Integrated Weed Management

Weed control is critical to safeguard the yield of the current crop and to keep weeds from reproducing and interfering in subsequent crops. Weeds reduce celery yields by competing for light, water, and nutrients; delaying maturity; and reducing the efficiency of harvest. Weed control is especially critical in the early part of the season when celery is most susceptible to weed competition.

Weed management in celery depends on good cultural practices such as field selection, crop rotation, preirrigation followed by cultivation, in-season cultivation, and herbicides.

All celery in production fields is grown from transplants. Even planted densely (37,000 to 44,000 plants per acre), celery in the early stages of growth cannot compete well with weeds. The poor competitive ability of young celery plants coupled with the frequent sprinkler irrigations needed for plant establishment favor weed growth.

Mechanical cultivation typically removes weeds from 80% of the bed while herbicides or hand weeding remove the rest.

Both winter and summer weeds can occur during the celery-growing season. Weeds that commonly require control are annual sowthistle, burning nettle, common chickweed, common groundsel, common purslane, lambsquarters, little mallow, London rocket, nettleleaf goosefoot, redroot pigweed, shepherd's-purse, and tumble pigweed.

Perennial weeds are usually not a major problem in the intensively farmed land used for celery, except for yellow nutsedge in the southern part of the state.

Irrigation Management

Throughout the season, manage irrigation carefully to improve the control of weeds and other pests:

• Keep furrows well maintained to prevent them from collecting water. Ponding water favors the growth of weeds and aids soilborne plant pathogens.
• Regularly monitor for uniformity and adjust sprinklers or drip systems to prevent uneven distribution of water. Check emitters for clogs and leaks. Poor irrigation uniformity stresses the crop and impairs its ability to outcompete weeds.
• Schedule low-volume irrigation to avoid excess water and off-target fertilizer movement, which can favor weed growth. Use CropManage or similar decision support tools to keep track of irrigation scheduling and fertilizer delivery to the crop.

MONITORING

For effective weed management, it is essential to identify and quantify the amount of each weed species in the field.

• Gather information about weed composition in the previous crop or after the last irrigation before planting. This information can help determine preplant weed control tactics and help estimate the cost of weed control.
• Monitor weeds during each crop cycle.
• Examine fencerows, roadsides, and ditch banks, as these are other sources for weed propagules.
• Note that wind-dispersed species such as common groundsel and sow thistle may move into the production field from surrounding areas.
• If managing non-crop areas, be aware that some weeds, such as horseweed and fleabane, may be glyphosate resistant.
• Pay special attention to the location of perennial weeds such as field bindweed or yellow nutsedge, and prepare to treat them if necessary.

WEED MANAGEMENT BEFORE PLANTING

Keep field records of weed species, soil texture, and subsequent rotational crops. Using a combination of weed management methods can minimize costs while achieving optimum weed control.

Crop Rotation

Rotate celery with either cool- or warm-season crops. The best rotation crops for celery are those for which effective weed management systems are available—such as strawberry, lettuce, or broccoli. Rotating to strawberries, if possible, is especially useful in fields where yellow nutsedge is a problem since preplant fumigation can help to manage this weed.

When using herbicides, always consider crop rotation schemes that allow for maximum flexibility with plant-back restrictions.

See Cover Cropping for Vegetable Production, ANR Publication 3517, and the UC Davis Cover Crops Database for more information.

Field Selection and Preparation

Choose field sites with as few weeds as possible.

• If previous crops had heavy weed infestations, repeated soil tillage may reduce weed seed density.
• For summer-planted celery, preirrigation of preformed beds followed by minimum tillage will often reduce initial weed competition. Preirrigation will also show the composition and location of germinating weeds in the field. This procedure must be planned in advance to accommodate planting schedules.

Herbicides

Use glyphosate or pelargonic acid to control small weeds that have emerged before transplanting celery. Apply oxyfluorfen to fallow beds up to 30 days before transplanting but the beds must be worked thoroughly before transplanting.

Apply prometryn to shaped beds at least 21 days before transplanting.

WEED MANAGEMENT AT PLANTING

Consider applying S-metolachlor, bensulide, and trifluralin before transplanting celery. Trifluralin must be incorporated 2 to 3 inches deep, which may limit its use in winter-sown celery because of wet soils. Applying S-metolachlor to shaped beds before planting is effective on some broadleaf and grass weeds and is particularly useful for controlling yellow nutsedge.

WEED MANAGEMENT AFTER PLANTING

The soil surface must be wetted below the planting depth with repeated sprinkler or drip irrigation. (Repeated applications of water during the period immediately following transplanting can limit access to the field.) After celery plants are established, extend the irrigation interval so that the field becomes accessible for weeding and cultivation.

Hand-hoe to remove remaining weeds between the plants after applying herbicides or tilling. Depending on weed density, this practice may involve 10 to 15 labor-hours per acre.

For maximum weed emergence before herbicide application, make sure celery transplants have new leaves developing from the crown (usually 3 to 4 weeks after planting).

Irrigate by sprinkler 24 to 48 hours after herbicide application to enhance the weed control of most herbicides. Drip irrigation is just as effective as sprinklers for weed germination if there is adequate bed-wetting.

Herbicides

• Herbicide applications after planting can provide preemergence and postemergence weed control. They are more selective in efficacy in clay soils than in sandy soils in controlling broadleaf weeds such as little mallow, redroot pigweed, lambsquarters, burning nettle, shepherd's-purse, and hairy nightshade. Prometryn and linuron provide effective postemergence weed control of many broadleaf weed seedlings. Both can be applied after transplanting, over the top of transplants. Apply prometryn 14 days after transplanting. Although prometryn and linuron can cause detectable plant injury, celery usually outgrows the injury without a significant reduction in yield or quality. In drip-only irrigated celery, linuron and prometryn applied 3 to 4 weeks after transplanting provide excellent control of most broadleaf weeds without significant crop injury.
• Prometryn is effective on many broadleaf weeds (including little mallow) when applied postemergence.
• Linuron provides partial control of emerged yellow nutsedge.
• S-metolachlor can be applied as a directed or broadcast application following transplanting celery and can control emerged shoots of yellow nutsedge.
• Sethoxydim or clethodim can be applied to control grass weeds.
• Carfentrazone-ethyl can be applied with a hooded sprayer to control the weeds in the furrows between the rows.