Agricultural pest management
Special Weed Problems
(Reviewed 9/08, updated 9/08)
YELLOW and PURPLE NUTSEDGE
Yellow and purple nutsedge appear similar to grasses but have leaves that are triangular in cross-section, whereas grass leaves are round. The flowers resemble those of grasses. Yellow nutsedge flowers are yellow in color, while purple nutsedge flowers are purple.
Yellow and purple nutsedge are 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 forms a bulb just under the soil surface. Leaves then grow from the 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 retreatment of the orchard takes place before new tuber formation. Because purple nutsedge is able to sprout from tubers deeper in the soil than the ones yellow nutsedge sprouts from, it is not as well controlled with MSMA.
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. Johnsongrass is 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 treat in order to reduce the underground portion of the plant using a translocated herbicide such as glyphosate.
UC IPM Pest Management Guidelines: Citrus
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