Currently registered preemergence herbicides will control only seedlings of annual and some perennial weeds. Repeated postemergence treatments are required to control perennials. While they are best controlled (preferably eradicated) before planting, if these weeds are still present after planting, a program is needed for their control. With these treatments, there is always some concern of injury to trees from careless application.
Some populations of annual bluegrass (Poa annua), horseweed (Conyza canadensis), annual or Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), and hairy fleabane (Conyza bonariensis) have developed resistance to glyphosate in California. For more information on treating herbicide-resistant weeds, see INTEGRATED WEED MANAGEMENT.
Primary species that are difficult to control are the perennial grasses and broadleaf weeds. In mature orchards with heavy shade, perennial weeds will be a minimal problem. It will be necessary to control seedlings of these weeds with preemergence materials or spot treatments of glyphosate (Roundup) or paraquat before they become established. Seed from the grass species can last at least 2 years in soil, while seeds of the broadleaf weeds may last 10 to 60 years in the soil, depending upon species. Therefore, frequent monitoring is necessary for continued control. Do not allow perennial plants to reestablish or to produce seed.
Bermudagrass is a vigorous spring- and summer-growing perennial grass. It grows from seed but, because of its extensive system of rhizomes and stolons, can also be spread during cultivation. It frequently becomes a problem in mowed orchards because mowing increases the amount of light that the stolons receive, thus stimulating their growth. This grass is very competitive with the trees for moisture and nutrients. Seedlings can be controlled with preemergence herbicides. If bermudagrass develops in localized areas, immediately spot-treat it with postemergence herbicides such as glyphosate (Roundup).
Curly Dock, Dandelion, and Johnsongrass
To achieve the best control with postemergence contact herbicides, cultivate the weeds to chop the stems, crowns, and rhizomes into small pieces, then irrigate to encourage regrowth and a lot of leaf area on the weeds. Spray before the weeds flower or set seed. Good coverage is important and spot treatment of regrowth may be needed. In young trees, fluazifop or sethoxydim can be used to control grasses without risk of tree injury. Curly dock can also be suppressed with 2,4-D applications, while dandelion can be controlled with 2,4-D. A permit is required for this material. Use 2,4-D with caution in tree nut orchards to prevent tree damage.
Dallisgrass is a common perennial weed and can be highly competitive in newly-planted orchards. Dallisgrass seedlings germinate in spring and summer; this species can also form new plants on short rhizomes that develop from the original root system. Seedlings can be controlled with cultivation or with preemergence herbicides. Dallisgrass has a clumpy growth habit that gives it a bunchgrass appearance. Like bermudagrass, it tends to become dominant in mowed areas because mowing stimulates seed set. It also grows in areas with standing water. The plants are heavy seed producers. 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 from stolons, rhizomes, or extensive roots. Because of the seed's longevity 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 when the soil is dry controls seedlings. If field bindweed appears in or around the orchard, spot-treat with high label rates of glyphosate, especially when the bindweed is actively growing.
Yellow nutsedge is a difficult-to-control perennial weed that reproduces from underground tubers, which survive for 2 to 5 years in the soil. The tubers are easily spread by cultivation equipment. Each tuber contains several buds that are capable of producing plants. One or two buds germinate to form new plants; however, if the plant is destroyed by cultivation or an herbicide, then a new bud in the tuber may be activated.
In established orchards, if nutsedge develops, spot-treat it with glyphosate (Roundup). Then re-treat it before the plant reaches the five-leaf stage. New nutlets do not form if the plants are re-treated at or before this stage. As with seedling bindweed, young nutsedge can be controlled by cultivating when the soil is dry.
Nutsedge is a particular problem in young orchards or around replacement trees because it does best in full sun conditions.
Common purslane is a prostrate summer annual that reproduces from seed, which germinate in April to early May. Common purslane grows into a plant with fleshy stems, which can root and continue to grow after cultivation or mowing if sufficient moisture is present. Common purslane can cause problems with both nut drying and pick-up during harvest operations.
This weed predominates in sunny areas of the orchard, especially if low rates of translocated herbicides (e.g., glyphosate) are used as preharvest sprays. If problems develop with this weed, use higher rates of glyphosate to control it.
A low-rate preemergence herbicide program can also effectively manage this weed and reduce the need for preharvest treatments. Pendimethalin (Prowl H2O) at 1 quart/acre applied with glyphosate in April to the area between the tree rows in the orchard can provide season-long control. Monitor the rates used and adjust them so that populations of winter annual vegetation such as annual bluegrass are preserved.
Hairy fleabane is an annual plant that normally emerges in late fall and early spring but can also emerge during winter if temperatures are relatively warm. This plant can withstand several mowings and still produce seed. It is not a good plant to have in a ground cover because its hard stem does not degrade easily and can cause harvest problems. In addition, it can interfere with moving sprinkler and drip irrigation lines. Simazine is an effective preemergence herbicide for hairy fleabane. Paraquat and saflufenacil can control this species when it is small, but once plants bolt (when they send up flowering stalks), they may not be completely effective. Glyphosate at 1 lb a.e./acre will control susceptible plants up to 13 leaves; for plants with 14 to 19 leaves 2 lb a.e/acre is required. Plants larger than 19 leaves are not adequately controlled. Most populations of hairy fleabane in California are not well controlled with glyphosate; some are also resistant to paraquat.
Little Mallow (Cheeseweed)
Little mallow is an annual or biennial weed that is sometimes not effectively controlled with many common preemergence herbicides. High rates of oxyfluorfen (Goal) can provide acceptable control. Once established, little mallow becomes woody and forms a thick crown and root, making it difficult to control mechanically or with postemergence herbicides. Plants that are less than 4 to 6 inches tall are easiest to control with a tank mix of oxyfluorfen plus glyphosate. Smaller plants can also be controlled with carfentrazone (Shark) or saflufenacil (Treevix). Repeated mowing is usually not an effective means of control.