Weeds compete with newly planted nursery stock. Deep-rooted perennial weeds can reduce vigor and render a mature product unmarketable due to laws on transporting noxious weeds. Consequently, weeds are controlled with herbicides either broadcast-applied in closely spaced crops, or within the tree or shrub row for wider spaced crops, with alleys being mowed or flailed.
Weed shifts Repeated use of simazine was first demonstrated, in nurseries, to cause a shift within a natural population of common groundsel to a resistant or tolerant biotype. Routine cultural practices also cause such a weed shift. Examples include prostrate weeds that tolerate flailing, deep-rooted or easily spread perennials that tolerate cultivation, and weeds that resist repeated applications of the same or similar herbicide treatments. A significant weed shift noted over the last decade is the rapid increase in marestail or horseweed in many nurseries. Speculation for the cause of this included glyphosate resistance, but recent screening of populations of the Willamette Valley and the Columbia Basin did not find elevated levels of glyphosate tolerance.
Preventing weed shifts Weeds that survive cultivation, mowing or flailing, specific herbicide treatments, or other routine cultural practices must be eliminated before the tolerant species or biotypes become established. Combine a variety of weed control practices or treatments; rotate fields, cultural practices, and herbicides; and spot treat with a hoe or registered herbicide when the weed first appears. Also, clean equipment when moving from an infested field.
Sod covers In specimen tree plantings, manage either native or planted grasses in row middles to reduce soil erosion and improve traffic conditions during wet weather. In addition, soil moisture and nutrient retention are improved along with increased water infiltration rates for many sods. New sod varieties, including dwarf cultivars that respond to minimal management practices such as drought, low fertility, or sublethal rates of postemergence herbicides, are being introduced into various horticultural cropping systems. Consult your local Extension agent, or request recent information about living mulches and their management.
Managing weedy vegetation Successful weed control in nurseries requires a comprehensive, year-round approach that uses and alternates a combination of weed control practices over several years. Developing these strategies requires knowledge of each weed and weed control practice. Weeds must be identified and information gathered about the effectiveness of each weed control practice. Consider costs and select herbicide combinations that may be applied together or in split applications that control all weeds present in the nursery. Note the action of each herbicide or how the chemical works in the plant. Then tank mix and alternate use of these products to reduce the chance of developing resistant species or biotypes. Often a combination of mechanical, herbicidal, and sometimes hand-removal or spot-treatment with herbicide sprays or wipers, will provide the most effective year-round control.
Soil-active (preemergence) herbicides Persistent, soil-applied herbicides can be applied to weed-free soil during winter when rain will activate the chemical; some compounds may be applied throughout the year. Apply lower rates on sandy soils having lower clay, organic matter, or cation exchange capacities. Existing vegetation can be controlled by mixing a postemergence contact or translocated herbicide. Consult label for listed species and duration of expected control.
Postemergence herbicides In nursery production, postemergence weed control requires precision. Postemergence treatments either selectively control susceptible weeds or are applied with selective equipment.
Frequent scouting to identify susceptible weeds and the correct stage of weed growth must be combined with appropriate weather conditions and labeled spray additives to maximize control. Consult labels for numerous precautions or information about crop or cultivar tolerances. Due to the multitude of nursery crops, always verify selectivity on a few plants before treating the entire block or field.
Note Herbicides must be applied at the correct rate and time to selectively control weed growth. For band applications under tree rows, reduce quantity of herbicide applied proportionally to the area within the row actually sprayed.