Most of these special weed problems can be minimized through an active preplant weed management program.
Herbicide resistance is the inherited ability of a weed to survive an herbicide dose that would normally control the species. Over-reliance on one herbicide mode of action can select for weed biotypes that have resistance to the particular herbicide and can lead to a rapid increase of that biotype in the orchard and a corresponding loss in weed control. To prevent, delay, or manage herbicide resistance, use a variety of weed control strategies including cultural practices like tillage as well as good herbicide practices such as herbicide mode of action rotation, tank mixtures, and full-rate applications.
Bermudagrass is a vigorous spring- and summer-growing perennial. It grows from seed, but its extensive system of rhizomes and stolons can also be spread during cultivation. It competes aggressively with trees for moisture and nutrients. Seedlings typically are controlled with preemergence herbicides or cultivation. If bermudagrass develops in an orchard or in localized areas, spot-treat it immediately with postemergence herbicides such as glyphosate (Roundup). Repeated applications during the weed's growing cycle are often required for complete control.
Dallisgrass is a common perennial weed found in olive orchards. It has a clumpy growth habit that gives it a bunchgrass appearance. It can be highly competitive in newly planted orchards; in established orchards it competes for soil moisture and nutrients. Seedlings germinate in spring and summer and form new plants on short rhizomes that developed from the original root system. Dallisgrass seedlings can be controlled with cultivation or with preemergence herbicides. Treatment with glyphosate has been successful in controlling dallisgrass infestations.
Field bindweed is a vigorous perennial weed that either grows from seed, which can survive for up to 30 years in the soil, or vegetatively from rhizomes and extensive roots. Because of the longevity of the seed in the soil, it is critical to destroy plants before they can produce seed. The plants may spread from stem or root sections that are cut during cultivation: however, cultivation controls seedlings. If field bindweed appears in or around the orchard, spot-treat it with high label rates of glyphosate. Repeat applications are often needed, as a single glyphosate application seldom results in complete field bindweed control.
Johnsongrass is a perennial weed that spreads from seed or from an extensive system of underground rhizomes. It grows vigorously in spring and summer when it overtops newly planted trees and competes for light, moisture, and nutrients. Severe setback of a young orchard can occur under these conditions. A postemergence application of fluazifop or clethodim can be used around newly planted trees. If johnsongrass develops in or around trees in an established orchard, spot-treat it with glyphosate to prevent the spread of its rhizomes.
Yellow nutsedge is a perennial weed that reproduces from underground tubers that survive for 2 to 5 years in the soil. The tubers are easily spread by cultivation equipment. Each tuber contains several buds capable of producing plants. One or two buds germinate to form new plants; however, if destroyed by cultivation or an herbicide, then a new bud is activated. In established orchards, if nutsedge develops, spot-treat it with glyphosate. Purple nutsedge is usually more difficult to control with herbicides than yellow nutsedge. It is more common in the San Joaquin Valley and southern California than in more northern parts of the state.
Blackberries (Himalayan and California) are vigorous perennial vines or canes that are often found around orchard margins and sometimes around the trunks of trees. They interfere with all cultural operations, especially pruning and harvest. For best control, spot-treat with glyphosate at the flower stage or after fruiting when there is good soil moisture and the plants aren't stressed. A re-treatment may be required on large clumps if regrowth occurs. If blackberry is growing near or up into an olive canopy, pull the vines out before treating so the herbicide will not get into the tree.
Little Mallow (Cheeseweed)
Little mallow is an annual or biennial plant that is sometimes not controlled with preemergence herbicides. In addition, plants larger than 4 to 6 inches are not controlled well with glyphosate. Mature plants are tall and woody with a large taproot that can be removed with a shovel or with cultivation. Oxyfluorfen effectively controls seedlings or young plants.