Difficult to Control Weeds: Rough Comfrey (Symphytum asperum)

History:

Traditionally grown as a silage crop in Russia, rough comfrey is a coarse, clumped and deeply rooted weed that will typically reach heights of 1 m. Roots and leaves of rough comfrey contain small amounts of toxic pyrrolizidine alkaloids that can cause liver damage when consumed in high quantities.

Lifecycle:

Reproduces by seed and broken root fragments, this makes this species very difficult to control and eliminate from a site.

Identification:

Stems: Erect, thick, angular, hollow, usually branched and covered with rough, bristly hairs (See Figures 2 & Figure 3).

Figure 2

Figure 3

Taproots: Thick, brittle, deep and covered with a thin blackish bark, but are white inside. The rootstock also contains much mucilage making them feel somewhat "slimy" (See Figure 1).

Figure 1

Leaves: Alternate, ovate to lanceolate, 5-30 cm long, sparsely covered with short bristly hairs.

Flowers: May-August. Corolla initially pink then turns deep blue or purple, calyx 2-4 mm long (See Figure 4).

Figure 4 - The flower head of comfrey.

General Cultural Control:

Although fragmented roots can generate new plants, repeated cultivation can eliminate weedy populations, especially when timed to prevent seed production. In addition, above ground parts die back in cold winter climates, therefore tillage operations that bring root stock to the soil surface in the fall may also contribute to control of this weed. Mowing before seed is produced can prevent establishment of new seedlings while depleting root reserves.

Control in Non-Crop Areas:

In 2002, field trials were conducted in Norwich, ON to evaluate control of rough comfrey in a non-cropped area (Table 1). Several herbicides were broadcast applied using flat fan nozzles and at a water volume of 80 L/ac. None of the herbicides tested provided complete control of rough comfrey. A 1.7 L/ac rate of Vanquish (a rate used for brush control on roadsides and rights of way) offered the best level of control. However this product and rate would not be appropriate for field and horticulture crop production systems as carryover, drift, and crop tolerance issues would likely be prevalent.

*Adapted from Page 284-285 of the 2002 Annual Weed Research Report, Department of Plant Agriculture, University of Guelph.

"Spot" application in non-crop areas:

Greater control of rough comfrey was achieved when "spot" applications with a backpack sprayer were made in early August. Both small and large plants were targeted and the smaller plants were generally more easily controlled than larger plants (Table 2). It is likely that spot applications resulted in better control because of increased spray coverage combined with a high product rate when individual plants were targeted with a backpack sprayer.

*Adapted from Page 286-287 of the 2002 Annual Weed Research Report, Department of Plant Agriculture, University of Guelph. All products were applied at their labeled rates in field crops.

Conclusions:

Based on limited field researd, it appears that rough comfrey is better controlled with an application of a non residual herbicide (i.e. glyphosate) using a backpack sprayer. The earlier the infestation of rough comfrey is targeted, the better the chance of getting acceptable control. It is likely that follow-up "spot" applications would be needed in the succeeding years to ensure that the weed infestation is kept in check.

Acknowledgements:

These trials were conducted with the financial assistance of OMAF through the OMAF / University of Guelph Research agreement.

References:

California Noxious Weed Index. Retrieved Jan 20, 2005 from http://www.cdfa.ca.gov/phpps/ipc/weedinfo/symphytum.htm

Tardif, F.J. 2002. Control of Rough Comfrey. Weed Research Annual Report. Department of Plant Agriculture. University of Guelph. Pp 284-286.

Related Links

• California Noxious Weed Index

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