A Weed is Just a Weed

By Amy Smith, UW Extension Educator

It seems too early to be thinking about next year’s weeds when fall has just begun, although with a little preplanning, producers can create a weed management program and reduce the number of weed seeds added to the soil seed bank each year. 

To create an effective management program, it is important to first understand the components of the system.

What is a weed?

There are various working definitions for plants considered “weeds.” 

One definition from S.D. Miller states, “A weed is a plant which is competitive, persistent, pernicious and therefore, interferes with human activities.”  

The second definition, author unknown, says, “A plant considered undesirable, unattractive or troublesome, especially one growing where it is not wanted.” 

Either of these definitions provide a solid foundation for plants called weeds. 

Weed characteristics

There are three different growth habits for weeds – annual, biennial and perennial. 

Annuals are any plants which complete their lifecycle within one year. 

There are two subcategories of annuals – summer and winter. Summer annuals germinate in early spring, mature and disperse seeds by early fall. Winter annuals germinate in fall, mature and disperse seeds by early spring. 

All plants designated as annuals have the same characteristics. They are considered fast growing, have a shallow root system, they are prolific seed producers and the seeds are generally very small in size. 

Examples of summer annuals are Russian thistle, ragweed and puncturevine, among others. Winter annuals include downy brome, commonly known as cheatgrass; pennycress and cranesbill, among others.

Biennials are plants which complete their lifecycle within two years. During the first year, biennial plants exhibit vegetative growth which is normally low to the ground and sometimes in a rosette form, as commonly observed in dandelions. 

The second year is when the plant bolts, sending up shoots with reproductive organs such as flowers, pollination occurs and the plant sets seeds. 

Additional characteristics include a tap root and the plants tend to be bigger in size than annuals, with medium-sized seeds. Examples of biennials include musk thistle and Scotch thistle.

Perennials are plants which complete their lifecycle in three or more years. These plants tend to be larger, as they take a minimum of three years to reach maturity. They have an extensive, fibrous root system which allows them to overwinter and grow back in the spring. Their flowers tend to be showier, although not always. 

They can reproduce by seed production and vegetative reproduction, including modified stems growing laterally above the soil surface (stolons), modified stems growing laterally below the soil surface (rhizomes), tubers (potatoes), bulbs or corms. 

Perennials are persistent and the hardest to control. 

Examples of perennials include all toadflax and Canada thistle.

Why control weeds?

The life purpose of a weed is to germinate, grow and reproduce. But, is it really this simple? Yes and no. 

One thing to keep in mind is how populations grow or increase their size over time. 

There are stages to colonization related to lifecycles of all organisms, including plants – dispersal, establishment, growth and maturation and reproduction.

Dispersal of plant seeds allows colonization of new habitats which is important in agriculture because of the continual disturbance found in fields or pastures. Despite the cyclical disturbances, there are numerous organisms which maintain resident populations found in the soil and are a direct result of dispersion. 

Dispersal is a difficult action to stop completely, as wind, animals, water or gravity all work together as mechanisms for dispersal. Think about the different methods of dispersal weeds use to scatter their seeds. 

Dandelions produce a fluff which carries their seed in the wind; sandburs get stuck in animal fur, tires or human clothes and are dispersed mechanically; kochia and Russian thistle plants dry and tumble across the landscape with the wind, and every time they hit the ground, tiny little seeds fall out and become part of the seed bank.

One might wonder how plants grow in what seems like barren ground. There is what is known as a seed bank, which is the top two to six inches of soil, where seeds from many species of plants are in a dormant stage until conditions are perfect for germination. 

Seeds can live in the soil for up to 50 years and will not all germinate at the same time and not until environmental conditions are suitable. Why is this important? The seeds found in a specific seed bank can tell a producer about the prior history and management of a particular piece of ground.

The adjacent table refers to several common weeds found throughout Wyoming, the number of seeds produced per plant and the number of years the seeds are able to germinate from the seed bank. 

Palmer amaranth is a relatively new weed species in Wyoming. However, it is making a name for itself quickly. 

One palmer amaranth plant can produce up to 450,000 seeds. Although they are only viable for approximately three years, it is easy to calculate how one plant can lead to an exponential number of seeds into the soil bank in a short amount of time. 

Palmer amaranth is also notorious due to fact it is already resistant to many common herbicides. Therefore, it is imperative for producers to be aware of the weeds present and create a management plan to control them.

Managing the seed bank

Although there is no fail proof way to know all of the species of weed seeds present in the seed bank, vigilant observations of seasonal patterns and species germination will provide producers with a good idea of weed species present. 

As with most pest species, an integrated approach is the best way to manage weeds present, whether in a cropping system or a livestock operation by using one or more of the following methods.

Mechanical or physical control is any method that physically controls weeds and is most effective when weeds are young. Examples include pulling, cutting and/or hoeing.

Cultural practices reduce weed establishment, reproduction, dispersal and survival. Examples include crop rotation, altered planting/harvesting and/or trap crops.

Biological control is the use of a weeds natural, living enemy to control reproduction and dispersal. Examples include using the musk thistle flea beetle to control musk thistle and goats or sheep to control tansy ragwort or leafy spurge. It should be noted this is not a quick control and only maintains populations – it does not eliminate a population.

Chemical control is the use of organic or synthetic chemicals to control weed populations. Examples include 2,4-D to control broadleaf weeds and Roundup Ready Corn to control weeds with the glyphosate product Roundup in corn crops. 

Timing is key

Timing of weed control is key to minimizing new deposits of weed seeds into the seed bank. 

One key strategy for weed control is to kill the weeds before the plant set seeds. The earlier the better, as many plants have the ability to set seeds, if they were flowering at the time they were pulled or severed. 

Control perennial weeds before they form new rhizomes, stolons, corms or bulbs. Try to keep crops ahead of the weeds. Weeds shaded by the crop canopy are less likely to produce seeds. 

Physically remove any weeds that escaped other control methods. 

Minimize weeds found in marginal areas around fields and irrigation canals and ditches by mowing, tilling, cultivating and/or grazing. Be mindful of weed-free seed or hay to minimize spread or introduction of weeds. 

Creating an effective weed management plan is essential to reducing the number of seeds deposited into the seed bank every year. 

Weed management plans should include correct identification of all weeds present, growth characteristics of the weeds present, dispersal methods of weeds present and how and when to best control the weeds present through a variety of methods.

Managing weeds is a challenging task. However, when producers effectively manage weeds, it helps reduce the number of weeds spread unintentionally through seeds, crops, wind, water, hay, wildlife, humans, etc., thereby helping the community. 

Amy Smith is an agriculture and natural resources educator with the University of Wyoming Goshen County Extension. She can be reached at asmit207@uwyo.edu or 307-532-2436.