Agricultural pest management

Special Weed Problems

(Reviewed 1/07, updated 1/07)

In this Guideline: More about weeds in avocado:

BERMUDAGRASS. Bermudagrass is drought-tolerant and thrives in hot, sunny locations. It is not as aggressive in dense shade. In cold locations it becomes dormant and turns brown during the winter. Bermudagrass reproduces from rhizomes, stolons, and seed. Depending on the situation, complete control of established infestations usually requires some combination of methods, including repeated dry cultivations and applications of a translocated herbicide.

DALLISGRASS. Dallisgrass is a bunchgrass that is highly competitive with young trees for water. Mature plants typically form loose clumps about 1 to 4 feet tall. Dallisgrass reproduces from seed and very short rhizomes. Seed are easily transported in water or by machinery. Dallisgrass can become dominant in mowed groundcovers because mowing stimulates seed production. If dallisgrass becomes established in a young grove, repeated applications of a translocated herbicide may be needed to provide control.

BARNYARDGRASS. Barnyardgrass is a summer annual that grows in dense, tall or spreading clumps. Several varieties that differ in growth habit and floral appearance occur in California. Each stout plant ranges from 6 inches to 6 feet tall. Plants often root at the lower nodes. Lower spikes of the flower head are spaced apart; top ones are crowded together. Management methods include cover cropping, cultivating, flaming, hand-weeding, mowing, mulching, and employing cultural practices that maintain and promote a dense tree canopy. Because barnyardgrass produces huge quantities of seed, in sunny locations it can be difficult to control without using herbicides.

CRABGRASSES. Large crabgrass, also called hairy crabgrass, is a low-growing annual. It has a papery ligule but no auricles, and there are small tufts of hairs where the leaf blade meets the sheath. Smooth crabgrass is a smaller species without hairs and it is less commonly a problem. Correctly distinguish crabgrass from other species so you can identify effective management. For example, large crabgrass roots deeply at the nodes, giving the appearance of stolons, which may cause crabgrass to be confused with perennials such as bermudagrass. Management methods include cover cropping, cultivating, flaming, hand-weeding, mowing, mulching, and employing cultural practices that maintain and promote a dense tree canopy. Established crabgrass plants are difficult to remove, but most preemergent herbicides are highly effective at preventing crabgrass seed germination.

LONGSPINED SANDBUR. Longspine sandbur seedlings closely resemble those of barnyardgrass. Seedling leaves are flattened and have a purplish tinge at the bottom. The most distinctive seedling characteristic is the bur from which the young plant emerges. This bur may be found by digging carefully around the roots. Management methods include cover cropping, cultivating, flaming, hand-weeding, mowing, mulching, spraying herbicides, and employing cultural practices that maintain and promote a dense tree canopy.

NUTSEDGES. Sedges resemble grasses, but grass stems are hollow, rounded, and have nodes (joints) that are hard and closed. Sedges have three-sided, solid stems that are triangular in cross-section. Yellow nutsedge is the most common weedy nutsedge in California. Purple nutsedge can be prevalent at wetter sites. Nutsedges, sometimes called nutgrasses, reproduce from tubers (incorrectly called "nutlets") that form on their rhizomes. The tubers are spread easily by cultivation and when moving infested soil. Nutsedges may become troublesome in groves where herbicides are used for total weed control. Most chemicals do not control nutsedges well and nutsedges spread quickly in the absence of competition. To prevent the formation of tubers, kill the young plants before they reach the 5-leaf stage

FIELD BINDWEED. Field bindweed, also called perennial morningglory, competes with trees for moisture and nutrients during summer months. Established infestations are nearly impossible to eradicate because plants produce perennial roots and seed can remain dormant for up to 60 years. Take care not to transport viable rootstock fragments on field equipment. Kill seedlings before they have 5 leaves. Treating plants with a translocated herbicide, then cultivating, and treating regrowth when flowers begin to form reduces infestations substantially if repeated over a period of years.

WILD CUCUMBER. Several species of wild cucumber may occur in California avocado. Cucamonga manroot (Marah macrocarpus) is probably the most common. These perennial vines develop a large tuber, which makes established plants difficult to eliminate. Wild cucumber vines have clinging tendrils. Stems climb up and entwine young trees and the sides of mature trees exposed along grove edges and roadsides. Cultivation, flaming, hand-weeding, or translocated herbicides must be applied repeatedly to kill regrowth until plants exhaust the energy stored in tubers.

PUNCTUREVINE. Puncturevine produces hard spiny fruit that can penetrate tires and are easily spread on shoes or tires. Plants are prostrate in open areas but somewhat erect in dense vegetation. Control puncturevine as seedlings when most methods are effective. However, the seed can germinate beyond the effective depth of some preemergent herbicides. Mature plants are much harder to control than the seedlings. A seed weevil and stem weevil generally control puncturevine in undisturbed and unirrigated areas, except for several years after freezing weather, which suppresses weevil populations. Avoiding cultivation and increasing weed drought stress by preventing irrigation water from watering puncturevine increases weevil's effectiveness in biologically controlling puncturevine.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Avocado
UC ANR Publication 3436


B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
C. A. Wilen, UC Statewide IPM Program, San Diego County
B. D. Hanson, Plant Sciences, UC Davis

Acknowledgment for contributions to Weeds:
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
A. Shrestha, UC Statewide IPM Program, Kearney Agricultural Center, Parlier
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County

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