Weed of the Month for May: Galinsoga

People in early spring weeding spinach that has been hooped and covered overwinter.
Photo Wren Vile

This is the first of my new monthly series of posts. All about weeds.

Sustainable (or Ecological) Weed Management: A Holistic Approach to Organic Weed Management

In the early days of organic farming, maximum use was made of frequent cultivation to kill weeds. Now we know that too-frequent cultivation risks causing soil erosion, and that each tilling or deep hoeing stirs air into the soil and leads to a burning-up 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!

Start with restoring and maintaining balance in the ecosystem. Develop strategies for preventing weeds and for controlling the ones that pop up anyway. An obvious point is to avoid adding new kinds of weeds 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!

Weeding in early June. Photo Lori Katz

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. Develop an understanding of weeds and the different types: annual/perennial; stationary perennials/invasive perennials; cool weather/warm weather; quick-maturing/slow-maturing; and what Chuck Mohler referred to as “Big Bang” versus “Dribblers”. In this monthly blogpost series, we’ll meet various types of weeds, and develop a broader understanding of how and when to tackle each kind.

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 begin competing for moisture and nutrients. These early weeds have the greatest potential for reducing crop yields if allowed to grow unchecked. We need to cultivate or otherwise control weeds before this 2- to 3-week grace period is over.

Weedy sweet corn. ideally, we would have cultivated two weeks and four weeks after sowing. Photo Bridget Aleshire

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. 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 period it is essential to control weeds to prevent loss of yield.

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 there are long-term reasons for removing weeds to improve future crops.

Know Your Weeds

Lettuce with weeds, easily hoed. Photo Bridget Aleshire

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 down 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. Some crops, like carrots and onions never cast much shade at any point of their growth, so that sun-loving weeds like purslane are more likely to thrive there, but not be a problem for crops which rapidly form canopies that shade the ground.

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

Galinsoga

This month’s Weed Character is galinsoga, a “Seed Dribbler”, that matures seed while still quite small plants, sheds some, makes some more, and can carry on for a long seed-shedding season.

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

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 rotavating, as seeds somehow manage to stay near the surface with rotary tilling. Chuck Mohler, author of the excellent book Manage Weeds on Your Farm, has tested this out with colored plastic beads.

We have two kinds of galinsoga: narrow-leaved and hairy. Both behave the same way. They thrive in highly fertile, freshly tilled soil, just the same as you hope your vegetable seeds will. Mostly we think about how to get rid of galinsoga (prompt hoeing or other cultivation before it flowers), or stop it germinating in the first place (mulches). Its flowers attract beneficial insects such as hoverflies, and it can be eaten by humans and livestock. Young leaves can be used in a soup or in mixed dishes. It doesn’t have a strong flavor. The plants contain flavonoids and phenolic compounds, and it has been found to have antioxidant and anti-inflammatory properties. Extracts from hairy galinsoga can coagulate blood. It is an alternate host for certain nematodes and over twenty insect pests. Hairy galinsoga is thought to have originated in Central and South America, and has become naturalized in North America and other temperate and tropical regions.

Galinsoga is a summer annual that belongs to the sunflower family (Asteraceae) and can invade vegetable gardens with dense infestations that crowd out crop plants. The secrets of galinsoga’s success are that its seeds germinate immediately they reach the soil (no dormancy period), it grows very fast, shading out other plants, it sets seed in as few as 30–40 days after emergence, and continues shedding seed as long as it is growing. It also has the knack of re-rooting if pulled and laid on the surface of the bed, if there is any moisture in the soil. Large plants seem able to transfer the water in their cells to their roots, helping re-rooting happen. Seed that is shed early in the year is capable of growing a mature plant very quickly. There can be multiple generations in one warm season. Fortunately, the seeds are short-lived, and have to be in the top 0.25″ (6mm) of the soil to germinate.

Hoe weeds while they are small and you’ll be rid of those with short-lived seeds in a few years. Galinsoga and Outredgeous lettuce.
Photo Pam Dawling

Galinsoga Identification

Hairy galinsoga (Galinsoga quadriradiata), has profuse hairs on stems and leaves. Narrow-leaved galinsoga ( Galinsoga parvifolia), is very similar, except it is not hairy and it has narrower leaves. Leaves are oval with serrated margins and distinct petioles. They are arranged opposite each other on the stems. The flowers have densely packed yellow disc florets and five tiny white ray florets, each with three scalloped teeth at the end. Seeds of hairy galinsoga germinate between 54°F-86°F (12°C-30°C) with an optimum temperature requirement of 68°F-75°F (20°C-24°C). Most of the seed germination occurs from May to June, after the last frost. It flowers abundantly from about late-May until late fall here in central Virginia. Fallen seeds can germinate immediately due to the absence of dormancy requirement. Take advantage of this phenomenon to eradicate hairy galinsoga from an infested field in three to four years by careful management.

Controls for Galinsoga

Prevention of Weed Germination

Hoeing or mechanical cultivation is effective if carried out repeatedly during the early stages of growth (before flowering). Mulches, such as thick (6-mil) black plastic, or straw, hay, leaves, woodchips over cardboard or newspaper, are effective to control galinsoga in small gardens if applied immediately after planting the crop and before the galinsoga germinates.  Tarping is the equivalent solution for larger areas.

Reduction of Weed Seeding

Grazing, or the mechanical equivalent, mowing, will take care of galinsoga in places you are not currently growing a crop. This weed is not usually found in lawns. It has no resistance to frost. Livestock will happily graze it.

Reduction of Viability of Seeds

Most weed emergence happens within two years of the seeds being shed. Not all seeds that are produced will ever get to germinate (I was very pleased to learn that seeds have many ways of not succeeding!) You can help reduce their chances, by mowing crops immediately after harvest, (to prevent more weed seed formation); then wait before tilling to allow time for seed predators to eat weed seeds that already produced. Seeds lying on or near the soil surface are more likely to deteriorate or become food for seed predators than buried seeds, so delaying tillage generally reduces the number of seeds added to the long-term seed bank. (Short-term, they may germinate!)

If they do not get eaten, dry out or rot, seeds on top of the soil are more likely to germinate than are most buried seeds, and small, short-lived seeds of weeds which have no dormancy period, such as galinsoga, will almost all die within a year or two if they are buried a few inches.

Putting it Together

Strategies include

  1. Inversion tillage such as moldboard plowing (seeds will die off deep in the soil within a year or so.)
  2. Mulching – the seeds will not germinate or be able to grow through the mulch, and will be dead by next year. Be sure to rotate the mulched crops around the farm, so that the benefits are not confined to one section;
  3. Grazing with small livestock, or harvesting galinsoga for human consumption, or mowing: especially mow as soon as the food crops are finished, if you cannot till right away.
  4. Tarping (mow first);
  5. No-till cover crops, with summer crops transplanted into the dying mulch;
  6. Stale seed bed techniques, including flaming;
  7. Plant flowers that attract beneficial insects, particularly seed-eating insects, and birds.
Front cover of manage Weeds on your Farm

Resources on Weeds

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

Buckwheat, a wonderful summer cover crop

Credit Southern Exposure Seed Exchange
Credit Southern Exposure Seed Exchange

The June/July issue of Growing for Market magazine has an article I wrote about buckwheat, one of our favorite summer cover crops.

Buckwheat is a fast-growing warm-season broadleaf annual that is a very useful cover crop. Its special strengths are in weed-suppression, attracting beneficial insects, improving the soil tilth (aggregate structure) with its fibrous roots, and extracting potassium, calcium and phosphorus from the soil to the benefit of following crops. Buckwheat is almost three times as good as barley in scavenging phosphorus, and more than ten times better than rye (a poor phosphorus scavenger). Buckwheat can do fairly well on poor soils, is tolerant of a range of soil pH and is an easy crop to deal with manually or with small-scale equipment. Because buckwheat is not related to any of the common food crops, it is simple to include in rotations.

There wasn’t space in the magazine to include all the great resources I found, so I’m including them here.

Resources in the article

e-Organic’s Buckwheat for Cover Cropping in Organic Farming

Principles of Sustainable Weed Management in Organic Cropping Systems

Virginia Association for Biological Farming Infosheet Seeders: Using Manually-operated Seeders for Precision Cover Crop Plantings by Mark Schonbeck and Ron Morse,

How to De-hull Buckwheat with the Country Living Mill

Buckwheat beginning to flower. Credit Southern Exposure Seed Exchange
Buckwheat beginning to flower. Credit Southern Exposure Seed Exchange

Buckwheat Resources

Cornell Buckwheat Cover Crop Handbook (thorough, but not organic) .

Buckwheat Cover Crop Guide – Jefferson Farm and Gardens

General Cover Crop Resources

 Agricultural Sustainability Institute at University of California Davis, Cover Crop Database

 USDA/ARS Cover Crop ChartThe crop “tiles” in the chart can be clicked to access more information about 46 cover crops.

NCSU Department of Horticultural Sciences Horticulture Information Leaflet 37, Summer Cover Crops,

SARE, Managing Cover Crops Profitably, the whole book is available as a free download

CEFS Organic Production: Cover Crops for Organic Farms, under Resources, Guides, Organic Production Guide:

Cornell University, Cover Crops for Vegetable Growers,

GFM-June July 2013-cover-300px

Useful sustainable farming links

My Number One Resource for many years has been ATTRA, National Sustainable Agriculture Information Resource, www.attra.ncat.org. Solid useful info on a range of topics. Very helpful people. Toll-free hot-lines in English and Spanish. Hundreds of helpful publications. Newsletters. Look also on their site for SIFT, (Small-Scale Intensive Farm Training Program) for new farmers. Here’s ATTRA’s  pest management page.

 

Fast becoming another favorite of mine is the newer and rapidly growing eOrganic, the Organic Agriculture part of the Cooperative Extension System.

Many state Extension Services have good websites. Some have particular strengths: Our own Virginia Tech  has lots about vegetables and diseases and pests (not necessarily organic). For locally relevant information, start with your local Extension Office after the EOrganic one. Then prepare for global warming and try one south of you. Cornell is good on fruit and Cornell Plant Pathology runs the Vegetable MD onlineNorth Carolina has good info for commercial growers of vegetables, fruits and flowers, including some publications specifically on organic methods. They also have publications geared more towards home gardeners. And they have another of my favorites: Debbie Roos’ site Growing Small Farms.

 

Southern Sustainable Agriculture Working Group has produced a series of Virtual Farm Tour DVDs. The series is called Natural Farming Systems in the South.

Sustainable Agriculture Research and Education (SARE) Grants and information, including free downloads of several really good books such as Managing Cover Crops Profitably. Click on the Learning Center tab.

Southern Exposure Seed Exchange, seeds for the south and lots more contacts and events. Click on the links button.

Growing for Market magazine. Monthly magazine packed with practical information for market growers.

If you want to join a discussion group, here’s the one I do: Market-Farming listserv

Farmscaping: Symbiont Biological Pest Management Company, Dr Richard McDonald, and more at ATTRA

Virginia Association for Biological Farming www.vabf.org Conference February 8-9, 2013 in Richmond, including a one day Farm School for new farmers and growers.