Canada Fleabane
Table of Contents
- Description
- Distribution
- Biology and Ecology
- Economic Impact
- Control
Description
Canada Fleabane (Conyza canadensis or Erigeron
canadensis) is a winter or summer annual with a short taproot, and
a rosette of dark green, sparsely hairy leaves with smooth or slightly
toothed margins (Figure 1). It produces one
or more erect, flowering stems, 10-180 cm tall, with numerous, narrow
leaves crowded together on the stem (Figure 2). The daisy-like
flower heads are very small (3-5 mm in diameter), and numerous on
short branches near the top of the main stem (Figure 3). The seeds are
1-2 mm long, almost transparent, and have an attached pappus, or parachute,
which carries the seed for long distances on the wind much like a dandelion
seed. Other common names of this weed are horseweed or mare's-tail (both
used in the United States), and vergerette du Canada.
Figure 1. Basal rosette
of Canada Fleabane. (Reproduced with permission from "Guide to Weeds
of Quebec" MAPAQ, 1999)
Distribution
Canada Fleabane is found south of latitude N 55 in all
provinces of Canada except Newfoundland. It is widespread throughout the
United States and is also common in Europe, Australia, and Japan. For
many years, it was found primarily in orchards, vineyards, along roadsides
and railways, and on abandoned agricultural land or forest clear-cuts.
With the reduction in tillage on many farms, it now increasingly occurs
as a weed of arable fields where there is no soil disturbance to interrupt
its life cycle. It can be found on all soil types, including organic soils
but it is most common on coarse textured soils.
Figure 2. Bolting
stem of Canada Fleabane. Note that as the flower stalk elongates, the
basal rosette deteriorates. (Reproduced with permission from "Guide
to Weeds of Quebec" MAPAQ, 1999)
Biology and Ecology
Most seedlings of Canada Fleabane emerge from late August
through October, and form rosettes that overwinter. A few seedlings emerge
in spring, from March through early May. Flower stalks begin to elongate
in May and plants bloom in mid-July. Seed production peaks in early August
and continues into September, after which the plants die. The number of
seeds produced per plant is proportional to stem height. A plant 0.4 m
tall produces about 2,000 seeds, while a plant 1.5 m tall produces about
230,000 seeds.
Figure 3. Canada
Fleabane inflorescence.
Seeds of Canada Fleabane are not dormant at maturity, and
germinate readily in autumn, with a smaller flush the following spring.
Germination is greatest when seeds remain on the soil surface. Studies
show that germination and emergence are reduced by burial and by crop
residue. The seeds form only a short-term seed bank. They can survive
for more than 1 year, but probably not more than 3 years under most conditions.
Economic Impact
Data on the effects of Canada Fleabane on crop yield and
quality are scarce. A Michigan study estimated soybean yield losses of
83% from 150 Canada Fleabane plants per metre squared in a no-till cropping
system. There are also reports that dense populations of Canada Fleabane
reduced sugar beet yields in Germany by 64% and the growth of new spring
buds of grapes in Italy by 28%. In these crops it occurred as a winter
annual. On the other hand, in carrots and onions on organic soils in Quebec,
where it grows as a summer annual after spring tillage, its effects on
harvesting efficiency are more detrimental than its effects on crop yield.
Plants that emerge in autumn generally attain a larger size, produce more
seeds, and are more competitive with crops than spring emerging plants.
Canada Fleabane is a wild host of the tarnished plant bug,
which causes injury to many crops. It is also an alternate host for the
alfalfa plant bug, and for aster yellows, a disease transmitted to a wide
range of host plants by the aster leafhopper.
Control
Rosettes of Canada Fleabane are easily controlled by either
fall or spring tillage, and therefore it is not generally a weed problem
in conventional tillage systems. Shallow disking usually provides adequate
control. Management practices that delay autumn emergence, such as increasing
crop residue cover, may reduce winter survival and therefore population
the following spring. A rye cover crop can reduce germination and emergence
of Canada Fleabane, possibly due to chemicals exuded by the roots. Mowing
pastures or hay crops can prevent or delay seed production. Including
spring barley as a rotational crop reduced populations of Canada Fleabane
in onions or carrots on organic soils in Quebec.
Control of Canada Fleabane with postemergence herbicides
is best in the fall or early spring when rosettes are small and actively
growing and before stem elongation. Non-selective herbicides containing
glyphosate or glufosinate ammonium are generally effective. In May or
June, when stems have started to elongate, plants are less susceptible
to herbicides and control is more variable. Good postemergence control
has been reported with chloransulam-methyl or chlorimuron-ethyl in soybean,
with products containing dicamba in corn, with bromoxynil/MCPA) in cereals,
and with clopyralid in many horticultural crops, as well as in non-crop
areas.
Preemergence control of germinating seedlings can be obtained
with products containing flumetsulam, metribuzin, pendimethalin, isoxaben
simazine, and isoxaflutole. In no-till soybeans, where Canada Fleabane
can be a problem, pre-plant application of glyphosate products plus flumetsulam
or metribuzin will provide control of both rosettes and later germinating
seedlings. Consult the latest edition of OMAF Publication 75, Guide
to Weed Control, for information concerning crop registrations and
rates of these products.
Canada Fleabane has evolved resistance to various herbicides
in many parts of the world, including Ontario. Populations in several
orchards in Essex County, Ontario, became resistant to paraquat after
years of continuous use. In many European countries, Canada Fleabane has
developed resistance to atrazine or simazine. In Delaware, U.S.A., Canada
Fleabane plants resistant to glyphosate were discovered in several no-till
soybean fields where glyphosate had been used repeatedly. In Ohio, resistance
to ALS inhibiting (Group 2) herbicides has been reported for Canada Fleabane.
Rotating herbicides as well as crops, and following other integrated weed
management practices, will help prevent the development and spread of
resistant plants.
