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.
- Conferences
$10.00
Crops Conference
When 02/04/2025Length 6 hours, 30 minutesEvent Format In-PersonRecap the 2024 crop production season, discover agronomic innovations, and earn valuable certification credits. Attendees gain expert insights, network with industry leaders, and enjoy a buffet lunch. - Videos
Poison Hemlock Identification
Length 2:08Learn how to identify poison hemlock, a toxic weed, growing in Pennsylvania. - Guides and Publications
Starting at $8.00
Mid-Atlantic Field Crop Weed Management Guide
Get strategies and peruse herbicide tables for managing weeds in corn, sorghum, soybeans, small grains, and forages. - Videos
Autumn Olive: Accurate Identification
Length 3:23Learn the distinguishing characteristics to help you accurately identify the invasive autumn olive (Elaeagnus umbellata) shrub. - Articles
Noxious Weed - Catsear
Horse owners should manage catsear presence in pastures and limit their horses' contact with the plant. - Articles
Purple Loosestrife in Pennsylvania
Purple Loosestrife is an aggressive plant that is invading our wetlands and replacing valuable wetland plants; eliminating food and shelter for wildlife; and choking waterways. - Articles
Pesticide Toxicity
Because pesticides can be toxic, they are also potentially hazardous to humans and animals. - Articles
Weed Control in Home Fruit Plantings
Weed control can be accomplished chemically or mechanically. Depending on the situation, each method has its advantages and disadvantages. - Articles
Home Orchards: Table 2.4. Plant Protection Materials
Informational table showing pesticides available for home garden use on various fruit crops. - Articles
Identifying Weeds in Home Fruit Plantings
To adequately control weeds in the small-scale orchard, you should learn proper weed identification. - Articles
Pests and Pesticides in Home Fruit Plantings
Can fruit crops be grown in the home garden without pesticides? The answer is yes, but fruit quantity and quality may decrease. - Articles
What Makes a Good Plant or Insect Sample?
Advice for collecting a good plant or insect sample for identification. - Articles
Juniper and Minute Cypress Scales
Juniper and minute cypress scales are pests of junipers, cedars, cypresses, false cypress, incense cedar, and related trees and shrubs in landscapes and nurseries. - Videos
Controlling Tree of Heaven: Why it Matters
Length 3:53Learn about the invasive tree-of-heaven, commonly found in Pennsylvania, and how you can use it to help control another invasive species, spotted lanternfly. - Articles
Weed Control in Cut Flowers
Managing weeds in cut flowers often requires different approaches depending on the specific weed, specific flower or woody stem, time of year and level of weed infestation. - Videos
Tree of Heaven: Native Look-alikes
Length 5:06Learn the identification characteristics of the invasive tree-of-heaven and be able to distinguish it from the native look-alikes, black walnut and staghorn sumac. - Videos
Tree of Heaven: Control Strategies
Length 6:37Tree-of-heaven is especially difficult to eliminate. Learn when and how to apply control measures, including herbicides, to eradicate this prolific tree. - Articles
Bull Thistle Identification and Management
Bull thistle (Cirsium vulgare) is a biennial that true to its name, has notable spines. This plant is much different than Canada thistle. - Articles
Yellow Woodsorrel
Yellow woodsorrel (Oxalis stricta) has foliage (composed of three leaflets) that resembles white clover or black medic. - Articles
Wild Onion and Wild Garlic
Wild garlic (Allium vineale) and onion (Allium canadense) are perennial plants that can be a weed in turfgrass and landscape beds. - Articles
Purple Loosestrife – A Noxious Weed
Recently, some folks pulled into the local nursery, asking about the pretty pink flowering plants that were blooming along the fields and roadsides. - Articles
Poison Hemlock
Poison hemlock (Conium maculatum) appears to have become more abundant over the past few years.