Agriculture: Citrus Pest Management Guidelines

Special Weed Problems

Yellow and Purple Nutsedge

Yellow and purple nutsedge appear similar to grasses but have leaf and flower stems that are triangular in cross-section, whereas grass leaves are round or oval. The flowers resemble those of grasses. Yellow nutsedge flowers are yellow, while purple nutsedge flowers are purple.

Yellow and purple nutsedge are more easily distinguished from each other by looking at their tubers. Yellow nutsedge tubers are nearly round and somewhat smooth. Purple nutsedge tubers are oblong and very rough and scaly. Purple nutsedge tubers are linked together by rhizomes (underground stems), whereas yellow nutsedge tubers are found only at the ends of rhizomes. Tubers of both species have three to seven buds that are capable of forming a new plant. Nutsedge plants develop from sprouts on a tuber; the sprout then forms a bulb just under the soil surface. Leaves and new rhizomes then grow from this basal bulb.

Populations of these two weeds can be reduced by applications of glyphosate at or before the five-leaf stage. If sprayed after this point, the plant may be killed, but it has already formed new tubers that can form new plants. Glyphosate kills the leaves and basal bulb, but the herbicide rarely travels down to the tuber in sufficient amounts to kill the tuber. The tuber's three to seven buds can resprout, necessitating careful attention so that re-treatment of the orchard takes place before new tuber formation. Repeat applications at 21- to 28-day intervals are often needed during summer to keep nutsdge controlled and to completely eliminate it over the period of 2 to 3 years. Rimsulfuron (Matrix) and flazasulfuron (Mission) have both pre- and postemergence activity on both nutsedge species. Norflurazon (Solicam) can be used as a preemergence treatment to suppress high nutsedge populations.

Johnsongrass

Johnsongrass can grow from either seed or rhizomes. Johnsongrass is a perennial grass with erect, usually solid stems that grow 2 to 8 feet tall. The seeds have a red to purple tint and remain viable in the soil at least 5 years. Rhizomous Johnsongrass can be controlled by repeated tillage during the dry summer months. However, the soil must be fairly dry; otherwise the rhizome buds may sprout. Rhizomes as small as 1 inch in length can sprout if they don't lose more than 60% of their initial weight to drying. After flowering, reserves are sent to the roots making this stage an excellent one to spray in order to reduce the underground portion of the plant using a systemic herbicide such as glyphosate. Where large clumps of Johnsongrass exist, it is often best to spot treat with a grass herbicide like sethoxydim (Poast) or clethodim (SelectMax), since they have little to no impact on citrus growth if contacted. Seedling Johnsongrass plants are readily killed with systemic herbicides like glyphosate (Roundup) or sethoxydim (Poast) before rhizomes are produced.

Sprangletop

Sprangletop is a summer annual grass commonly found in citrus groves and is widespread in California. Sprangletop seed is commonly spread through district irrigation canals and ditches, so it infests most orchards. It most commonly emerges as furrow irrigation water is applied, requiring vigorous attention. Screens placed at intake pumps can help reduce the seed load into a field. It is also commonly found infesting microsprinkler-irrigated orchards.

Several labeled preemergence herbicides effectively control this weed, including indaziflam. As preemergence herbicides breakdown over time during the irrigation process, escapes are common. Spraying with glyphosate or a grass-selective herbicide is effective as new flushes of plants emerge. Sprangletop is not controlled effectively with postemergence herbicides once plants become mature and begin producing seed.

Hairy Fleabane

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, and its seed is readily disseminated by the wind. Preemergence herbicides like rimsulfuron, indaziflam, and bromacil can effectively control it. Hairy fleabane's multi-branched, often woody stems and lack of significant leaf area make it difficult to control with postemergence herbicides, although saflufenacil (Treevix) and paraquat (Gramoxone) can do a good job if these weeds are sprayed at a young age (before the rosette stage).

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 unless cut. Horseweed can become a prevalent weed where repeated uses of oryzalin and oxyfluorfen are used preemergence to control other weeds. Control of emerged plants should begin when these weeds are in the seedling or rosette stage of growth. Delaying herbicide applications beyond this can result in significant regrowth. Mowing is not recommended, since cutting plants off above the soil line can result in the sprouting of lateral buds at the base of this weed.

Text Updated: 04/19