Problem Weed Series for Apples: Pigweed

Problem weed: Redroot pigweed
Other names: Amaranthus retroflexus, redroot, rough or tall pigweed, green amaranth
Growth habit: Annual
Spreads by: Only by seed

Reasons for concern: Usually the first weed escape from soil-applied herbicides; may grow up to 2 m tall; peak germination in spring during critical period for trees and vines; competes strongly with trees and vines for moisture and nutrients; irritating to workers

May be confused with:

1. Green pigweed: which has brighter, shinier green leaves with less hair on the stem, and a thinner inflorescence with a long terminal spike. This pigweed may be the most common pigweed in southwest Ontario, and in field crops has developed herbicide resistance.
2. Prostrate pigweed: growing prostrate near the ground, with flowers in the axils of leaves, and no large inflorescence. Frequently looks dried up even when green.
3. Tumble pigweed: looks similar to prostrate pigweed, but grows more erect. The dried flower heads break off and are blown around by wind, spreading seed.
4. Water hemp: (New weed found in Essex & Lambton counties in 2002 so keep your eyes open!) Young plants look very similar to green and red root pigweed but have very narrow (lanceolate) and somewhat shiny leaves that are completely hairless. These weeds appear to have triazine resistance, so could be a problem in orchards and vineyards where Sencor or simazine are used.

Chemical control: Pigweeds are well controlled with spring burndown applications of glyphosate, Amitrol, Gramoxone or Ignite when weeds are small. Soil-applied herbicides like simazine, Sencor/Lexone, Casoron, linuron, Dual Magnum, Treflan, Devrinol, and Kerb usually control pigweeds for 8-12 weeks. Mid-summer burndown herbicides can work well if coverage is adequate, and weeds are not too tall, and in the tree/vine canopy. 2,4-D Amine can be very effective on pigweeds, but must be used with care. Do not apply 2,4-D within 80 days of harvest, or after harvest when juice apples are still on the ground. Wait for calm, cool conditions to apply to avoid both physical drift and vapour drift.

Cultural control: Mechanical control methods (cultivation, flaming, mulching) are very effective on pigweed if done early. Mowing needs to be done very close to the ground, as side branches will grow and flower from mown plants.

More than meets the eye! (by Jan Schooley, Ginseng & Medicinal Herb Specialist, OMAFRA, Simcoe)

The redroot pigweed, which most farmers despise, originated in tropical America and records show its use by the Aztecs both as a grain and as a religious offering to the Aztec sun god. The Spanish conquistadors brought the ancestor of pigweed to Europe where it made its way to North America. Early settlers used it as a vegetable, as a medicine to stop internal bleeding, as a laundry additive because of the saponin content and as a red pigment for dyes. Today it is more annoying than it is useful, although it has some important cousins, but given its colourful past it is definitely more than meets the eye!

References and Pictures:

• Publication 505, Ontario Weeds, p. 66

Related Links

• OMAFRA Factsheet - Pigweeds (Redroot, Green and Smooth)
• Ontario Weed Gallery