A series of field experiments was conducted during 1975 and
1976 to examine peppermint tolerance and Canada, thistle response to
Dowco 290 (3, 6- dichloropicolinic acid). Peppermint oil yield and
Canada thistle density were considered most important in evaluating
experimental results.
Peppermint tolerance to Dowco 290 was studied in weed-free
peppermint (cv. Mitcham).. Rates of 0.25 lb/A or more reduced peppermint
oil yield. Time of application appeared to be less critical
than rate of herbicide as oil production decreased with increasing
rates. Mint injury was sufficiently severe at high rates to cause yield
reductions the year of treatment, but recovery was good and no reduction
in oil yield was found 1 year later.
Spring application of Dowco 290 to peppermint (cv. Mitcham and
Todd's Mitcham) infested with Canada thistles resulted in erratic oil
yield but good thistle control. Oil yield was difficult to evaluate because
of the variability of the peppermint stand. All rates of Dowco
290 tested gave good short-term thistle control and rates of 0.5 lb/A
or more gave excellent seasonal control,
There was no advantage to split applications of Dowco 290 (fall
plus spring) over a single application in the spring. An excellent
combination of good thistle control and high oil yield was obtained
from sequential treatments in the spring when 0.125 lb/A was applied
10 weeks prior to harvest and 0.063 lb/A was applied 2 weeks later.
Trans location of Dowco 290 through underground thistle parts
was demonstrated by treating parent plants and observing the response
in a connected daughter plant. The herbicide was not lethal at
rates that translocated to connected plants, but it did cause abnormal
floral development.
Dowco 290 was sufficiently active when applied at 0.25 lb/A to
control all underground plant parts of test thistles even when treated
plant parts were removed as soon as 1 hour after treatment. Root
mortality was measured by a modified tetrazolium test.
Seedling Canada thistle plants were slightly more sensitive to
Dowco 290 than plants which developed from mature rootstock. After
seedling plants had developed a more complex root system, regrowth
from rootstock of plants grown from seed and from mature rootstock
was similar
