Redroot (Amaranthus powellii)
Redroot is a fast-growing summer annual weed common in New Zealand’s warmer regions, especially the North Island. It thrives in disturbed or bare soil through late spring and summer, dying off with frost in autumn.
It germinates later than many other annual weeds and prefers hot, dry conditions. In lawns, redroot appears mainly around edges, driveways, or newly sown areas with thin grass cover. It produces dense, upright growth that can shade or crowd out young turf.
This species is sometimes called Prince of Wales Feather, Powell’s Amaranth, or Pigweed.
Identification
Redroot is a tall, upright plant that can exceed 1 metre in height in fertile soil. Leaves are oval with a pointed tip and often have a slight red or pink tint. The stems range from green to reddish-pink and feel firm and fibrous.
The flower head is greenish-brown and spiky to the touch, producing large numbers of shiny black seeds. Seedlings are identifiable by a small notch at the tip of the first true leaf.
It’s often mistaken for similar weeds such as prostrate amaranth (Amaranthus deflexus), which grows flat along the ground, or purple amaranth (Amaranthus lividus), which is smaller with purple-tinged foliage.
Why It’s a Problem
Redroot competes strongly with turf and garden plants for light and nutrients. Once mature, its deep taproot makes it difficult to remove, and each plant can produce thousands of seeds that remain viable in soil for years.
In lawns, it thrives in compacted or poorly maintained soil and quickly dominates bare patches, giving the area a coarse, weedy look through summer.
Management and Prevention
Prevention is key since redroot is easy to control when young but much harder once mature.
Cultural control methods:
- Maintain a dense lawn: Overseed thin areas to prevent seed germination.
- Mow regularly: Frequent mowing keeps seedlings short and stops seed formation.
- Fertilise and water correctly: Healthy grass outcompetes redroot by shading the soil surface.
- Hand-remove seedlings: Uproot small plants before they flower, especially in garden beds.
- Avoid bare soil: Redroot seeds germinate rapidly in disturbed or open ground.
Chemical Control
Redroot is susceptible to most common herbicides when treated early.
Effective options include:
- Mecoprop/ioxynil/bromoxynil mixtures (Image): Provides selective control in turf.
- Triclopyr/picloram mixtures (NZLA Gold): Strong results on established weeds.
- Glyphosate or glufosinate: Ideal for spot-treating driveways, paths, and edges.