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.

More on Insectary Flowers; Vegetable Crop Resources, Especially Weeds

 

Borage flowers attract many beneficial insects. Spot the honeybee! Photo Raddysh Acorn

More on Insectary Flowers (to attract beneficial insects)

A reader responded to my post Growing flowers to attract aphid predators in early spring

“Isn’t too cold for the predators to be around, Pam? unless they hibernated in the greenhouse. but even so, it’s still cold in there at night. We have some aphids too in the tatsoi and some of the lettuce, so thank you for all the tips, and the life cycle. I had not quite realized that the cycle was so short. I grow borage in the hoophouse but in the ground – the plants get large and gorgeous with clouds of blue flowers in March and April – much bigger and healthier than anything I try to grow outside. The honeybees absolutely love it and they attract are a lot of other insects too.”

Yes, it has been still too cold for predatory insects to be around, until this week, when ladybugs greet us around every corner. Our idea with the flowering plants was that by starting the plants in the fall, we’d have actual flowers earlier than if we started in “spring”, and that perhaps the extra stresses would even cause the plants to flower earlier. Apart from the borage, none of the others have flowered yet (Feb 23). We likely need to fine tune our sowing dates. We sowed at the very beginning of September and the very end of October. That two-month gap probably has better sowing dates! We noticed that some of our plants were not very cold-hardy. Some died and some had to be pruned of dead bits. Since then, we started more flowers in our greenhouse on February 1. Another thing we’re noticing since early February is that the plants in pots dry out very fast. It’s probably better to get the flowers in the ground in the hoophouse and greenhouse as soon as they are big enough, as suggested by the results of my reader quoted above, with borage.We had thought that having them in pots would enable us to move them into trouble spots.

Vegetable Crop Resources, Especially Weeds

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

A newly released handbook from Sustainable Agriculture Research and Education (SARE), Manage Weeds On Your Farm: A Guide to Ecological Strategies by Charles L. Mohler, John R. Teasdale and Antonio DiTommaso, is set to help us all. I haven’t read it yet (although I am looking forward to that!), so this is not a review, But these are three big names in weed science, and SARE is well-known for providing solid information on sustainable farming.

Silver Queen sweet corn with a wilting pulled amaranth plant in the center
Photo Kathryn Simmons

I had the great good fortune to attend a workshop by Chuck Mohler years ago, and got some realizations that forever changed my approach to weeds. Top of the list is that some weeds, such as pigweed (amaranth species), don’t distribute any seeds until they have grown very big. Until that point they are not threatening next year’s farming efforts. We used to get huge pigweed plants in our sweet corn, and fatalistically did nothing once we were in there harvesting, somehow believing it was “too late”. No, it’s not! They hadn’t seeded. We started to make a practice of pulling the huge pigweed every two days while harvesting corn. Often it was necessary to stand on the base of the corn plant to hold it in place, while pulling the weed. Then all we had to do was drop the pigweed between the rows. Sweet corn ripens in hot weather and the weeds soon died, rather than re-rooting. All those big leaves sucked the moisture right out of the plants. Be extra careful if you have spiny amaranth. We have twice eliminated this weed form our gardens, by diligent hand-pulling, only to have it reappear a few years later!

Galinsoga – a fast growing, fast-seeding weed of cultivated soil.
Photo Wren Vile

Conversely, galinsoga forms seeds very soon after germinating, while still small. This weed is one to strike early and repeatedly. It readily re-roots in damp soil. Our strategy when we are too late to hoe and have to hand-pull them, is to shake off as much soil as possible, then to either twist and break the stem (if there are not many), or “shingle” the weeds, laying them down with the roots of one on top of the leaves of the previously pulled plant, providing a surface of roots all exposed to the air, and none touching the soil. This works quite well. Timely hoeing is much better, of course!

Manage Weeds has chapters on How to Think About Weeds, Cultural Weed Management, Mechanical and Other Physical Weed Management Methods, Profiles of successful managers,  and then the alphabetical rogues gallery of grass weeds and broadleaf weeds.

This book and all the online information from SARE is free of charge. You can buy print copies if that suits you better. Other good resources from SARE, while you’re at their website, include several other books:

Building Soils for Better Crops

Managing Cover Crops Profitably For many of us, this is the “Cover Crops Bible”

Systems Research for Agriculture

Crop Rotation on Organic Farms 

There are also podcasts, bulletins, videos, Topic Rooms and interactive pages to explore.

Winter Vegetable Production Methods

For those who missed the Pasa Sustainable Agriculture conference, here is my slideshow Winter Vegetable Production Methods, From the Field to the Hoophouse

Winter Vegetable Production Methods, From the Field to the Hoophouse Dawling 60 mins 2022 2.11 9am