Managing Pigweed in the Home Garden

The genus Amaranthus is a large genus of weedy annuals containing 65 to 75 species, including annual weeds, agricultural grain, and ornamental cultivars. However, it is the annual weed species commonly known as pigweeds that provide agriculture and gardeners alike with identification and control challenges.

Pigweeds include annual weed species native to Pennsylvania: smooth pigweed (Amaranthus hybridus), Powell amaranth (A. powellii), and redroot pigweed (A. retroflexus). All are exasperating field weeds in an agricultural setting and can be equally problematic when found in your garden. A fourth species, Palmer amaranth (A. palmeri), is native to the southwestern United States and was identified in Pennsylvania in 2013. This is the only species of amaranth currently listed on the Pennsylvania Department of Agriculture Noxious Weed List. Smooth, Palmer, and redroot amaranths can be found growing together in the same location; they are difficult to identify to species and are known to hybridize. Adding to the difficulty are cultivars sold in seed catalogs and garden centers that can escape to grow and establish in areas in which they were not originally planted.

Pigweeds can be tall, growing to 3 to 6 feet. Stems and leaves may be smooth or hairy. Leaves are alternate on the stems and vary from oval to diamond-shaped and taper to a point. Leaf margins may be smooth or wavy. Redroot pigweed is named for a red taproot and lower stem; however, other species, including smooth pigweed and Powell amaranth, can also have a red taproot.

Growing in full sun, pigweeds tolerate a variety of soils, growing in clay, loam, or sand. Germination and emergence occur over weeks, with an aggressive spurt of growth in May through June. This is when an observant gardener will first notice the plants in their garden or landscape. Seedlings of the three common pigweeds are difficult to identify and can be confused with other weeds. 

Flowers are present from July to September. Small and inconspicuous, the greenish flowers are formed on the ends of the main stems and branches. Some flowers may bloom from the leaf/stem joint (axil) on the upper branches. The flower stalks have a stiff, rough texture in redroot pigweed (also known as rough pigweed), and though similar in most aspects, smooth pigweed flower stalks are softer; in Powell pigweed, the flower stalks are not as branched and are thicker.

Seeds are formed from August to October. One plant can produce an average of 13,000 to 35,000 seeds. Pigweeds have evolved multiple processes that allow seeds produced in a given season to germinate at different times over the next several years. This allows pigweed to survive in the soil seed bank, taking three years to reduce the seeds in the soil to 50 percent and up to 20 years to reduce them to 99 percent. Because of these survival strategies, it is best to control pigweeds before seeds are mature.

The seeds have very little storage for nutrients to feed the germinating seedlings, which are quickly reliant on nutrients in the soil. Germination takes place in the top 1 inch of the soil, so seedlings can be readily removed by hand. Watering the soil prior to hand pulling or hand pulling after rain will help make the task easier. Cutting, burial, or a flame weeder can also remove seedlings. Shading out seedlings by using mulch will also reduce the number of seedlings. Cutting larger plants at the base of the stem before seeds are developed will help minimize the number of future seedlings and the amount of seeds in the soil seed bank.

For large infestations, pre-emergent, broad-leaf herbicides can help manage existing seed banks. Post-emergent applications of herbicides are more difficult, as there is some thought that pigweeds have evolved resistance to these types of herbicides, including glyphosate. If using post-emergents, use them early in the season and do not use them anywhere around vegetable gardens or sensitive plants.  In all cases, when using any type of chemical control, read and follow label instructions. For additional questions or assistance, contact your county’s Extension office.