Agricultural pest management
Special Weed Problems
(Reviewed 9/16, updated 9/16)
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, it can be laborious to control without using herbicides.
Bermudagrass is a drought-tolerant perennial grass and thrives in hot, sunny locations. It is less 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, management of established infestations usually requires some combination of methods, including repeated dry cultivations and applications of a translocated herbicide. Complete control is difficult.
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 distinguishing crabgrass from other species will help to identify effective management strategies. 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. Crabgrasses tend to grow in moist areas due to its shallow root system. Managing irrigation, system leaks, and drainage will help reduce establishment and vigor. Most preemergence herbicides for grass management are highly effective at preventing crabgrass seedling establishment.
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.
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.
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.
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. Grass specific herbicides such as fluazifop are not effective in controlling nutsedges. Yellow nutsedge is the most common nutsedge in California, but purple nutsedge can be prevalent at warmer 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 herbicides 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. Where herbicides are used, repeated applications are often needed.
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. Puncturevine is best managed at the seedling stage. Mature plants are much harder to control than the seedlings. There are species of seed and stem weevils that can generally suppress puncturevine in undisturbed and unirrigated areas, except if several years of freezing winter weather suppress weevil numbers. Avoiding cultivation and increasing drought stress through irrigation management can increase the weevils' effectiveness in biologically controlling puncturevine.
Several species of wild cucumber may occur in California avocado. Cucamonga manroot (Marah macrocarpus) is 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.
UC IPM Pest Management Guidelines: Avocado
B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
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