Weed Management for Organic Celery Production

Effective weed control and prevention in organic vegetable productions depends on an integration of sound cultural practices, careful cultivation, and hand labor. Preventing the production of weed seed or propagules of perennial weeds in the field before planting, coupled with in-crop control, will deplete the seedbank and reduce future weed management costs. 

The first step in developing a weed management program is to survey the planting site and identify the existing weeds, their densities, and distribution in the field. Become familiar with each weed’s growth and reproductive habits to choose the most effective management options. For help in identifying common weeds, see the weed photo pages that are linked to the weed list in the section Common and Scientific Names of Weeds.

It is beneficial to plant organic celery in fields with low weed pressure or to use weed management methods that reduce weed pressure before planting. It is also important to keep areas near production fields free from weeds with windblown seed. Transplants give celery a head start competing with germinating weeds while a vigorous celery canopy development limits resources for weeds. The goal of organic weed control techniques is to give the crop an advantage over the weeds in order to produce the crop as economically as possible.

Equipment sanitation can minimize or eliminate the introduction of weeds into organic fields. After using equipment in weedy fields, pressure-rinse it to remove soil and help prevent weed seed or perennial structures from moving to new sites.

WEED MANAGEMENT BEFORE PLANTING

Crop Rotations and Field Sanitation

The previous crop can significantly affect weed pressure in a celery crop. A previous crop that has had effective weed control means that fewer weeds will germinate from the seedbank during celery production.

Keep the areas surrounding the field free of weeds that have aerially dispersed seeds, such as groundsel and sowthistle.

Preplant Germination of Weeds

Preplant germination of weeds (pregermination) involves the use of irrigation or rain to stimulate weed seed germination before planting the crop. The emerged seedlings are then killed by shallow cultivation or flaming. The time of year, irrigation system, and the interval between irrigation and weed control all affect how well this technique works. Preplant germination can take place before or after beds are formed but must take place before a crop is planted.

Done properly, this method removes up to 50% of the weeds that would have otherwise emerged in the subsequent crop:

1. Irrigate the field to germinate weeds at least two weeks before planting to assure that the weed spectrum does not change before planting. Changes in the weed spectrum may occur because of changes in the season or weather, such as from winter weeds to summer weeds.
2. Allow weeds to emerge and the field to dry enough to permit shallow tillage.
3. Cultivate as shallowly as possible to avoid bringing up dormant weed seed from deeper soil layers.
4. Shape the beds and apply water to stimulate another flush of weeds.
5. Kill the weeds using shallow cultivation, flaming, or applications of organic herbicides. Cultivate as shallowly as possible to avoid bringing up dormant weed seed from deeper soil layers.
6. Plant the crop immediately on these beds.

Deep Plowing

Deep plowing is a tillage technique that buries weed seed or propagules of perennial plants below the depth at which they can germinate or sprout in the case of perennial weeds. The viability of buried weed seed declines over time; longer intervals between deep plowing and subsequent deep plowing (i.e., 3–5 years) are preferred to avoid bringing up large numbers of viable weed seed back to the soil surface.

Consider sampling underlying soil for analyses of salts and nutrients before inverting the soil with deep plowing.

Cover Crops

Cover cropping is an important cultural practice in organic production. Cover crops can provide a variety of benefits to crop production but can, potentially, both increase or decrease weed numbers in vegetable production systems.

In noncompetitive cover crops, weeds can produce seed and add seed to the seedbank. Slow-growing winter cover crops (legumes and mixes of cereals and legumes) can be particularly problematic; many allow significant weed growth and set seed early in the growth cycle of the cover crop. Fast-growing winter cover crops, such as cereals and mustards, provide complete ground cover in the first 30 days of the cover crop cycle and are better able to compete with weeds. Competitive cereal and mustard cover crop varieties include Merced rye (Secale cereale), white mustard (Sinapis alba), and Indian mustard (Brassica juncea). An adequate seeding rate is also an important factor in providing for rapid ground cover. It is important to monitor the cover crops, particularly in the first 40 days following seeding, to make sure that they are not creating a weed problem for subsequent plantings. The competitive ability of cover crops with weeds along with other benefits are discussed in detail in the Cover Cropping for Vegetable Production handbook.

Soil Solarization

Solarization can be used in areas with sufficient solar radiation such as the Central Valley and low desert. In coastal regions where celery is produced, solarization typically does not generate soil temperatures lethal to most germinating weeds. For further information, contact your local farm advisor, or see UC ANR Publication 21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds.

WEED MANAGEMENT AFTER PLANTING

Cultivation

Cultivation is one of the most effective postplant weed control practices that can be performed. Organic vegetable crops are typically cultivated at least twice. Traditional interrow cultivation can remove weeds in 80% of the field while leaving a 4-inch-wide uncultivated strip around each seedline. New intelligent cultivators and laser weeders can cultivate in the seedline near the crop plants and remove nearly all of the weeds between crop plants. Intelligent cultivators will miss some weeds obscured by the leaves of celery transplants. As a result, they generally do not remove all of the weeds but rather remove a large percentage, thus increasing the efficiency of subsequent weeding operations. In addition to intelligent cultivators, mechanical devices such as finger weeders and torsion weeders can also remove a majority of the weeds very close to the crop without damaging it.

Hand Hoeing

Hand hoeing is essential in organic vegetable crops. Typically, celery is weeded 20 to 40 days after transplanting.