Many of these special weed problems can be minimized by managing them before planting pistachio orchards.
Hairy fleabane is a summer annual that reproduces from seed, although under certain environmental conditions it can grow like a biennial. It germinates from fall through spring, and matures and produces seed from July through September. Hairy fleabane is a member of the sunflower family; its seed is readily disseminated by the wind. The preemergence herbicides flumioxazin (Chateau), indaziflam (Alion), isoxaben (Trellis), penoxsulam plus oxyfluorfen (Pindar GT), and rimsulfuron (Matrix) provide effective control. Hairy fleabane is difficult to control with postemergence herbicides because its stems are multi-branched, often woody, and lack significant leaf area. Hairy fleabane plants are most susceptible to control with postemergence herbicides or mechanical control when they are in the seedling stage. Effective postemergence herbicides are glufosinate (Rely 280), paraquat (Gramoxone), and saflufenacil (Treevix). Due to widespread distribution of glyphosate-resistant hairy fleabane biotypes, do not use glyphosate (Roundup) as a primary herbicide for control.
Horseweed (Mare's Tail)
Horseweed is a summer annual weed, closely related to hairy fleabane, with similar growth and reproductive characteristics. Unlike hairy fleabane, horseweed grows as a single stalk. Horseweed can become a prevalent weed in orchards where oryzalin (Surflan) and oxyfluorfen (Goal) are used as preemergence treatments over several years in a row. Control emerged plants in the same manner as hairy fleabane. Delaying treatments when plants are bolting will likely result in poor control and regrowth. Mowing is not recommended because cutting plants off above the soil line can result in the sprouting of lateral buds at the base of this weed. Due to widespread distribution of glyphosate-resistant horseweed biotypes, do not use glyphosate (Roundup) as a primary herbicide for control.
Field bindweed is a vigorous perennial weed that grows from seed, which can survive as long as 30 years in the soil, or more commonly, from reproductive stolons, rhizomes, or extensive roots. It is important to control this weed before it has the chance to establish and produce seed. While cultivation can be used to control seedlings, cultivating mature plants can spread reproductive structures. Once field bindweed appears in an orchard, spot treat with high label rates of glyphosate (Roundup). Two or more treatments may be needed to eradicate the new infestation.
Yellow and purple nutsedge are perennial weeds that reproduce mainly from underground tubers. Yellow nutsedge is the most common of the two species found in pistachio orchards. Most tubers of yellow nutsedge will survive in the soil for less than five years and are found primarily in the top eight inches of the soil profile. Yellow nutsedge grows and reproduces best under sandy, well-irrigated conditions. The tubers have several buds that can each give rise to additional plants. Under normal conditions, one or two buds sprout to form new plants; however, if killed by cultivation or an herbicide, then new buds are activated. Rimsulfuron (Matrix) provides some control in nonbearing and bearing orchards. Treat emerged plants with glyphosate (Roundup) or paraquat (Gramoxone) before they reach the 4- to 5-leaf stage. Repeated applications at 3- to 4-week intervals will be required during summer as new plants emerge.
Little Mallow (Cheeseweed)
Little mallow is a winter annual or biennial weed that is controlled with preemergence herbicides like oxyfluorfen (Goal), flumioxazin (Chateau), and penoxsulam plus oxyfluorfen (Pindar GT). Once established, little mallow becomes woody and forms a thick crown and root, making it difficult to control mechanically or with postemergence herbicides. Plants that are less than 4 to 6 inches tall are easiest to control with a tank-mix application of oxyfluorfen (Goal) plus glyphosate (Roundup), glufosinate (Rely 280), or saflufenacil (Treevix). Repeated mowing is not an effective means of control.