Weed of the Week: shepherd’s purse

Reducing tillage has increased cash flow, boosted bottom lines and improved soil health.

One of the downsides has been minor weeds becoming major problems.

Some fall annual weeds have thrived where steel now fails to find them.

Worse, pests like shepherd’s purse have managed to escape some of the handiest herbicides, such as Group 2 products, by becoming immune to their effects.

Shepherd’s purse, or capsella bursa-pastoris, is one of those broadleaf weeds that remained easily controlled until the past few years.

Researchers at Agriculture Canada’s research centre in Lacombe, Alta., have identified a variety of fall annuals that haunt fields early in the season, robbing the top layers of the soil of the most recently deposited and converted nutrients.

The research has shown that early season weed removal is critical to high yielding crops.

Shepherd’s purse, stinkweed, cleavers, flixweed and narrow-leaved hawk’s beard all fall into that category and have all become serious field pests since the abandonment of cultivation.

Shepherd’s purse is part of the mustard family. It is usually a low growing pest, from 15 to 50 centimetres, and the stems are hairy. However, if left alone all season it can reach 90 to 100 cm tall.

Starting off as a rosette on the ground, the lobed basal leaves form the base for the stem, with stem leaves clasping it as they grow.

Flowering typically begins in July, provided the pest escapes the spring burn-off herbicide application. Flowering lasts all season.

Shepherd’s purse is prolific, with many small white flowers clustered together at the top of the stem. These spread out by several centimetres along the main stem as the plant grows.

The weed gets it name from the shape of its seedpods: small, flat, triangular pockets with notched tops. Each pod contains about 20 orange, oblong seeds, and a mature, uncontrolled plant will produce as many as 38,500 of these.

Research at Utah State University has found buried seeds might remain viable for up to 20 years.

The pest can flourish in non-herbicide tolerant broadleaf crops and in Group 2 treated fields where it has developed resistance.

Like most fall annuals, the best control begins when the plants are seedlings, after harvest. Seeds typically remain dormant until the year following distribution.

The weed is especially challenging in non-herbicide tolerant canola, where there are no registered choices for control.

Post-emergent applications of Odyssey and imazethapyr are effective in peas, but these too are Group 2 products.

Basagran, a Group 6 chemical, can be effective as long as the weed is in its early stages. It also works in other pulse crops, such as dry beans.

Metribuzin (Sencor), a Group 5 chemical, can be used in peas with trifluralin, a Group 3 chemical, but there are several restrictions based on soil organic matter and texture.

Several traditional non-Group 2, post emergent chemistries are effective in cereals as long as full rates are used because mature weeds can be tough to kill.

The weed is easily controlled by glyphosate or glufosinate in spring burn-off applications or herbicide tolerant crops.

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