Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 177

Figure 4.12 Spyders.(Redrawn from Schweizer,Westra & Lybecker,1994.)
cm) than most disk hillers, and this allows them to work closer to some crops.
Also, when cutting soil away from the row, they leave a loose soil layer next to
the row rather than a smooth shoulder, and this probably reduces soil drying
rather than encouraging it.
Basket weeders consist of two sets of rotating wire cages.The forward cages
are ground driven; the rear cages are driven by a chain connected to the
forward cages, and turn twice as fast. Penetration is shallow, 2 to 5 cm, but the
soil is thoroughly worked. Consequently, few small weeds escape substantial
damage even if they are not completely uprooted. The implement is unsuited
to stony ground because rocks bend the baskets out of shape and can become
caught between adjacent wires.
Two types of brush weeders are currently in use. One consists of power-
take-off driven polypropylene brushes on a horizontal shaft; these work par-
allel to the crop row (Figure 4.13). They uproot small weeds, and shear off
larger ones (Pedersen, 1990). The soil flow is primarily parallel to the row,
which, in conjunction with narrow tunnel shields (6 to 20 cm wide), allows
cultivation very close to small crop plants. Another type of cultivator is
required once the crop plants grow too large to move easily through the
shields. A second type uses pairs of unshielded brushes on rotating vertical
shafts.Melander (1997,1998) used this type to control weeds within 1.5 cm of
onion rows without damaging the crop. Fogelberg & Dock Gustavsson (1998,
1999) showed that a vertical-axis brush weeder could be used for in-row
weeding of young carrots because the weeds were more prone to uprooting
than carrots when both were in the two-to-four leaf stage. Brush weeders
resist clogging with large weeds and debris (Geier & Vogtmann, 1987), and