Posted: August 14, 2026
Until recently completing the Master Gardener program, I never really thought about weeds.
Alaska fireweed populating a field after a fire. Courtesy of Leslie Rowley
I certainly plucked unwanted plants from ornamental landscapes, but did not really understand the definition of weeds. The word "weed" had such a bad connotation in my mind that I began to identify plants around my home to determine if they were weeds. I began to wonder if my favorite flower, fireweed (Chameaenerion angustifolium) was a weed. After all, the common name of this beautiful flowering plant contained the word "weed". The Master Gardener program taught me that weeds were a bit more complicated than good versus bad. Weeds can be native or nonnative plants, and the characterization as a weed often depends on our preferences. State and federal law designates and regulates certain weeds as noxious weeds that are harmful to the environment, agriculture, and us.
In the broadest sense, a weed is any plant that is growing where it is not wanted. The inclusion of the word “weed” in the name of the plant is not dispositive of whether the plant is a weed and is unwanted. This definition can be rather subjective and based upon the likes and dislikes of the observer. For example, a homeowner may love the look of yellow dandelions in the lawn while another homeowner considers them a weed that must be eradicated. The basic definition of weeds does not hinge on whether the plant is aesthetically pleasing, has medicinal uses, or provides some benefit to the environment, humans, or animals. If the plant is unwanted where it is growing, it is a weed.
A more detailed definition of a weed is a plant that adversely impacts the use, economic value, and aesthetic aspect of the lands and waters it infests. This definition expands upon the basic definition. Like the basic definition, these plants are unwanted not only because of their subjective aesthetic value but also their potential to interfere with the use and economic value of the land or water where they grow. A golf course considers dandelions a weed because the plant impacts the appearance of the course and players' games by altering the course of the ball. A farmer may be economically injured if the forage his cows eat impacts the flavor of the milk or if there are weeds in the product he is raising for sale. Thorny or poisonous weeds may also limit the use of a playground or yard. Weeds can lower the quality and quantity of agricultural products, resulting in lost revenue.
Weeds can be native or nonnative to North America. Native means the plant occurs naturally in a particular area or was present prior to Europeans settling the continent. Nonnative plants are those that were introduced to North America either purposefully or accidentally. From colonial times until the present, global travel and commerce have introduced many nonnative plants to North America. Some plants that were intentionally brought here escaped landscaping and gardens and are now known to be very aggressive and troublesome to nature and humans. Examples of nonnative plants include: English Ivy (Hedera helix), Chinese and Japanese wisteria (Wisteria sinensis and Wisteria floribunda), dame’s rocket (Hesperis matronalis), and garlic mustard (Alliaria petiolata). These plants are also invasive. Native plants can also be troublesome, grow aggressively, and cause harm to the environment or humans. Poison ivy (Toxicodendron radicans), pokeweed (Phytolacca americana), ragweed (Ambrosia artemisiifolia), greenbrier (Smilax rotundifolia), and violets (Viola) for example are native but can disrupt the environment and cause humans much discomfort.
Some weeds are known to be invasive. In 2016, President Barack Obama signed Executive Order 13751 entitled Safeguarding the Nation from the Impacts of Invasive Species. This order amended earlier rules but still requires federal agencies to prevent and control invasive species. This includes preparing Invasive Species Management Plans to inventory, map, and prioritize efforts to control invasive species. Invasive species are defined as "a non-native organism whose introduction causes or is likely to cause economic or environmental harm, or harm to human, animal, or plant health." As the definition suggests, not all nonnative species are invasive. Queen Anne's lace (Daucus carota) is a nonnative plant but is not considered invasive because it does not cause harm, although those with sensitive skin may be irritated by touching the plant. Pennsylvania recognizes many invasive plants but does not regulate them, preferring to rely upon education. The Pennsylvania Department of Conservation and Natural Resources Invasive Plants website is a good resource for information about what plants the state considers invasive. While invasive species are not regulated, Pennsylvania characterizes and regulates 13 plant species known as noxious weeds.
The federal government and many states, including Pennsylvania, regulate certain plants and noxious weeds. In Pennsylvania, noxious weeds are plants identified by Federal Law or "a plant part or plant in any stage of development that is determined to be injurious to crops, livestock, agricultural land, or other property, including forest land and bodies of water." The list can be found on the Pennsylvania Department of Agriculture website and includes plants that were introduced for ornamental or medicinal uses and were later found to be harmful. These include Canada thistle (Cirsium arvense), giant hogweed (Heracleum mantegazzianum), purple loosestrife (Lythrum salicaria), mile-a-minute vine (Persicaria perfoliata), kudzu (Puerania montana var. lobata), and multiflora rose (Rosa multiflora), among others. These plants are illegal to sell, propagate, or transport, with some exceptions or by permit.
My beloved fireweed is native to Alaska, and while it can grow aggressively if you have it in your yard, it is not noxious. This beautiful flower is often the first plant to grow after a fire and was among the first to emerge after the eruption of Mount St. Helens. It quickly colonized the urban landscape in London after it was bombed during World War II, earning it the name bombweed. This pink to purple fireweed is like a sign of hope after a disaster and has many culinary and medicinal uses. Fireweed is also an important plant for wildlife and pollinators. You can read more about fireweed on the Fish and Wildlife Service website. Of course, while fireweed may be a native plant with value, it could also be considered a weed if it is planted where it is not wanted.
So, how can you identify plants on your property to know if they are native, nonnative, invasive, or regulated by federal or state law? Accurate plant identification is key because many plants have look-alikes that are not harmful. There are numerous applications online and resources to assist with plant identification and information, such as the Pennsylvania Department of Conservation and Natural Resources. Emailing detailed pictures or visiting the Penn State Extension Office with sample plant parts and pictures is also an option.
Leslie Rowley is a Penn State Master Gardener from Adams County, Penn State Cooperative Extension of Adams County is located at 670 Old Harrisburg Road, Suite 204, Gettysburg. 717-334-6271.
The Master Gardener Hotline is open Monday, Wednesday, and Friday from 10 a.m. to 2 p.m. Please send an email (with pictures if possible) to adamsmg@psu.edu with your gardening questions, call us at 717-334-6271 or stop by Penn State Extension, 670 Old Harrisburg Road, Gettysburg.
Interested in becoming a Master Gardener or learning more about the program? Contact Mary Ann Ryan, mar35@psu.edu, for more information or an application. Applications are being accepted through August 31, 2026.
