Pigweeds can plague gardeners
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 amaranth (A. hybridus), Powell amaranth (A. powelli), and redroot amaranth (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; 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 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% and up to 20 years to reduce to 99%. 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 one-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 reduce seedlings. Cutting larger plants at the base of the stem before seeds are developed will help. Herbicides can be used to remove amaranth; however, because physical methods mentioned above are very effective, herbicides aren’t recommended. If herbicides are used, make certain to read and implement all of the instructions for application properly.
For more information or questions on pigweeds, call the Butler County Master Gardener Garden Hotline at 724-287-4761, ext. 7 or email the Master Gardeners at: butlermg@psu.edu.
Mary Alice Koeneke is a Penn State Extension Butler County Master Gardener.
