Book Review: Manage Weeds On Your Farm

Front cover of Manage Weeds on your Farm

Book Review: Manage Weeds On Your Farm: A Guide to Ecological Strategies, by Charles Mohler, John Teasdale and Antonio DiTommaso. SARE Handbook 16, 2021, 416 pages, color photos, drawings, charts, $24.00

This immense book is a game-changer! A resource enabling us to understand weeds better and deal with them smartly, exploiting their weaknesses, making best use of natural and created resources.

About 300 pages comprise a directory of major agricultural weeds of the United States and Canada: about 20 grasses and sedges; about 45 types of broadleaf weeds. Many of the weeds in North America came from Europe, so the book’s usefulness is not restricted to this continent. The focus is on weeds of arable farmland. Gardens fit in this category, although the physical tools will be smaller! This part of the book is not merely to help you identify weeds, but to develop a management plan for each one.

The first part of the book is 120 pages of agricultural gold – an exploration of concepts of ecological weed management. Understanding the biology of weeds is vital to successful ecological management. (Note that I’m avoiding the use of the word control, as the authors do.) The book is “intended to provide the information you need to grow crops without synthetic herbicides, great expense or back-breaking work.” Good information is an efficient tool. Understanding more about how the biological world works will enrich your life!

This is not a book many of us will read cover to cover. It’s a toolbox. Read the first section, then seek out the profiles of your most problematic weeds and make a plan for each one. In each profile there is an identification section with good photos, a management section, a concise summary, referring back to cultural and mechanical strategies, and an ecology section with specific information leading to the recommendations in the management section.

Chapter 2, How to Think About Weeds, starts with the reminder that weeds die from various causes (any of which we can use to advantage). Seeds may fail to germinate, or get eaten. Seedlings die from drying out, disease, competition from other plants, or lack of light, or being eaten, mowed or turned under.

“The goal of ecological weed management is to arrive at a balance between birth and death that keeps the density of weed populations low most of the time and reduces them quickly when density starts to increase.”

You need to increase the death rate and reduce the germination rate of the weed seed bank, or else the population continues to increase. This demonstrates the value of understanding which tools to use in which situation. Keeping on hammering with a wrench will not work well!

Seed size is one of the characteristics of weeds that affect their successful management. Smaller seeds are easier to kill, because the seed does not provide much food for the seedling. Large-seeded crops and transplants can out-compete small-seeded weeds, if the timing of cultivation is right. There is a good explanation for why tillage prompts seed germination, which can give weeds the upper hand. Environmental cues such as soil temperature, the difference between night and day temperatures, oxygen levels, even a brief flash of light, can indicate if the seed is near the surface and whether there are competing plants up there. The cues can be very specific. Velvetleaf and tall morning-glory germinate in response to a sudden absence of certain volatile compounds which are vented from the soil during tillage. Understanding this Secret Life of Plants can help us figure strategies for specific weeds.

Different weeds germinate in different seasons, and crop rotation between spring, summer and fall crops will disrupt weed lifecycles and prevent any one taking over. Another consideration is that the same percentage of the seeds still in the soil will die each year. This means that if no fresh weed seed is added, the seed bank declines rapidly in the first few years, leaving some seed persisting for years.

The main cause of seed death is probably that seeds germinate in unfavorable conditions and then die. Secondly, seeds are eaten. Lastly, some seeds rot and decompose. Small seeds deep in the soil are unlikely to germinate. It takes a big seed to provide the resources to grow a shoot that can reach a long way to the surface. Galinsoga seeds rarely emerge from deeper than ¼” (6mm). Few seeds can germinate from deeper than 2” (5cm).

June-planted potato emerged through hay mulch. Potatoes are a C3 crop
Photo Pam Dawling

Nowadays we are learning about two photosynthetic pathways, C3 and C4. C3 plants thrive in cool, moist conditions, not needing full daylight to maximize their photosynthesis. Most cool-season grasses and broadleaf weeds use the C3 pathway.  They can increase photosynthesis (grow more) as CO2 concentration in the atmosphere increases. C3 crops like potatoes, pumpkins and soybeans will probably do better against C4 weeds as CO2 concentration increases in the climate disaster. C4 plants perform best at high temperatures, with more sunlight enabling more photosynthesis. Bermuda grass, foxtails, pigweeds, and common purslane use the C4 pathway. But C4 crops such as corn will have a harder time with C3 weeds. If your climate becomes warmer and drier, C4 weeds and crops will be favored over C3 weeds and crops. This effect may be stronger than the effect of increased CO2.

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

Other factors influencing growth include frost tolerance, drought tolerance, and the presence or absence of mycorrhizal fungi. The majority of flowering plants do form mycorrhizal associations, but many weeds and some crops do not. Brassicas, chenopods (spinach, beets, lambsquarters, amaranths), smartweeds and sedges do not. Mycorrhizae assist the growth of host plants by providing nutrients and a good growing environment. When conditions favor mycorrhizae, those crops are more competitive against non-mycorrhizal weeds.

The diameter of the roots also has a role. Large-seeded crops tend to have large diameter roots, while small plants tend to have small diameter roots, which can grow longer faster. Pigweed (small seeds) after 28 days of growth has a root-length:weight ratio eight times higher than sunflower (large seeds). Pigweed roots are better at gathering nutrients, because they explore more of the soil, and can absorb more nutrients (because the ratio of surface-area:volume is greater).

Some weeds flower near the end of their lifecycle, after growing quite large, in a “big bang” (pigweed and lambsquarters). Removing these weeds early in life prevents the competition from these large plants that reduce the crop yield. If you miss that opportunity, killing the weed later in life (before it seeds) will help future crops.  Other weeds are “dribblers” – they start to set seed while still small. They can hide among the crop plants, making seed whenever conditions are favorable. Failing to remove these weeds early in life will potentially reduce yields for many years. This is how galinsoga can be such a nuisance in vegetable farms, surviving where the soil is frequently cultivated, and sometimes neglected long enough for seeds to mature. It’s always worth hand-pulling a large galinsoga as you walk by, as the largest plants produce the most seeds.

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

All plant species have natural enemies (diseases and pests that have co-evolved to live in balance), plus the occasional alien plant enemy that could devastate the population. Consequently, there are few natural enemies of weeds other than imported ones. Bio-herbicides are rare. But there are less obvious natural enemies of weeds. The authors measured the mortality of lambsquarters and redroot pigweed in the absence of human intervention. 80% or more of the lambsquarters emerging after tillage died before maturity. Fungi and insects were the likely predators. Results with pigweed were similar.

There is a chart of edible weeds for those inclined to engage in direct weed eradication, and the chart includes cautions about toxic parts of each plant.

The chapter summary lists ten important lessons. Dealing with roots and rhizomes of perennial weeds; rotating between spring, summer and fall-planted crops; influencing when weed seeds germinate and when they die; using transplants; using slow release nutrients to feed your crops rather than the weeds; avoiding over-fertilization; preventing weeds from seeding; reducing arrival of new weeds on your farm.

Wheelhoe, Courtesy of Valley Oak

Chapter 3 is about cultural weed management. Ecological weed management involves “many little hammers”, using multiple strategies together in a complementary way. Crop rotation is one that involves advance planning. Spring weeds can be destroyed while preparing the soil for summer planting, reducing future pressures in spring crops. Good stands of overwintering cover crops, especially mixtures, can inhibit winter and spring weed germination. The diversity of field operations associated with particular crops is as important as the diversity of the crops themselves.

Growing healthy competitive crops is a fundamental part of weed management, and involves many aspects, starting with using high vigor, fast-germinating seeds. Planting the crop at an appropriately dense spacing will reduce weed opportunities. Any crop that produces multiple harvests (kale, tomatoes, squash) can be planted closer than most recommendations without loss of yield, whereas those with a single harvest (cabbage, lettuce, corn, root crops) will get smaller if planted too close. Planting 50% closer is usually worth trying, for a higher total yield, when smaller individual units are acceptable. Thus may involve more time harvesting, and bigger seed purchases. The reduction of weeds may benefit many subsequent crops.

Other factors not yet mentioned include row spacing, row orientation (plants get more light in rows that run N-S), choice of fast-growing or large-leaved varieties (Danvers are better at shading than Nantes type carrots), planting date (avoid the period when the dominant weed species is likely to grow vigorously), intercropping (practice with caution, avoid having two crops in competition), nutrient and water supply.

No-till cover crops, where the residue remains on the soil surface, will inhibit many weeds, and provide many other ecological benefits. Organic no-till isn’t the answer for every situation. It keeps soil cool and somewhat compacted, and doesn’t release its nutrients quickly, so it isn’t good for early spring crops, or early warmth-loving crops. To sow the necessary good stand of cover crops, tilling is required. This means no-till can have a valuable place in your rotation, but continuous organic no-till is not likely to work.

Tarping is a method of covering the soil with large opaque tarps for several weeks, to germinate and then kill emerging weeds by depriving them of light. This provides a seedbed ready to plant. Tarping can also be used to kill mowed cover crops or crop residues. Tarping can be useful in the transition from tilled to no-till farming, while weeds are still a big challenge.

Solarizing with clear plastic. Photo Pam Dawling

Solarization is another soil-covering practice, this time with clear plastic and the goal of heating the soil to kill weed seeds, pests and disease organisms in the top layer of soil. This method works in hot weather in areas with a good amount of sunlight. It works best when the plastic is laid tightly over well-prepared beds, providing good soil contact. The edges are buried to hold in the hot air. It takes several weeks to kill weed seeds, even when conditions are right.

A flock of chickens can do a good job of weed management, if penned in the vegetable garden early enough to allow 90 days after their removal before the crop is harvested (above ground crops) and 120 days for in-ground crops. These are commonsense food safety precautions required for Organic certification.

There are two main approaches to weed management. The first is to remove enough weeds so that crop yields are not compromised in an economically significant way. The second is to minimize weed seed production, aiming for very low weed populations, meaning little weed management work in the future. This preventive weed management requires more precise attention in the early years, including removing weeds that are not, in themselves, causing measurably lower yields. Either approach can be successful, but the preventive strategy is a good one for people who are growing older (!) and want less work in the future, while maintaining an income and satisfying work.

Chapter Four covers mechanical and other physical weed management methods. “The effect of tillage or cultivation on a weed population depends on the interaction between the nature of the soil disturbance and the ecological characteristics of the weed.” In other words, to control a particular weed, we need to know the features of that weed and choose methods of cultivation and tillage that will exploit the weaknesses of that weed, and take account of the weather, the soil conditions and the crop stage. Timing determines success, and the greatest success comes from using a planned sequence incorporating several operations.

There is a very clear explanation of vegetative reproduction of perennial weeds and how to thwart that process. Tilling chops up roots, which grow into new plants. Partial damage to perennial roots stimulates sprouting of dormant buds. The best chance of success comes from exhausting the root or rhizome pieces. With most perennial weeds, carbohydrates flow from the storage organ into the leaves until they produce enough food to return some to the root. The ideal stage to kill such plants is when the pieces of the storage organs drop to their minimum weight after growing new leaves. Generally this is after three or four leaves have grown.

Tilled fallow is a time without crops, when the plot is tilled often enough to stop weeds proliferating. Most annuals take 5 weeks to set seed, and so once every three weeks is a good tilling frequency, for management of both perennial and annual weeds. This will inevitably damage the soil structure. Growing a fast cover crop (buckwheat or a mustard) between tillages will reduce the damage.

Buckwheat cover crop in flower.
Photo Pam Dawling

A discussion of ten Principles of Mechanical Weeding follows. A useful chart of two dozen weeding implements and tools provides information on when and how they are best used, which crops they are most suited to and what their limitations are. The chart is followed by pages of clear drawings of various cultivators, with explanations of when they are most useful.

Often one goal is the creation of a surface layer of small aggregates allowing good air circulation and decreasing germination of new weeds. This is widely called a “dust mulch”. Weeding early, shallowly and often, is widely shared advice. Shallow soil disturbance can eliminate a large percentage of annual weeds, without bringing new seed to the surface. Small weeds do not re-root easily, as they have only small reserves of energy. Weeds over 2” (5cm) tall are more likely to re-root.

After the profiles of five farms with great weed management strategies, explaining their overall approach to weeds, comes the directory of weeds, including information on resources, naming, ecological information, recommendations for management and the limitations of those recommendations (for example, whether or not they have been field-tested).

There are summary tables of summer annual weeds, winter annual weeds, and perennial weeds, each subdivided into broadleaf weeds and grasses, with information on characteristics. To help with visualizing seed sizes from the weights given, they helpfully tell us that a lettuce seed is likely to weigh 1mg. The tables are followed by 3-4 page profiles for each weed, including several clear photos of the weed at different stages of growth, management suggestions, ecology and a handful of references for further reading.

There are tips on developing management plans for weed species that are not in the book. Some weeds are a big problem in a small geographical area, and of not much consequence elsewhere. Record your own observations, using the questions provided to focus your attention and identify the weed. Each taxonomic level (family, genus, species) can provide actionable information. There are some great resources for weed identification, leading me to find one from Virginia Tech https://weedid.cals.vt.edu/.

There is hope for dealing with even the worst weeds! “Competitive cover crops are effective for suppressing bermudagrass.” Example: A dense fall sowing of winter rye, barley or oats, harvested for forage in spring, with the stubble plowed under to allow sowing of a very competitive summer cover crop like cowpeas. The dense shade following the late spring soil disturbance will suppress the grass.

The directory is the main part of the book, and the part where you will want to search out your worst problems and form a plan. Keep this book in a place you can always find it when needed, for the rest of your farming life!

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