Pest Management Guidelines
Special Weed Problems
(Reviewed 3/09, updated 3/09, corrected 5/09)
Bermudagrass is a vigorous spring- and summer-growing perennial. 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).
LITTLE MALLOW (CHEESEWEED)
Little mallow is an annual or biennial weed that is sometimes not effectively controlled with 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 application of oxyfluorfen plus glyphosate. Smaller plants can also be controlled with carfentrazone (Shark). Repeated mowing is not an effective means of control.
Dallisgrass is a common perennial weed found in orchards. It can be highly competitive in newly-planted orchards. Dallisgrass seedlings germinate in spring and summer and form new plants on short rhizomes that develop from the original root system. They 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 controls seedlings. If field bindweed appears in or around the orchard, spot-treat with high label rates of glyphosate.
Hairy fleabane is an annual plant that normally emerges in February, but it can emerge in December if winter temperatures are warmer than average. 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 glyphosate both can control this species when it is small, but once plants bolt (sending up flowering stalks), they will not control it. Glyphosate at 1 lb a.i./acre will control plants up to 13 leaves; for plants with 14 to 19 leaves 2 lb a.i./acre is required. Plants larger than 19 leaves are not adequately controlled.
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 that are 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.
Common purslane is a prostrate summer annual that reproduces from seed, which germinates 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 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. Oryzalin (Surflan) 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.
UC IPM Pest Management Guidelines: Almond
WeedsA. Shrestha, Kearney Agricultural Center, Parlier
J. H. Connell, UC Cooperative Extension, Butte County
Acknowledgment for contributions to Weeds:
T. S. Prather, Department of Plant, Soil, and Entomological Sciences (PSES), University of Idaho