Weed Identification and Control

A weed can be any plant growing where you don’t want it to. They compete with other plants for nutrients, water, and light, as well as harbor diseases and pests. On this page, you’ll find information on weed identification and control, including catsear, marestail, purple loosestrife, pokeweed, pigweed, poison ivy, crabgrass, hemlock, purslane, and multiflora rose. You’ll also find tips on using various herbicides and integrated pest management.

Weed Control

The actual definition of a weed varies. Many people consider it to be any plant that grows where you don’t want it. An alternative clarification is that they are plants whose undesirable qualities outweigh their good. Whatever definition you choose, correctly identifying major weeds can be a crucial first step toward effective weed control.

Some weed species can look very similar at certain growth stages, yet have entirely different life cycles, modes of reproduction, effects on crops, and responses to control tactics. Understanding weed ecology, biology, and identification will help you design a management system to minimize their impact.

Lawn Weed Identification and Management

Weeds can appear in a lawn for a number of reasons. It could be because the soil is compacted, poor soil conditions, improper mowing practices, frequent, shallow watering, or not fertilizing correctly.

Weeds are not welcome in a lawn because they compete with desirable grass species for moisture, light, and nutrients. They also affect the aesthetics of turfgrass and its uniformity.

One of the most common weeds that occurs in home lawns is moss. It tends to grow where turfgrasses are thin and weak. One particular moss species, Bryum argenteum, is becoming a common weed on Pennsylvania golf courses.

Annual grasses such as crabgrass and goosegrass can also be an issue in many Pennsylvania lawns. You can obtain satisfactory control of summer annual grass weeds by both cultural and chemical methods as long as you consider the plant’s life cycle. Control starts with the creation of the seedbed.

Weed Control for Food Crops and Forages

Weeds in field crops and forages can reduce yields by competing for water, sunlight, and nutrients. Weeds in field crops also lower the quality, increase the risk of disease and insect problems, create harvesting problems, and cause premature stand loss. Some weeds are unpalatable to livestock, or in some cases, poisonous.

Managing weeds is one of the biggest challenges for producers of agronomic crops, forage crops, fruits, and vegetables. You can, however, control them through an integrated weed management approach.

The initial focus of weed management and control should be on cultural practices before chemical weed control. Food crops and forage weed management starts with identification.

Timing is also vital for successful weed management and control. Biennial and perennial weed control are most effective in the fall, before they overwinter. If left until the following year, weed control will be more challenging. It’s essential to monitor and control annual weeds after silage harvest, but it depends on the weeds that you are targeting, as not all annuals set seed at the same time.

The methods of weed management and control a producer chooses will depend on the type of production system they use. Managing weeds in conservation tillage systems requires a planned approach. Organic producers can use many of the same weed management techniques as those used in conventional systems, but the focus is more on nonchemical control strategies.

Managing Invasive Plants

We consider a plant species to be “invasive” if it has moved into a habitat and reproduced so aggressively that it displaces some of the original plant species. The United States is under constant threat of invasive species. They can adversely affect biodiversity, cause the ecosystem’s instability, decrease land productivity, poison livestock, reduce wildlife habitat quality, diminish aesthetic value, and affect human health.

Managing invasive plants such as the tree-of-heaven, multiflora rose, and pigweed is therefore essential. In the home garden, there are certain invasive shrubs you should avoid. They include burning bush, privet, Japanese barberry, and the butterfly bush.

There is lots of information available to help you identify invasive species. The publication Invasive Forest Plants of the Mid-Atlantic is a full-color publication with descriptions of 25 of the most common invasive plants in Pennsylvania. Penn State Extension runs a workshop to help you learn about the impact of invasive plants and control measures using herbicides. A Master Watershed Steward and Master Gardener in Montgomery County also shares her experience of discouraging invasive species in her own garden.

Poisonous and Noxious Weeds

Noxious and poisonous weeds are a big concern for livestock producers. They can be a problem in hay or pastures and come in all shapes and sizes.

• Milkweed is a native perennial herb with milky sap and leaves. The smallest of doses, if ingested, may cause toxicosis and possibly death. Cattle, sheep, and horses are most susceptible and milkweed does not lose the toxicity when dry.
• Poison hemlock is commonly found along roadsides, fields, and wet areas. It is poisonous to animals and humans.
• Noxious pigweed is a problematic weed for field crop growers. You should verify identification and report it to your local Penn State Extension Office and to your professional crop advisor. An integrated management approach and aggressive management of the weed will prevent seed production and spread.
• Jimsonweed is an annual that is poisonous to animals and humans. It grows well in landscape beds or open and undisturbed areas.
• Poison ivy is a native woody, perennial vine. All parts of the plant contain resinous compounds called urushiols. When it comes into contact with the skin or you inhale it, it can cause inflammation, itching, and blistering.
• Purple loosestrife is found along waterways, marshes, and wetlands. It became popular as an ornamental plant because of its pretty pink flowers and attraction to foraging bees. But we now consider it a noxious weed across the US, particularly in the northeast.

Herbicide Application and Herbicide Resistance

Management and control of weeds have traditionally involved the use of herbicides. You can apply them using several methods and at various times during the year.

Fall is a good time for herbicide applications in orchards. If the weather over the winter has been mild, you can apply post-harvest pre-bud swell herbicides in the spring. Some winter annual weeds are easier to manage in the fall, particularly when it comes to wheat and barley establishments. Herbicide application times and restrictions for corn can vary, but there is lots of help available online. The same applies to soybean herbicide applications.

Timing is critical with certain residual herbicides such as early pre-plant (EPP) herbicides. If you apply them at the wrong time, you may require additional weed control in the form of a post application.

You must take many other factors into consideration when using herbicides, such as soil persistence or residual soil life. If you spray burndown herbicides close to vineyards or other sensitive areas, you must be aware of herbicide drift and drift-related damage. When you use burndown herbicides, the air temperature before, during, and after application can affect the control.

You have to use the proper herbicide formulations to prevent damage to non-target plants. Herbicide carryover is another issue to be aware of.

Drift related damage could mean you face a potential lawsuit. It’s possible to reduce spray drift and other considerations by making changes to the spraying system.