in Lupinus sp. L. annus L. variicolor L. arboreous (Annual) (Herb) (Shrub) Reproductive tissue 61 18 20 Seeds 29 5 6 Stems — — 50 Roots 3– 4 40 — Source: Adapted from Pitelka (1977). cantly affects one’s ability to control both herbaceous and woody plants. One can conclude from Pitelka’s data that in order to achieve a high degree of chemical control, the plants must be sprayed either before or after the flowering stage, but not during the initial reproductive phase. Categories of Woody and Herbaceous Weeds For the sake of simplicity, noxious plants will be categorized and discussed in the following categories: (1) annuals (summer annuals/winter annuals/ biennials), (2) hard-to-kill herbaceous perennial weeds, (3) suffrutescent (half) shrubs, (4) cacti, and (5) woody plants (sprouting and nonsprouting). Consideration of long- and short-shoots is very important when planning herbicidal control of woody plant resprouts. Long-shoots in woody plants are primarily vegetative shoots, whereas, short-shoots are primarily reproductive shoots (discussed in more detail later in the chapter). Annuals Annuals are relatively easy to control chemically. However, they should be sprayed during the vegetative stage. After the plants become reproductive and bolt (produce a flower stalk), they are very difficult to control. Many annuals, at least in the southern and southwestern United States, germinate and become established during late summer, fall, and early winter if soil water is available. They remain vegetative throughout the winter as rosettes and bolt in the spring when air temperatures become warm. This gives them a head start on the warm-season grasses and creates weed problems, such as common broomweed (Amphiachyris dracunculoides). Common broomweed , for example, and other annuals are easily controlled if sprayed anytime before the plant bolts (usually mid- to late spring). After the plant bolts, it is difficult to impossible to control chemically. Cold air temperature at the time of spraying can be important if the plant is not sufficiently physiologically active. Ueckert et al. (1980) found that bitterweed (Hymenoxys odorata) was difficult to kill if sprayed when air temperatures were 70 F. Perennial Herbaceous Weeds Many perennial herbaceous weeds such as silverleaf nightshade (Solanum eleagnifolium), field bindweed (Convolvulus arvensis), blueweed sunflower (Helianthus ciliaris), and leafy spurge (Euphorbia esula) have the reputation of being difficult to kill. Most of our problems stem from improper timing of herbicide application. Herbicide application typically occurs when the plants are actively growing, which usually coincides with the flowering/ reproductive stage. Silverleaf nightshade, for example, has traditionally been sprayed during the reproductive phase when the plant is in the purple flower stage. Top-kill is obtained but root-kill is not. If silverleaf nightshade is sprayed after the plant...