Sown crops show slow initial emergence and growth, and consequences are
very sensitive to weed competition. Transplanting allows a more thorough
mechanical weed control and gives to the plant a competitive advantage against
weeds; Late-season threshold (minimum weed free period) was 5 weeks after
emergence(Weaver et al; 1987).
The critical periods for weed control
was between 30 and 45 days after emergence (Duranti and Carone, 1983) 24 and 36
days after emergence(Weaver,1984; Weaver and Tan, 1983),24 and 36 days
after transplanting (Friesen,1979;Weaver
and Tan,1983).
The critical weed free period for common
lambsquarters and pigweed in cucumbers appears to be 14-32 days after
emergence. Interference from weeds during that period of growth greatly reduces
yield and quality. it also appears that,if as little as 5% of the weeds are
left to compete with cucumbers, a yield reduction of more than 50% can occur.
There is weed interference with cucumber growth by direct competition for
light, nutrient and soil moisture and there is evidence that interaction
involving allelopathetic substances released from weed may inhibit cucumber
growth and development. One of the greatest problem with weeds is their ability
to interfere with hand and mechanical harvest of cucumbers. Therefore critical
period of interference appears to be between two (2) and four (4) weeks after
emergence. When as few as5% of the weeds are allowed to remain in cucumber
longer than 3weeks yields are often reduced significantly. In a1999 North Carolina
60% of growers responding to a survey use herbicides. Common cocklebur are the
most troublesome weeds in spring and fall crops. From the survey, cucumber are mechanically cultivated 3 times and hand
weeded twice .Ethalfluralin (curbit) is used, pre emergence on 54% of the
acreage and nptalam (alanap)is used pre emergence on only 10% of the acreage.
Sethoxydin (poast) is used post emergence on 9% of the cucumber acreage. Use 0f
the naptalam or sethoxydin or more acreage would lessen the negative effect
from weeds onn the total yield of cucumber in North Carolina. Objective of
their study was to evaluate new herbicides in cucumber; ethlfuralin has given
the best (greater than 90 pre emergence control of common lambsquarters ,
annual grasses and pigweed).
The minimum weed free period (when the
crops must be kept weed free to prevent yield losses to weed competiion) in
cucumber, squarsh and other cucurbit crops has been estimated as the first 4 to
6 weeks after planting (friesen,1978;Nobble,2009;Peet,1996;Stall,2009;Rorse,horticulture
Dept;Virginia tech Pers.commum). vine- forming cultivars usually pass through
their minimum weed free period when the
vines begin to occupy the inter-row spaces. Delays in crop development lengthen
the minimum weed free period, and may result from;
1.
Chose
of crop varieties not well adapted to local climate and soil
2.
Poor
seed quality
3.
Cool
or wet soil conditions
4.
Inadequate
soil moisture, nutrients or organic matter
5.
Heavy
pest or disease pressure.
The crop is most
vulnerable to weed competition during the emergence and establishment phase,
and is also easiest to keep clean through cultivation. The phase of rapid
vegetative development is most challenging for weed control in cucumber ,melon
and winter squarsh, because the spreading vine
make it increasingly difficult to cultivate without damaging the crop. Although cucurbit became less
vulnerable to weed competition after the vine run out (or when bush varieties
become less well established),the crop may take a few more weeks ot close
canopy, weeds emerging in a mature cucurbit crop usually have little direct
impact on yield, and a well developed crop canopy can suppress the growth of
newly emerging weeds. Weeds that emerge during this time may complicate harvest
by concealing fruit or hampering manual picking with prickly foliage or
entangling vines , and promotes fungal diseases by limiting air circulation.
Horsenettle and black nightshades are host for the cucurbit powdery mildew
fungus(erysiphe cichoracearum), and many common weeds(pigweed, nightshades,
purelane, bindweeds, etc) can carry cucumber mosaic virus (CMV).closer row and
within-row spacing promotes canopy closure and weed suppression.