Weeds of the Month for July: Pigweeds (Amaranth species)

 

Redroot pigweed
Photo by: D.G. Swan, WSU CAHNRS and WSU Extension

In May, I wrote about galinsoga, a warm weather annual that dribbles out seeds from an young age. In June, I wrote about docks, tap-rooted perennials that are best tackled early in life. This month is the turn of pigweeds, warm weather annuals that put out seeds in a “Big Bang” as Chuck Mohler describes in Manage Weeds on Your Farm. Pigweeds are fast-growing, tall, erect-to-bushy weeds that respond to high levels of nutrients as found in gardens and crop fields. Like corn, they use the C4 photosynthetic pathway, which means they thrive in high temperatures and high levels of light. They avoid getting shaded by growing fast, and they tolerate drought. Their vulnerability is that they do not produce seeds until they have been growing for some time and have reached a noticeable height.

Green Amaranth/Calaloo, grown for cooking greens.
Photo Baker Creek Seeds

Pigweeds are amaranths, a family that includes valuable food crops used for grains and greens, as well as dreaded weeds such as Palmer Amaranth, Waterhemp and Spiny Amaranth. Pigweeds are frost-tender, but if you have a long enough frost-free period, one plant can produce over 200,000 seeds.

Species of pigweeds

Mark Schonbeck has written a profile of pigweeds on the eOrganic site. He provides a list of eight unwelcomed pigweed species:

Silver Queen sweet corn with wilting pulled pigweed amaranth. Corn is a C4 crop, amarnath (pigweed) a C4 weed. Photo Kathryn Simmons.

Our experience with redroot pigweed in sweet corn

When I first encountered pigweed, I was told they were more-or-less impossible to deal with. Once I learned from Manage Weeds on Your Farm that, as tender annuals, they die with the frost, and as Big Bang weeds, they produce no seeds during most of their growth (unlike galinsoga!), I started a program of pulling them in our sweet corn patches. We used to grow a lot of sweet corn, and this tall crop, taking about ten weeks to mature the harvest, is a good habitat for pigweeds. We cultivate our sweet corn two weeks and four weeks after sowing, then don’t come back until the corn is ripe. This is an excellent window for pigweed to mature in! We started pulling the huge pigweeds each time we harvested corn. Inevitably, the weeds were right next to the corn plants, where our hoeing had failed to dislodge them. We found that if we put one foot against the base of the corn stalk and grasped the pigweed firmly and pulled up, we could usually tug them out. This won’t work if your soil is a tight clay, and if the pigweed breaks, the stem will branch and regrow. Most times, we harvest a patch of corn for two weeks and then disk or till it under. Usually we are turning the crop and weeds under before the regrown pigweed has had a chance to set seed.

Here is a video about Redroot Pigweed:

https://www.youtube.com/watch?v=Muqv3D-EujI

Economic weed thresholds of pigweed

Mark Schonbeck reports:

“Economic thresholds (weed levels that cause a 5% yield loss) for pigweeds emerging with corn and soybean crops have been estimated as low as 0.2–0.5 plants per 10 square feet, and 2–4 plants per 10 square feet for weeds emerging after crop seedlings have several leaves (Costea et al., 2004). In Ontario, redroot pigweed that emerged in corn at the 3–5-leaf stage at populations of 0.5–2.5 weeds per foot of row reduced yields 10–30%, whereas pigweed emerging at the 7-leaf stage had little effect (Knezvic et al., 1994)”

Our experience was that our corn yields were good, and we had found a successful method to manage the levels of pigweed over several years. We were cultivating between the rows and undersowing with soy at 4 weeks after sowing the corn, so the pigweeds growing were only those very close to the corn stalks. I do picture that an understory of pigweed would have a serious effect.

Spiny Amaranth

Spiny amaranth – a weed to exterminate by careful pulling.
Photo Pam Dawling

Spiny amaranth has been designated the world’s 15th worst agricultural weed. We have twice eliminated spiny amaranth from our gardens for a number of years, and then had it return. The seeds can remain viable in the soil and regrow when they are exposed to light. Although not as tall-growing as redroot pigweed, spiny amaranth has other ways of succeeding against those who would remove it from their crops: The nasty 0.5” (13 mm) spines (borne in opposing pairs at each leaf node) are not very visible among the branches, and are close enough together to make grasping the stem difficult.

The leaves of spiny amaranth are a darker green than redroot pigweed and have a V-shaped mark in a different color. Spiny amaranths can produce even more seeds than redroot pigweed: 235,000 each. Beware importing animal manure from other farms, and even gravel. I have seen a spiny amaranth germinate in a fairly large gravel pile brought in for road repairs.

Six methods of tackling pigweeds

Mark Schonbeck lists six methods of tackling pigweeds in organic production systems, and recommends using a combination of:

  • Cultivation, flame weeding, and manual removal 2-3 weeks after emergence
  • Stale seedbed
  • Mulching
  • Crop rotations that vary timing of tillage and other operations [our sweet corn was one year in 3 or4]
  • Cover crops and competitive cash crops
  • Measures to prevent or minimize production of viable seeds

Pigweed Flea Beetle

Disonycha glabrata – Pigweed Flea Beetle.
Photo from Bug Guide.net

Pigweeds even have their own striped flea beetle, Disonycha glabrata from the Chrysomelidae family. It is a large flea beetle. More the size of a striped cucumber beetle, but with a reddish thorax. It feeds on Redroot Pigweed (Amaranthus retroflexus) and other amaranths, so we never even try to grow salad amaranths as the leaves get riddles with holes. The first larval stage lasts 3.6 days, the second 2.6 days, and the third 2.9 days.  It spends 13.5 days in the soil.

Another Weed Management Resource

The book Steel in the Field, available as a downloadable pdf from SARE

Steel in the Field: A Farmer’s Guide to Weed-Management Tools. 1997. Edited by Greg Bowman. Sustainable Agriculture Research and Education (SARE). This book is a farmer’s guide to weed management tools using cultivation equipment.