Canada Fleabane

Agdex#:	640
Publication Date:	10/02
Order#:	02-067
Last Reviewed:
History:
Written by:	Susan Weaver - Agriculture & Agri-Food Canada

Table of Contents

1. Description
2. Distribution
3. Biology and Ecology
4. Economic Impact
5. 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)

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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)

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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.

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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.

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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.

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