How to Manage Pests
UC Pest Management Guidelines
Weed Management For Organic Lettuce Production
Effective weed control in organic lettuce depends on an
integration of good cultural practices, careful cultivation, and hand labor.
Preventing the production of weed seed in the field before planting will reduce
subsequent weeding costs during crop production.
The first step in developing a weed management program is to
survey the planting site and identify the weeds that are there. Become familiar
with each weed's growth and reproductive habits in order 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.
Lettuce is produced in three distinct production districts
(coastal valleys, San Joaquin Valley, and the desert) during the cooler times
of the year (San Joaquin Valley and desert) and from spring to fall along the
coast. It is typically grown on double row, 40-inch wide beds, but there is an
increasing amount of acreage of 80-inch wide beds with 5 to 6 seedlines. In
addition, baby lettuce is grown on 80-inch wide beds with 24 to 32 seedlines on
It is important to locate plantings 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
areas infested with weeds with windblown seed. Some growers use transplants to
give the lettuce crop a head start on the weeds. Transplants reduce the number
of days to harvest and establish a canopy more quickly. They generally only
need one weeding instead of two, as with direct-seeded lettuce, to get through
the growth cycle. The goal of organic weed control techniques is to reduce weed
pressure and/or give the crop an advantage over the weeds in order to produce
the crop as economically as possible.
WEED MANAGEMENT BEFORE PLANTING
Crop rotations and
The previous crop can significantly affect weed
pressure in the lettuce crop. A previous crop that has had excellent weed
control generates fewer weed seed that germinate in the lettuce crop. In
addition, it is important to keep the areas surrounding the lettuce field free
of weeds that have aerial dispersed seeds such as groundsel and
germination of weeds before bed shaping.
Preplant germination of weeds (pregermination)
involves the use of irrigation or rain to stimulate weed seed germination
before planting lettuce. The
emerged seedlings are then killed by shallow cultivation, flaming, an organic
herbicide, or a combination of these treatments. Germinate and remove weeds as
close as possible to the date of planting to assure that the weed spectrum does
not change before planting the vegetable crop. Changes in the weed spectrum may
occur as a result of changes in the season or weather. The time of year, irrigation system,
and the interval between irrigation and weed control all affect the efficacy of
this technique. Waiting 14 days
after the time of preplant irrigation allows weeds to emerge and for the field
to dry enough to permit use of shallow tillage to control emerged weeds. Done
properly, this method removes up to 50% of the weeds that would have otherwise
emerged in the subsequent crop. If time permits, repeat the
preplant process to further reduce weed populations.
germination of weeds after bed shaping.
are shaped and ready to plant, water can be applied to stimulate a flush of
weeds, thereby depleting the quantity of weed seed in the top inch of soil. The
flush of weeds can be killed by shallow cultivation, flaming, or applications
of organic herbicides. Be careful to not till too deeply or additional weed
seed may be brought to the surface from deeper layers. The crop can be planted
immediately on these beds. This technique is called "stale" seedbed
weed control and can provide substantial control.
control after planting.
Flaming or organic herbicide treatments can be used to kill
the flush of weeds anytime between seeding the crop and its emergence. This
technique is particularly effective on crops that have slow seed germination.
It is not widely used on lettuce because it is generally quick to germinate
(i.e. 4-5 days in summer) and does not allow enough time to conduct pre-crop
emergence weeding operations. In addition, flaming and organic herbicides are
effective on small (i.e. less than 2 true leaves) broadleaves but not effective
on grass weeds.
Deep plowing is a tillage
technique that buries weed seed or propagules of perennial plants below the
depth at which they can germinate.
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
in order to avoid bringing up large numbers of viable weed seed back to the
use of cover crops is a key cultural practice in organic production. Cover
crops provide a variety of benefits to crop production but can potentially both
increase or decrease weed pressure in vegetable production systems.
Unfortunately, annual weeds frequently become established at the time of the
cover crop, and depending upon the species of weed, they can grow and set seed
unnoticed in cover crops. Often weed plants decompose before the end of the
cover crop cycle making their detection difficult. In such cases, the cover
crops act as nurse crops to weeds, making substantial contribution to the seed
bank. Slow-growing winter cover crops (legumes and cereal/legume mixes) can be
particularly problematic in this manner; many allow substantial weed growth and
seed set 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 crops 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 your cover crops, particularly in the first
40 days following seeding, to make sure that they are not creating a weed
problem for the subsequent lettuce plantings.
Solarization can be used in areas with sufficient solar radiation. Typically,
the soil is irrigated and then
covered with clear plastic for 4 to 6 weeks during the hot summer period. The effectiveness depends
mainly on the heat that can be generated under the plastic during a certain period. Soil solarization can
reduce populations of weed seed in the soil, as well as provide partial control of root knot nematode
and soilborne fungal pathogens. For further information, contact your local farm advisor or see UC
ANR Publication #21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and
WEED MANAGEMENT AFTER PLANTING
is one of the most effective postplant cultural practices that can be carried
out. On double row, 40-inch beds it is possible to cultivate 80% of the bed
(assuming a 4-inch wide uncultivated strip is left for each seedline).
Direct-seeded lettuce is frequently cultivated at about the two to three leaf
stage and again 2 weeks later. The first cultivation removes early emerging
weeds and later cultivations remove weeds that germinate following thinning.
The goal of cultivation is to remove weed seedlings as close to
the seed row as possible without disturbing the crop. New precision guidance systems for cultivation (i.e. EcoDan
and Robocrop) can help improve the accuracy of cultivation operations. More
precise cultivation allows for reducing the width of the uncultivated band and
thereby removing a higher percentage of the weeds. Uncontrolled weeds in the
seedline are removed by hand or other mechanical means. High density plantings
of baby lettuce on 80-inch beds with 24 to 32 seedlines are difficult to
effectively cultivate, and weed control in these systems relies on cultural
practices in the previous crop to reduce weed pressure.
Removal of weeds from the seedline can be achieved by the use of
specific weeding implements such as finger and torsion weeders. These devices
are more suited to transplanted lettuce because their mode of action is more
aggressive to the crop. They will not generally remove all of the weeds but
rather remove an increased percentage of the weeds that will make subsequent
hand weeding operations more efficient.
Hand hoeing is generally necessary in organic
lettuce. Direct-seeded lettuce is thinned and weeded 30 to 40 days after
seeding. At this time the plants are tender, and careful hand weeding is
necessary in the seedrow to safeguard the crop. Typically the initial hand
weeding and thinning operation is followed 2 weeks later by a final hand
weeding before the crop canopy closes. The use of transplants will generally
reduce the number of hand weedings to one time per crop. At present there are
no successful techniques that can substitute for careful hand weeding of direct-seeded
crops, but successful employment of the above mentioned techniques can help
make hand weeding operations less time consuming and more efficient.
UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
R. F. Smith, UC Cooperative Extension, Monterey County
S. A. Fennimore, Weed Science/Plant Sciences, UC Davis/Salinas
M. LeStrange, UC Cooperative Extension, Tulare County
Acknowledgment for contributions to Weeds:
D. W. Cudney, Botany, UC Riverside
W. E. Bendixen, UC Cooperative Extension, Santa Barbara County
C. E. Bell, UC Cooperative Extension, San Diego County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
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