A single harvest reduced biomass to 10 to 25
watermilfoil in Wisconsin.
cent of the original level; three harvests 1 month apart essentially
Reduced growth in the following year was most
all plant material.
apparent in plots harvested three times in the preceding summer, and was
apparently due to cutting of root crowns.
Wile et al.
Wile,
and Conyers and Cooke (1983) also found excellent control in
and Beggs
northern lakes if the cutter bar was operated in the sediments so as to cut
Cooke and
(1986) investigated the effects of harvesting
root crowns.
frequency and technique on regrowth of a dense infestation of
spicatwn.
They found that season-long control of this plant could be achieved
with one harvest if the plant root crowns were also harvested.
By way of con-
trast, Anderson (1984) followed the regrowth of Eurasian watermilfoil, in
Reservoir, Ohio) as Cooke and
another area of the same reservoir
after the plants had been harvested with the traditional
method in which
to 5-cm stumps are left and few if any root crowns are
Anderson found that the milfoil biomass in the harvested area
removed.
equaled the original biomass and the biomass of a control area within 21 days.
While the technique of cutting and removing root crowns may be more
consuming and can produce damage to cutter blades, the harvested area may not
Further testing of this approach is needed.
require a second harvest.
Some macrophyte species are more affected by harvesting than others.
Nicholson (1981) has suggested that harvesting promoted milfoil growth in
Chautauqua Lake (New York) because this plant can spread and become estab-
lished from fragments.
Other species, such as the pondweed Potamogeton, which
can be a severe nuisance in reservoirs, are susceptible to harvesting because
they emphasize sexual reproduction and regenerate poorly from fragments.
Pondweeds therefore might be replaced by milfoil in harvested reservoirs where
both species are present.
Efficiency is related to the density of plants, the size of the area to
be harvested, the number of harvesters available, the presence of obstacles in
the water, and the distance of disposal sites from harvesting areas.
For
example, some lakes in British Columbia
Cooke et al. 1986) have
very narrow bands of dense milfoil beds along their length.
Harvesting was
Personal Couununication, 1986, P. R.
British Columbia Ministry of
Environment, Vancouver, BC.
118
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