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.

Gathered Info on Weeds

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

Following on from my recent seasonal posts on How to succeed with transplanting crops and Direct sown vegetable crops, I reckon people are now thinking about weeds.

Below is material excerpted from the Sustainable Weed Management chapter in my book Sustainable Market Farming, which was also published in Growing for Market as New Ways to Think About Weeds, back in 2011.

Do you need a justification for having some weeds visible among your crops? Do you crave a system to help you get a grip on your to-do list, so you’re not overwhelmed? Sustainable (or Ecological) Weed Management does all this! In the earlier days of organic farming, maximum use was made of frequent cultivation to kill weeds. Now we know that too-frequent cultivation can cause soil erosion, and that each tilling or deep hoeing stirs air into the soil and speeds combustion of organic matter. The practice of sustainable weed management is about effectiveness – including removing weeds at their most vulnerable stage, or at the last minute before the seed pods explode – and ignoring weeds while they are doing little damage. Work smarter, not harder!

Strawberries need a lot of weeding if grown on bare soil.
Photo Twin Oaks Community (Renee)

A Holistic Approach to Organic Weed Management

As always, strive to restore and maintain balance in the ecosystem. Develop strategies for preventing weeds and for controlling the ones that pop up anyway. An obvious way to prevent weeds is to avoid adding new kinds to any part of your fields. Remove the hitch-hikers from your socks out on the driveway, not when you notice them as you squat to transplant onions! We use our driveway as a convenient place to “roadkill” particularly bad weeds by letting them die in the sun. Beware of Trojan plant swaps (nice plants in soil concealing nasty weed seeds)!

Weeds are not a monolithic enemy, but a diverse cast of characters. Applying biological principles is not an attitude of war, but more like ju-jitsu, using the weaknesses of the weeds to contribute to their downfall. This chapter aims to develop our understanding of weeds and the different types: annuals and perennials; stationary perennials and invasive perennials; cool weather and warm weather types; quick-maturing and slow-maturing types; and what Chuck Mohler (Manage Weeds on Your Farm: A Guide to Ecological Strategies) refers to as “Big Bang” types versus “Dribblers”.

One factor to consider is how vulnerable the crop is to damage from that weed at that time. Weeds that germinate at the same time as a vegetable crop usually do not really affect the crop’s growth until they become large enough to compete for moisture and nutrients. If allowed to grow unchecked, however, these early weeds have the greatest potential for reducing crop yields. We need to cultivate or otherwise control weeds before this 2-3 week grace period is over.

The critical period for weed control for the crop is the interval from the end of the initial grace period until the end of the minimum weed-free period, which is approximately the first third to one half of the crop’s life. In other words, the most important time to weed a crop is from 2 weeks after sowing until the crop is half-way to being finished. For vigorous crops like tomato, squash and transplanted brassicas this is four to six weeks; less vigorous crops like onion or carrot need weed-free conditions for eight weeks or more. During that time it is essential to control weeds to prevent loss of yield.

A carpet of weeds, but the crop is easily seen! A good time to hoe!
Photo Bridget Aleshire

Weeds that emerge later have less effect, and ones that emerge quite late in the crop cycle no longer affect the yield of that crop, although their seeds give a reason for removing them to improve future crops.

Know Your Weeds

Learn to identify the major weeds on your farm, and any minor ones that suggest trouble later. Observe and research. Start a Weed Log with a page for each weed. Add information about your quarry’s likes and dislikes, habits and possible weak spots. Find out how long the seeds can remain viable under various conditions, and whether there are any dormancy requirements. Note when it emerges, how soon it forms viable seed (if an annual), when the roots are easiest and hardest to remove from the soil (if a perennial), what time of year it predominates, which plots and which crops have the worst trouble with this weed. Monitor regularly throughout the year, each year. Look back over your records and see if anything you did or didn’t do seems to have made the problem worse or better.

Next think about any vulnerable points in the weed’s growth habit, life cycle, or responses to crops or weather that could provide opportunities for prevention or control. List some promising management options. Try them, record your results, decide what to continue or what to try next.

Most weeds respond well to nutrients, especially nitrogen. If you give corn too much nitrogen, even as compost, the corn productivity will max out and the weeds will use the remaining nutrients. Sun-loving weeds like purslane are more likely to thrive among crops (like carrots and onions) which never cast much shade at any point of their growth. They won’t be a problem for crops that rapidly form canopies that shade the ground.

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

A few weeds, such as giant ragweed, emerge only during a 3-week interval, while others, such as pigweed and velvetleaf, can germinate during a two-month period, if temperatures are warm enough. Galinsoga seeds are short-lived and germinate only near the soil surface, but velvetleaf seeds can lie dormant for years deep in the subsoil, and germinate whenever they get brought close to the surface. Clearly, different strategies work best with different weeds.

Red Root Pigweed is a “Big Bang” weed – the plant grows for a long time, and then all its seeds ripen at once, as the plant starts dying. Most seeds come from a few large plants – pigweed-monsters that mature late in summer can shed 400,000 seeds! Pulling the largest 10% of the weeds can reduce seed production by 90% or better. We used to ignore pigweed growing in our sweetcorn, once it escaped two cultivations, believing anything that big must already have done damage. Now we pull while harvesting. Some pigweeds are as tall as the corn, but most don’t have mature seed heads. Since starting this a few years ago, we have noticed a considerable drop-off in the number of pigweeds we have to deal with. This is different from the “Seed Dribblers” like galinsoga, which mature seed while still quite small plants, shed some, make some more, and can carry on for a long seed-shedding season.

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

Another useful piece of information is that a constant percentage of the seeds left from one year’s shedding dies each year. For lambsquarters in cultivated soil it’s 31% per year (only 8% in uncultivated soil). The number of seeds declines rapidly at first, but a few seeds persist for a long time. The percentage varies widely among species.

While seeds survive better deeper in the soil, they don’t germinate better down there. Larger seeds can germinate at deeper levels than small seeds. If you are trying to bury seeds deep, use inversion tillage; don’t rely on rototilling, as seeds somehow manage to stay near the surface with rotary tilling. Chuck Mohler has verified this with colored beads.

Most of the weeds in cultivated soils are annuals, but some of the worst ones are perennials, either stationary (tap-rooted) perennials like docks and dandelions, or wandering/invasive perennials with tubers, rhizomes or bulbs (Bermuda grass, quackgrass). Stationary perennials in their first year act like biennials – leaves, roots, but no flowers or seeds. In annually tilled areas, they get killed in year one and don’t often establish. Wandering perennials are a more difficult problem, and understanding apical dominance is important in tackling them – see Reducing the Strength of Perennial Weed Roots and Rhizomes in the book’s chapter.

Burdock is a large perennial weed with a huge root. Photo Bridget Aleshire

The chapter also includes information on Prevention of Weed Germination, Reduction of Weed Seeding, Reduction of Viability of Seeds, Reducing the Strength of Perennial Weed Roots and Rhizomes, and Two Examples (galinsoga and nut sedge).

Knowing and understanding the particular weeds that are giving you the worst problems enables you to design an approach that includes removing weeds at the most important point in their life-cycle, before they do their worst damage. While focusing on that you can relax and ignore weeds that are not doing much harm.

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I have written here on my website about weeds previously. See my blog posts

Resources on Sustainable Weed Management