There are plants that like to spread out, taking over bare spots and even crowding out neighboring plants. It would be tempting to call these plants “invasive.” They will often invade space reserved for other plants and can even choke or starve them out. However, the word “invasive” is usually reserved for a plant species that a governmental body — city, state or federal — has identified as posing an economic or environmental threat. In Idaho the list of such “invasive” plants includes Yellow Toad flax, Salt Cedar, Scotch Broom, Purple Loosestrife, and Invasive Knotweed. Idaho nurseries and landscape centers are prohibited from selling those plants.
The plants I’m concerned with here have not been outlawed. They are sold at nurseries. Many are marketed as “fast growers,” “good at filling bare spots” and “low maintenance.” They are strong plants that can take care of themselves even in poor soil. I will call them Pac-Man plants.
You recall Pac-Man, the arcade game that became a craze in the 80’s. Pac-Man had a cartoonish yellow head with a very large mouth and a voracious appetite. The player accumulated points by navigating Pac-Man around as Pac-Man gobbled up rows of Pac dots.
Pac-Man plants spread out, gobbling up territory and often outcompeting nearby plants. We like to think of plants staying where they are, rooted to the soil. So how can plants spread out? To understand, we need to look at life from Nature’s perspective. Nature isn’t interested in individuals. Nature focuses on the survival of the species, and for nature, species survival is all about reproduction.
Pac-Man plants reproduce shamelessly, using varied approaches: self-seeding, sending down a deep tap root, sending lateral roots called runners that wind along the surface of the soil, roots called stolons just under the surface or roots called rhizomes even further underground. These plants may use one or a combination of reproductive strategies. They are not team players, and you should use them with caution. You may end up with too much of a good thing. Several years after planting, you may rue the day you put them in the ground.
Mint easily earns Pac-Man plant status. When my wife and I moved into our new home here in Twin Falls, the raised beds behind the house hadn’t been used for at least two years. Mint had taken over one of those beds, and I wanted the mint out. I tried everything. I tried tearing out the roots which had formed a dense mat. That root mat was so thick that when I finally got it out, the soil level dropped 3 inches. Despite my efforts, mint started to reappear within several weeks. I spent two more weeks pulling up mint but finally gave up and sprayed with Round Up. Now, three years later, I’m still pulling mint out of that raised bed. That is one tough plant.
Mint isn’t the only plant that can cause headaches. Siberian elm, podded honey locust, white mulberry, and Russian olive are all hard to get rid of once established. They will also aggressively colonize open or neglected areas. In the garden, Sweet Annie, Nicotiana, Bellflower, Bee Balm , Grape Hyacinth, Tiger Lilies, Chinese Lantern, Bells of Ireland, Yarrow, and Reed Canary Grass are just some of the plants that can overrun the area you have chosen for them.
Ground covers are particularly notorious for being rampant expanders. When looking for a ground cover, often one of our main concerns is how quickly it will cover the ground. Nursery personnel are quick to introduce us to varieties known for quickly filling in bare spots. One variety noted for vigorous growth is Vinca, commonly known as Periwinkle. Other aggressive ground covers include White Nancy, Golden creeping Jenny, Cypress spurge, Crown vetch and Japanese spurge. The worst is probably Snow in the Mountain or Bishop’s Weed. Its green leaves on rapidly growing stems will colonize your bare shade and then start challenging neighboring plants.
Any time you add a new plant to your landscape, ask yourself: Will this plant pose a threat to its neighbors? Remember: All life forms are competing for limited resources. Some are more competitive than others. Will this plant outcompete other plants that you value? Talk to the people at your nursery center. Ask pointed questions. Call the County Extension Agent or any Master Gardener you know. Always get the facts before you plant. When a new plant goes into your landscape, you want it to be a good neighbor.