It may look pretty, but field violet is becoming a troublesome weed.
November 15, 2007 By Rosalie I. Tennison
Just when growers think they have effective controls for most weeds, they find
something new in their crop. Field violet is becoming a problem in reduced tillage
fields in central Alberta and there are indications that the weed is spreading
quickly. Now, research completed at the University of Alberta is providing more
information on field violet and is offering some guidelines for control.
Field violet looks similar to what gardeners refer to as a 'Johnny-jump-up';
however, the weed is distinguished by its petals, which are shorter than its
sepals and blandly coloured. Often, infestations will form a mat of stems, which
may hamper crop establishment and harvest. Native to Europe, field violet may
have been introduced through gardens.
"We began hearing that field violet was becoming a problem weed a few
years ago," says Rory Degenhardt, the researcher who led the Alberta study.
"Our research led us to believe that the greatest threat is its potential
to spread across the province. We have anecdotal evidence from growers suggesting
that field violet competes with crops for moisture and nutrients, which reduce
yield; however we observed only minor yield loss in our experiments."
Grow competitive crops
Degenhardt and his colleagues conducted a two year study to learn more about
field violet and to determine if there are herbicides that will effectively
control it. Much of what was learned centred on the timing of crop establishment
and how that could affect the establishment of the weed. The researchers documented
the development of field violet in wheat and canola and they discovered that
if the crop is able to establish rapidly and vigorously, the weed will have
less impact on crop development.
Degenhardt believes that field violet may be an even bigger problem in some
of the weakly competitive pulse crops, although his research did not cover these
crops. He also adds there is documentation that field violet has become a serious
problem in horticultural crops when the weed escapes control methods.
According to Degenhardt, field violet is cold, shade and drought tolerant and
can exhibit annual, winter annual and perennial life cycles. The plant can produce
thousands of seeds that may persist in the soil for many years. Emergence can
occur intermittently throughout the growing season, however there are often
emergence peaks in early summer and the fall. Field violet plants are hardy
and can survive a variety of adverse conditions.
Even though Degenhardt says he found little indication that yield is significantly
affected by field violet, he suggests that a dense carpet of the weed could
delay crop maturity and reduce the quality of the harvested product. He adds
more research is needed to determine exactly how yield is affected.
"If the crop gets a good start, field violet is a weak competitor,"
explains Degenhardt. "Therefore, cultural practices to improve crop competitiveness,
such as altering the seeding date to exploit spring moisture or selecting competitive
cultivars, may be all that is required."
Herbicides show potential
In well-established infestations of field violet, Degenhardt suggests using
a broadleaf herbicide to control the weed. He says photosynthesis-inhibiting
products, such as Basagran and Lorox, have little activity on the weed, which
would make those products poor choices for control. He says the most effective
products in wheat are Attain, Ally, Refine Extra and Sundance. Sencor also provided
moderate control, although it was not as effective at reducing seed production
from field violet.
Control of field violet in canola is limited to glyphosate in Roundup Ready
cultivars. An application of Vantage Plus at a 1/2L/ac rate offered season-long
control in Roundup Ready canola. Degenhardt found no products that provided
in-crop control in conventional canola and found that Liberty only suppressed
weed growth in Liberty Link (glufosinate tolerant) canola.
"The best treatment for conventional and glufosinate tolerant canola cultivars
is a pre-seeding application of Vantage Plus at 1/2L/ac because it gets the
winter annual field violet and gives the crop a head-start," explains Degenhardt.
"If the weeds come up later, they can't compete."
Degenhardt offers some useful advice when planning a weed control program.
"When it comes to controlling field violet, growers should consider their
entire weed spectrum," he advises. "Field violet is not very competitive
in either wheat or canola, so base your herbicide selection on your knowledge
of all species present in the field."
Before growers dismiss field violet as a weak, but pretty, plant, they should
take note of it in the field and monitor its distribution. More information
is needed about the effects that field violet can have on crops, but growers
would be wise to know where it is and control it when possible. In this manner,
it can be curtailed before an annoyance becomes a much larger problem. -30-