“Clumping” is a horticulture term that describes plants that spread slowly to form a cluster of new plants. Few, if any, gardeners object to clumping perennials that increase themselves for free in an orderly manner.
“Spreading” perennials grow rapidly and produce many offspring. The fine line between “clumping” and “spreading” depends on whether the plant outgrows its assigned spot. If a plant creates extra work to remove unwanted offspring or competes with its neighboring plants, then it is “spreading.”
Personally, spreading inside a landscape bed is acceptable but spreading into the lawn, or more than 6 feet from the original spot, is not.
While the word “invasive” may accurately describe the behavior of any aggressively spreading plant, invasive specifically refers to non-native plants that spread into native habitats and are likely to cause harm (bit.ly/2MXfarK).
Spreading plants produce two specialized organs that grow underground. Stolons are horizonal stems, and rhizomes are roots that grow horizontally. Both stolons and rhizomes serve the same purpose: allow the plant to reproduce and colonize a larger area.
A good example of a spreading plant is swamp crinum (Crinum americanum). This native is well adapted to moist soil and even tolerates flooding. The first swamp crinum I bought bloomed the first year. Since this perennial is deer-resistant, I bought two more the following year.
When I planted them, I noticed the original plant was sending out stolons. Ten years later, the bed has 120 plants. Up to a dozen bloom in summer or early fall, depending on rainfall.
Swamp crinum’s tendency to spread outside the bed into the lawn is annoying. Mowing doesn’t faze the six plants growing a foot outside the bed, only shortens them temporarily. Their crowns are at least 4 inches deep in the ground, so digging them out would make holes in the lawn.
Another spreading native plant is swamp sunflower (Helianthus angustifolius). This perennial is known as narrowleaf sunflower in the Midwest, where it is “well-behaved” in the drier soils. In moist soil here in the Southeast, however, it spreads rapidly during late winter.
I started with one plant in a bed along the drainage easement at the back of the yard. That plant produced 12 plants the first year and more than 150 the next. Clearly, some had to be removed. Although deer severely limit the height of swamp sunflower, they don’t slow the spread, since the plants start as flat rosettes that don’t attract the attention of deer.
A non-native spreader is perennial mum. 'Gethsemane Moonlight' is a late fall bloomer with pale yellow, daisy-like flowers with narrow petals. In three years, my 1-gallon plant spread into a loose clump measuring 4.5 by 5 feet with more than 115 shoots.
The most vigorous spreader I have is wild ageratum (Conoclinium coelestinum), also known as blue mistflower, a fall-blooming, lanky version of annual ageratum.
Moist soil encourages rampant spread. The stolons of wild ageratum will grow under steppingstones. The Georgia Native Plant Society suggests cutting it back after flowering to prevent unwanted seed spread, which I plan to do this fall (bit.ly/2MZFb9P).
Spreading perennials can co-exist with gardeners who like neat and tidy beds. There are two basic approaches to corralling spreading plants. One technique involves on-going effort, whereas the other technique is a lot of work initially.
The first technique is to regularly remove any new plants that appear outside the allotted space. Although it may be tempting to leave them, these plants will eventually produce their own offspring, and by that time, the “daughter” plants will be well established and may be difficult to remove.
The second technique is to place a vertical barrier in the soil at the edge of the allotted space. The barrier must be tall enough and placed sufficiently deep to prevent stolons or rhizomes from growing underneath it. Landscape edging may work, but some types are too shallow.
A large, e.g. 15-gallon, plastic pot with the bottom cut out may be a better choice. It would allow a one-gallon perennial to spread but still contain it within a reasonably large space.
A third option for gardeners who have the room is to let spreading perennials occupy as much space as they want.