Gardeners need to be informed about plant growth habits.

What distinguishes the average garden thug from a true invasive plant? While that English ivy (hedera helix) may have taken over more of your yard than you bargained for or that common orange daylily (hemerocallis fulva) has started to bully less strenuous perennials into submission in your flower garden, are these plants true invasive species or merely aggressive growers?

There is a great deal of debate within the horticultural community on how to determine whether a plant has become invasive. “Some invasive plants lists are based more on opinion than scientific fact,” comments Mike Bryan, nursery program manager for the Michigan Dept. of Agriculture. “In Michigan, the state Legislature works closely with the state’s Department of Agriculture and Michigan State University to determine which plants are on the state’s official invasive plant list. These plants are officially added to the list by statutes passed in the Legislature.”

“Michigan is basing their invasive plant list on the definition stated in the 1999 “Federal Executive Order on Invasive Species,” which states that an alien species (plant, animal, insect, bacteria and fungi) whose introduction does or is likely to cause economic or environmental harm or harm to human health,” says Robert Schutzki, associate professor in horticulture at Michigan State University and member of the Michigan Invasive Plants Council. “Alien species” applies to non-native as well as native species outside their natural range or ecological boundaries.

There are hundreds of non-native plant and insect species now in Michigan. As many as one-third of the plants growing in Michigan may be non-native. Many of these plants were brought into the state to be used as ornamental plants or herbs such as garlic mustard, purple loosestrife and buckthorn, which have turned out to be true invasive (also known as noxious) plants.

“Because a plant is an aggressive grower does not mean it’s invasive,” Schutzki comments. “One of the key elements of an invasive plant is whether it jumps the ‘gap’ on its own, which means it has moved to a new location on its own and not been put there by a human.” He also suggests discovering whether the plant’s seeds are subject to long distance dispersal by wind and wildlife, and whether growing conditions are ripe for the plant to aggressively colonize natural areas.

“Many plants that are considered invasive in the South or in the West would not be considered invasive in the Midwest due to our climate,” Schutzki adds.

You will see some species on invasive plant lists, such as barberry, which can lead people to think it includes all barberry bushes. Actually, the barberry listed as an invasive plant is an old species that has not been sold for some time. There are many new cultivars of barberry that are fine to plant in your landscape.

So, what is a gardener to do while the “invasive vs. aggressive” debate is waged? Schutzki advocates that first and foremost, gardeners need to be informed. If you want to put a plant in your garden or landscape, find out its growth habits. The words “colonize,” “naturalize,” or “ground cover” in association with a plant means it’s probably an aggressive grower.

Secondly, consider where you are locating a plant, especially if it is an aggressive grower such as English ivy or a naturalizer such as the common orange daylily. You need to put these in a space that contains them, or where their spread will cause no problems. If you are putting in excessive time and effort to keep a plant under control, then there is a good chance you have the wrong plant in the wrong place and it is best to remove it.

Gardeners who live close to natural areas, forest preserves or waterways have an extra responsibility to monitor aggressive plants in their landscapes since these can quickly spread to the natural areas and destroy those habitats. If you live near a wooded or natural area, don’t plant an aggressive grower on the border of an area where it can get out of control—you want to put it in a space that will contain it.

Thirdly, dispose of plant material wisely. Think twice about returning certain organic materials to a natural area. Often, it’s the aggressive growers that gardeners are digging out of their gardens. Don’t dump them in natural areas. Either give them to friends who understand what they are getting, or compost or burn them. If necessary, bag them up for trash disposal.

For lists of invasive plants also known as prohibited and restricted weeds in Michigan, visit the Michigan Invasive Plants Council website and the Michigan Department of Agriculture

Rita Henehan, a freelance writer and photographer from White Pigeon, is a member of Midwest Energy Cooperative. She is an Advanced Master Gardener and author of “The Michigan Gardener’s Companion: An Insider’s Guide to Gardening in the Great Lakes State.”