Problem Plants | Invasive, Noxious, Dangerous

Plants provide us with food and fiber. They give us air to breathe and they cool our yards. They are a source of food and shelter for many animals, insects and birds. There are some plants, however, that cause problems for us. Those plants will be covered in these web pages.

Invasive plants may be the first that come to mind, and they are included in these web pages. But there are other plants that are problematic as well, such as those that are noxious weeds or are very susceptible to diseases or pests.

Since The Morton Arboretum is located in Illinois, plants that have legal standing in the state of Illinois have their own sections: ‘Exotic Weeds’ and ‘Noxious Weeds’. Plants that may have some legal determination in another midwestern state can be viewed on the Midwest Invasive Plant Network.

This webpage also covers plants not recommended by The Morton Arboretum due to susceptibility to extremely serious disease or insect problems. A final group of plants covered here is dangerous weeds. These plants are weeds that may be dangerous to ingest or handle. They have been selected based on information reported to the Plant Clinic at The Morton Arboretum.

The lists of plants included in these categories may change over time as new information is obtained.

Invasive and Potentially Invasive Plants

Invasive plants are recognized as a serious problem for natural environments, farms, residential neighborhoods and commercial areas. The definition of the word “invasive” is often misunderstood, leading to confusion about which plants should be removed and which should be preserved. An invasive species is more than just a plant that spreads easily or rapidly. An invasive species is one that is non-native to the location being considered and whose presence negatively impacts (or is likely to impact) ecological, economic or human health.

There is no universally accepted list of invasive plants. Local, state and federal agencies have lists that differ. In part, this is due to the fact that a plant that is invasive in one state may not be invasive in another. Many states do not update legislation about invasive plants on a regular basis.

For these reasons, it is often wise to look outside one’s own location to see what other states are legislating. For instance, individuals living in northern Illinois may want to look at invasive species lists in Wisconsin and Indiana to see which plants are considered invasive in those neighboring states. Invasive species lists for Midwestern states can be found on the website for the Midwest Invasive Plant Network. The website for the National Association of Invasive Plant Councils serves a similar purpose for other parts of the country.

The Morton Arboretum and other public gardens are working together to track and document non-native plants that are escaping from their cultivated collections. These organizations are using a standard methodology to better determine which plants are actually spreading from cultivation. The woody plant species presented on this webpage were chosen based on data gathered at The Morton Arboretum. Species described as invasive fit the definition given above.

Those described as “potentially invasive” have established populations that often occur at considerable distance from the collection where they originated on the grounds of the Arboretum, and/or are growing in uncultivated areas on or outside the Arboretum’s property. The “potentially invasive” label may also be applied to a species in an established population that is consistently managed or controlled to prevent spread within the Arboretum.

The list of woody plants that The Morton Arboretum considers invasive or potentially invasive based on Arboretum data includes the species listed below. The list will expand in the future as more data becomes available. At this time, the list is limited mostly to woody plants, as those plants are the ones being documented at the Arboretum.

There are many non-woody plants that are considered invasive or potentially invasive. At this time, The Morton Arboretum is not collecting data on these plants. Information on the invasiveness of non-woody species is available through other sources, including the Midwest Invasive Plant Network and the Invasive Plant Atlas of the United States. The atlas is a collaborative project of several organizations, including the National Park Service, the University of Georgia Center for Invasive Species and Ecosystem Health, and the National Association of Invasive Plant Councils. Two non-woody species have been included in these web pages, as they are widely regarded as potentially invasive in the Midwest.

Woody species considered invasive:

Acer platanoides (Norway maple)

Ailanthus altissima (tree of heaven)

Alnus glutinosa (common, black, or European alder)

Berberis thunbergii (Japanese barberry)

Celastrus orbiculatus (Oriental bittersweet)

Elaeagnus umbellata (autumn olive)

Euonymus alatus (burning bush)

Frangula alnus (syn. Rhamnus frangula) (alder buckthorn or glossy buckthorn)

Ligustrum vulgare (common or European privet)

Lonicera japonica (Japanese honeysuckle)

Lonicera maackii (Amur honeysuckle)

Lonicera morrowii (Morrow’s honeysuckle)

Lonicera tatarica (Tatarian honeysuckle)

Lonicera xylosteum (fly honeysuckle)

Morus alba (white mulberry)

Pyrus calleryana (Callery pear)

Rhamnus cathartica (common buckthorn)

Rosa multiflora (multiflora rose)

Ulmus pumila (Siberian elm)

Woody species considered potentially invasive:

Acer tataricum subsp. ginnala (Amur maple)

Koelreuteria paniculata (golden rain tree)

Phellodendron amurense (Amur corktree)

Populus alba (silver poplar)

Rhamnus davurica (Dahurian buckthorn)

Rhamnus japonica (Japanese buckthorn)

Robinia pseudoacacia (black locust)

Non-woody species considered invasive:

Phalaris arundinacea (reed canary grass)

Phragmites australis (common reed)

Plants Not Recommended Due to Serious Disease or Insect Problems

There are some ornamental plants, once standards in the landscape, that are no longer recommended by The Morton Arboretum. These plants are so susceptible to serious diseases or insects that attempting to maintain them in good health is cost-prohibitive and often ineffective.

Why have these species become so problematic? In some cases, a non-native insect or disease may have made its way into the country and caused damage to a desirable species. For example, the arrival of the emerald ash borer has led to the death of hundreds of millions of ash trees.

Weather also plays a role. Changes in climate have led to unusual weather patterns that are not always conducive to good plant growth. This puts plants under stress and has favored the spread of some problems, such as canker diseases and bronze birch borer, that attack trees under stress.

Another factor in the increase of disease and insect problems is overplanting. In the past, Austrian pine (Pinus nigra) had tremendous value in the landscape. It is an evergreen with long, dark green needles and a full form. It is tolerant of salt, alkaline soils and drought. It could be used as a specimen tree, windbreak or screen. This led to it being planted extensively in both residential neighborhoods and on corporate properties. However, Austrian pine is very susceptible to the fungal disease diplodia tip blight, and having a large number of host trees together allows this disease to spread more rapidly.

The list of trees that The Morton Arboretum no longer recommends due to serious disease and insect problems includes the trees below. The list may expand in the future as climate continues to change and as new pests arrive in North America.

Betula pendula (European white birch)

Betula pubescens (Moor birch)

Betula utilis (Himalayan birch)

Fraxinus americana (white ash)

Fraxinus excelsior (European ash)

Fraxinus nigra (black ash)

Fraxinus pennsylvanica (green ash)

Fraxinus quadrangulata (blue ash)

Fraxinus tomentosa (pumpkin ash)

Juglans cinerea (butternut)

Pinus nigra (Austrian pine)

Pinus resinosa (red pine)

Pinus sylvestris (Scots pine)

Populus nigra (black poplar)

Ulmus rubra (slippery elm)

Noxious Weeds

Since The Morton Arboretum is located in Illinois, plants that have legal standing in the state of Illinois as “noxious weeds” are presented here. Plants that may have some legal designation in another Midwestern state are listed by the Midwest Invasive Plant Network.

Under the Illinois Noxious Weed law, certain plants are legally designated as noxious weeds. The law defines a noxious weed as “… any plant which is determined by the Director, the Dean of the College of Agricultural, Consumer and Environmental Sciences of the University of Illinois and the Director of the Agricultural Experiment Station at the University of Illinois, to be injurious to public health, crops, livestock, land or other property. ‘Noxious weed’ does not include industrial hemp as defined and authorized under the Industrial Hemp Act.”

The law requires that “Every person shall control the spread of and eradicate noxious weeds on lands owned or controlled by him and use such methods for that purpose and at such times as are approved and adopted by the Director of the Department of Agriculture.”

The plants covered by this law are:

Ambrosia artemisiifolia (common ragweed)

Ambrosia trifida (giant ragweed)

Cannabis sativa (marijuana)

Carduus nutans (musk thistle)

Cirsium arvense (Canada thistle or field thistle)

Pueraria montana var. lobata, formerly Pueraria lobata (kudzu)

Sonchus arvensis (perennial sow thistle)

Sorghum halepense (Johnson grass)

Sorghum x almum (sorghum-almum)

Dangerous Weeds

Many plants can be classified as weeds because they can reproduce efficiently and spread into farm fields and home gardens. These weeds compete with desirable crops and plants. Some weeds also can pose a hazard to human health. Poison ivy, for example, may be dangerous to handle or simply touch. Others, such as poison hemlock, may be dangerous or even fatal to ingest. This page is limited to these weeds.

The dangerous plants selected for this webpage are those that are often reported to the Plant Clinic at The Morton Arboretum as commonly found growing in residential neighborhoods and natural areas. While the list contains non-native plants, there are also native plants that can be dangerous. Many of these weeds have become more widespread in recent years, making it possible for more people to come in contact with them.

There are also many desirable landscape plants that have poisonous parts. For more information on those plants, contact Poison Control at 1-800-222-1222 or www.poison.org. That organization has an extensive database on poisonous plants; The Morton Arboretum does not.

For plants that are toxic to pets, the American Society for the Prevention of Cruelty to Animals (ASPCA) has an Animal Poison Control Center, 888-426-4435, as well as a website about plants that are toxic to pets.

The Morton Arboretum’s list of dangerous weeds may expand in the future.

Conium maculatum (poison hemlock)

Datura stramonium (jimsonweed)

Heracleum mantegazzianum (giant hogweed)

Pastinaca sativa (wild parsnip)

Phytolacca americana (pokeweed)

Solanum dulcamara (bittersweet nightshade)

Toxicodendron radicans (poison ivy)

Toxicodendron vernix (poison sumac)

Urtica dioica (stinging nettle)