Weed of the Week: round-leaved mallow

A look through provincial and U.S. state crop protection guides can yield many things. However, when searching for control methods for round-leaved mallow, the suggestions are fewer than for many broad-leaved weeds.

Round-leaved mallow is generally an annual weed, but if allowed to grow for more than a few weeks, it can seem as though it established a year or two earlier. In some cases, the weed can overwinter, in which case it can be tough to kill.

The mallow has been found in about five to 15 percent of fields. Round-leaved mallow tends to be more prevalent in the black soil zone, but it can be found almost anywhere, except for the lightest of dirt.

As a seedling, the cotyledons are heart-shaped with rounded tips and easily visible veins. Before long, the first true leaves appear. Those are the characteristic round, jagged leaves that makes the plant recognizable.

Mallow spreads its multiple, densely haired branches for up to a metre across the field floor to gain access to the sun beneath the canopy.

In cases of heavy mallow infestations, the weed can rise up with a main stalk for several inches. Leaves can be smooth or sparsely haired with a wrinkled texture. Stem nodes have small green stipules.

Tiny blue or white flowers can be seen in small clusters. Disk-like fruit develop, inside of which seeds are arranged like sections of a Mandarin orange.Seeds are rough, brown and angular.

A large, white taproot gives the plant resistance to most forms of control, including all but the most aggressive tillage.

Researchers at the University of Saskatchewan have taken a closer look at the weed while working on related projects because there were few registered chemical controls.

In part, the lack of registered chemicals has come because there are fewer problem acres than what exists for most prairie broadleaves, so pesticide companies haven’t worked out registrations for the mallow.

However, mallow can be tough to kill.

Researchers found that Attain, Prestige, Frontline 2,4-D, Spectrum and Refine Extra plus eight ounces of 2,4-D ester offered the best control of the pest in wheat.

This was likely due in part to the residual control the products offer, as the weed flushes a great deal in the spring.

Products can be very effective be-yond the three-leaf stage, and researchers noted that at six to eight leaves, mallow was killed, meaning that producers could expect control from later applications.

Glyphosate and Liberty are very effective for spring burnoff, fall applications, chemical fallow or in herbicide tolerant cropping systems.

Some of the registered products for mallow control include Triton, Attain, Prestige XC, thifensulfuron/tribenuron and MCPA, Varro, and products with a mix of dichlorprop and 2,4-D, like Turboprop 600, Dichlorprop-D and Estaprop. Products registered for suppression include Refine Extra, Tundra, Infinity and Dyvel DS.

In broadleaved crops, there are few options with the exception of Clearfield and other herbicide tolerant canola.

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