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Introduction
Dandelion is a troublesome perennial weed in annual crops, and since the 1980s the relative abundance of dandelions has increased significantly in western Canada. Its distribution in fields is not necessarily related to the level of tillage practiced within that field but more likely to the cropping history, particularly if that history includes perennial forage in the rotation in which dandelion remained uncontrolled (Frosese and Van Acker 2003). Dandelion is very competitive weed; if remaining uncontrolled, it has the potential to cause significant yield losses in cereal and oil-seed crops (Frosese and Van Acker 2003; Hacault and VanAcker 2005).
Dandelion populations consist of several different genotypes. High seed producing types tend to invade disturbed areas while the lower seed producing, larger, more competitive types, invade pastures and turf. While both types can be found in direct seeded fields, the larger, more competitive types tend to be more difficult to control.
Description
Dandelion is an almost stemless lactiferous perennial herb. The leaves are clustered in a basal rosette; the leaf margins of dandelion are deeply lobed and toothed, generally 5 – 40 cm long and 0.7 – 1.5 cm wide. The leaf surface may be smooth or sparsely hairy. Dandelion inflorescence is bright yellow, up to 2 – 5 cm in diameter, and is produced at the top of hollow leafless stalks. The seeds are brown with a long tip and pappus and are disseminated by wind.
Growth and Reproduction
Dandelion has a deep, penetrating taproot that may be widely branched. Buds grow from the uppermost area of the root, producing a crown, which can divide to form numerous branches capable of regenerating, developing new shoots.
Dandelion is a prolific seed producer; each plant can produce 54 to 172 seeds per head, and a single plant can produce 2170 to 23,000 seeds (Stewart-Wade et al. 2002). Most seeds germinate soon after dispersal. Dandelion seeds have no seed dormancy when produced and may remain viable up to 9 years.
Dandelion seeds germinate at or near the soil surface under cool moist conditions (Boyd and Van Acker 2003) when the soil temperature is at least 10 °C, usually from mid June to mid July. The seeds germinate best under a protective crop canopy. During the first growing season, seedlings produce only basal rosettes that quickly develop a taproot with well-developed crown. These rosettes over-winter, resume growth in the early spring of the second growing season, and then produce an inflorescence that ripens quickly and produces white, round "puffballs" of wind-dispersed seeds. The main flowering period is April to May with some flowering in late summer. Some plants may flower any time during the growing season.
This weed can also reproduce vegetatively. Roots of the mature plants that are cut during tillage or seeding operations are capable of producing shoots and roots within 1-2 weeks. However, dandelions mainly spread through seed dispersal in the fields.
Control Measures
In direct seeding systems, dandelions should be managed using a systematic approach to prevent seedlings from developing into mature plants. This approach includes selective use of herbicides along with integrated weed management practices that prevent seedling establishment.
Cultural Options
Monitoring of fields and field margins – Dandelions produce large quantities of seed, and the seed is spread by wind. Therefore, monitoring non-infested fields for dandelions and controlling dandelions before they set seeds in the field margins and fencerows is important.
Crop competition – Dandelion germination may be suppressed with good crop competition. It is important to use certified seed and competitive crop cultivars for planting.
Herbicide Options
Dandelion seedlings and rosettes are most susceptible to systemic herbicides like glyphosate, glyphosate + Express TNG (tank-mixture), PrePass, 2,4-D and MCPA. There are five situations where dandelions can be controlled/suppressed with a herbicide application in a direct seeding system – pre-seeding (weed burn-down); early spring in-crop; pre-harvest; post-harvest, and termination of perennial forage.
1. Pre-seeding Application:
Glyphosate applied prior to seeding is registered for dandelion control in all crops. An application at 0.9 kg a.i/ha is registered for dandelion rosettes that are less than 15 cm (6 inches) in diameter; rates of up to 1.8 kg a.i./ha are recommended if dandelions are greater than 15 cm in diameter. The best results occur from applications earlier in spring because younger dandelions develop rapidly; later they become more difficult to control.
Research carried out at the University of Manitoba has shown that pre-seeding applications of glyphosate have variable results on dandelions. Froese et al. (2005) found that pre-seeding applications of glyphosate applied at 0.9 – 1.8 kg a.i./ha reduced dandelion shoot biomass by only 60 – 64%. In another multi site-year study in MB, a pre-seeding application of glyphosate at 1.35 kg a.i/ha also showed variable results on dandelion control in the first year (2003); the dandelion biomass was reduced by more than 95%; however, in 2004 trials, dandelion control varied from 65 – 95 % (Hacault 2006).
The other herbicides registered for dandelion control prior to seeding of barley, oats and wheat are glyphosate + Express TNG and PrePass. Similarly, the performance of these herbicides on dandelions is inconsistent. In a University of Manitoba study, Hacault and Van Acker (2006) have shown pre-seeding applications of PrePass reduced dandelion biomass by 60 – 90%, whereas a glyphosate plus Express TNG tank mixture reduced dandelion biomass by 73 – 95 %.
2. Early Spring In-crop Application
Dandelion spring rosettes can be controlled in-crop with several herbicides. However, dandelion seedlings germinate after the in-crop application window and, therefore, should be controlled in fall (prior to harvest or after harvest).
Dandelion Suppression in Cereals -- In-crop suppression of dandelions relies on good crop competition. Several herbicides are registered in cereals for in-crop suppression of dandelions (Table 2). From the long-term control standpoint, however, these herbicides are inadequate. While they do provide suppression of top growth or control of over-wintered rosettes, there is no control of seedlings that usually emerge after in-crop herbicide application; by harvest time, they are in rosette form ready to over-winter. In a system where more than one application is needed for dandelion control, the chemicals listed in Table 2 can be successfully utilized for initial in-crop suppression; then, prior to harvest or post-harvest, dandelions can be controlled with glyphosate-based products.
Dandelion Suppression in Herbicide Resistant Canola – Herbicide resistant crops offer a new option for in-crop suppression of dandelions. This technology allows dandelions to be sprayed later in the season than previously possible. Single or sequential applications of glyphosate at 0.45 kg a.i./ha are registered for dandelion suppression in Roundup Ready canola. Froese et al. (2005) found single in-crop applications of glyphosate applied at 0 – 3 leaf stage of Roundup Ready canola reduce dandelion shoot dry matter by only 32%. Repeat applications of glyphosate applied at 0.45 + 0.45 kg a.i/ha provided only 57% control of dandelions.
Liberty is also registered for the control of dandelion rosettes in Liberty Link canola.
Table 1. Registered Herbicides for In-crop Dandelion Suppression in Cereals
|
Products
|
Barley
|
Wheat
|
Oats
|
Rye
|
Type of Control*
|
|
Spring
|
Durum
|
Winter
|
|
2,4-D
|
X
|
X
|
X
|
X
|
X
|
X
|
Top growth
|
|
2,4-DB
|
|
X
|
X
|
|
X
|
|
Top growth
|
|
Attain
|
X
|
X
|
X
|
|
|
|
Over-wintered rosettes
|
|
Banvel + 2,4-D
|
X
|
X
|
X
|
|
|
|
Top growth
|
|
Banvel + MCPA
|
X
|
X
|
X
|
|
|
|
Top growth
|
|
Curtail M
|
X
|
X
|
X
|
|
X
|
|
Over-wintered rosettes
|
|
Frontline 2,4-D
|
|
X
|
X
|
|
|
|
Top growth
|
|
Frontline
|
X
|
X
|
X
|
|
X
|
|
Over-wintered rosettes
|
|
MCPA
|
X
|
X
|
X
|
X
|
X
|
X
|
Top growth
|
|
Prestige
|
X
|
X
|
X
|
|
|
|
Over-wintered rosettes
|
|
Prevail
|
X
|
X
|
X
|
|
|
|
Over-wintered rosettes
|
|
Spectrum
|
X
|
X
|
X
|
|
|
|
Over-wintered rosettes
|
3. Pre-harvest Application
Pre-harvest applications of glyphosate applied at 0.9 kg a.i./ha are registered in western Canada for dandelion control in barley, canola (including rapeseed), dry beans, flax (including Solin), lentils, oats, peas, soybeans and wheat. Inconsistent effects of pre-harvest glyphosate on dandelions have been reported in the literature. Research carried out in zero-tillage cropping at Indian Head, SK, has shown that pre-harvest glyphosate applied at 0.9 kg a.i/ha provided greater than 90% control of dandelions in lentil and wheat crops. Similarly, based on 18 field experiments across Canada, a pre-harvest application of glyphosate at 0.9 kg a.i/ha provided an average of 87% control. The range of control in these trials were 75 – 96% (Monsanto’s in-house Research, 1995). In contrast, a pre-harvest application of glyphosate in MB provided only partial control of dandelions in direct seeded wheat (Froese et al. 2005).
4. Post-harvest Application
Dandelions are low growing plants and, under proper residue management practices, their growth is not affected by harvest operations. Dandelions can tolerate moderate frost and remain actively growing in late fall (mid September). According to University of Manitoba studies, fall (prior to harvest or after crop harvest) is the best time to control dandelions. There are two reasons for this:
1) there is an increased translocation of herbicides into the roots along with net assimilates as plants get ready to over-winter; 2) most of the dandelions (seedlings that have emerged after in-crop herbicide application and plants that have emerged from root-stocks of the surviving rosettes) have reached their peak emergence (Van Ackers et al 2005). A number of herbicides, such as glyphosate, glyphosate + Express TNG tank-mixture, glyphosate + dicamba (tank-mixture) and 2,4-D are registered for fall dandelion control in stubble.
Glyphosate or mixture of glyphosate + Express TNG are very effective on dandelions. In University of Manitoba field trials, a post-harvest application of glyphosate at 0.9 kg a.i./ha rate provided excellent (92 - 99%) control of dandelions (Froese et al. 2005). Fall application of glyphosate (0.45 kg a.i./ha) and Express TNG (7.5 a.i/ha) mixture also provided superior control of dandelions than glyphosate alone (Hacault 2006).
Dandelion Control in Perennial Forage
There are no herbicides available for control of dandelions in alfalfa. In grass pasture and hay fields, dandelions can be suppressed or controlled by 2,4-D, dicamba, dicamba + 2,4-D (tank mixture), Escort, Grazon, MCPA and Remedy. If there is a rotation out of hay/forages, glyphosate applied at 1.35 – 1.8 kg a.i./ha, 3 – 7 days prior to the last cut can be used for dandelion control/suppression (Table 2).
Another herbicide, Amitrol 240, is registered at rates of 1.7 to 6.6 L/acre for alfalfa and clover renovation and dandelion control.
Table 2. Dandelion Control with Glyphosate – 3 – 5 days prior to last cut of Hay/Forage Rotation. Summary of 13 field experiments
|
Treatments
|
Rate (Kg a.i/ha)
|
Dandelion Control (%)
|
|
Mean
|
Range
|
Consistency of 85% + Control
|
|
Glyphosate
|
0.9
|
70
|
50 – 90
|
31
|
|
Glyphosate + 2,4-D
|
0.9 + 0.5
|
68
|
47 – 93
|
31
|
|
Glyphosate
|
1.35
|
82
|
63 – 93
|
82
|
|
Glyphosate
|
1.8
|
85
|
68 – 95
|
92
|
|
Untreated
|
-
|
0
|
0
|
0
|
Source: B. C. Legassicke 1996. ECW Meeting, Victoria.
Summary
Dandelions are best controlled in direct seeding systems using a systemic approach to prevent the development of mature plants. Cultural measures like crop competition are important, but control of over-wintered rosettes and seedlings relies on the application of a systemic herbicide such as glyphosate. In annual crops, dandelions can be best controlled by fall applications (pre-harvest or post-harvest) of herbicides or with a combination of fall post-harvest, spring pre-seeding, and in-crop control measures.
References
Boyd, N.S. and R.C. Van Acker. 2003. The effects of depth and fluctuating soil moisture on the emergence of ten annual and five perennial weed species. Weed Sci. 51: 725-730.
Froese, N. T. and R. C. Van Acker. 2003. Distribution and interference of dandelion (Taraxacum officinale) in spring canola. Weed Sci. 51:435-442.
Froese, N. T., R. C. Van Acker, and L. F. Friesen. 2005. Influence of spring tillage and glyphosate treatment on dandelion (Taraxacum officinale) control in glyphosate-resistant canola. Weed Technol. 19: 283–292.
Hacault, K. M. 2005. Emergence timing and control of dandelion using fall or spring applications of glyphosate and florasulam in spring wheat fields.
Stewart-Wade, S. M., S. Neumann, L. L. Collins, and G. J. Boland. 2002. The
biology of Canadian weeds.117.Taraxacum officinale G.H. Weber ex Wiggers. Can. J. Plant Sci. 82:825-853.
Van Acker, R.C. and Hacault. K.M., Froese, N. T and
L. F. Friesen 2005. Managing dandelion in Canadian cropping systems. In Proceeding: 2005, Western Canadian Agronomy Workshop. Portage La Prairies, MB. Web Page:
http://www.ppi-ppic.org/ppiweb/canadaw.nsf/87cb8a98bf72572b8525693e0053ea70/b6df09d1198c1c2006256ff60050ec16/$FILE/vanAcker - dandelion.doc. Accessed June 10, 2006.
