Extension Educator, Horticulture
I look at my wheelbarrow and what do I find? A pile of plants with exotic names and rapturous descriptions. A pile of plants whose source was one small lovely plant in a plastic pot. A plant I willingly and lovingly added to my garden as I dreamed of luxurious blooms requiring only my admiration. But now I cuss and curse, tug and tear to remove this plant transformed from beauty to beast. How can this be? Their pictures are not on the most unwanted list on garden center walls. Their profiles are not in the weed identification books.
Between the digging and pulling I reflect on my plight, "Why didn't somebody warn me of this plant's alter ego?" Or during periods of deep self-deprecation, "I'm supposed to be a horticulturist. Why didn't I know this gorgeous plant would turn on me like a rabid dog?"
Even though my garden is experiencing these space invaders, spreading is not necessarily a bad plant attribute. In my garden mayapples and purple coneflower can stretch their legs all they want. Sometimes we want groundcover plants to spread. Perhaps a slope is prone to erosion or is too steep to mow. Or we just don't want to mow. Ground covers are an appropriate solution in certain landscaped areas. However, right plant in the right spot has never been as true as with groundcovers.
The battle starts when we mix plants prone to misbehaving with plants of a more focused mind in our flower beds. The unruly plants throw a party and quickly overwhelm the neighborhood with their bad behavior. Or worse yet they move beyond the neighborhood into the local natural area.
For us to predict a plant's propensity to roam, we first need to understand how plants spread. A plant strolls or sprints depending on how well a garden's soil and environmental characteristics match the plant's needs. Poor growing conditions such as too much shade can keep sun-loving plants in bounds.
Some garden plants such as Ribbon grass, Phalaris arundinacea 'Picta', are aggressive no matter what the environmental conditions. Others such as the prolific reseeder Brazilian verbena, Verbena bonariensis, are open range plants and need bare space, even if it's your mulched path, to become aggressive. Few bare spaces equal few verbenas.
Physiologically plants can spread by rhizomes, stolons and seeds. Some spreaders use more than one method. Rhizomes are underground stems usually horizontally growing and often look like roots. Since they are stems, they have buds and nodes where future stems arise. Rhizomes are sneaky. Since they are underground we often don't notice their spread until they pop up on the other side of the sidewalk.
Stolons are horizontal stems that creep or dash along the soil surface. They root at the tips or at the leaf nodes. Each place they root a new plant forms but remains attached to the mother plant. Plants that spread by stolons are easier than rhizomes to remove by pulling since stolons are closer to the soil surface.
Many space invaders overwhelm by producing a gazillion seeds. We enjoy the plants then next year there is a carpet of seedlings from just a few mother plants. Sweet Annie, Artemesia annua, is a good example as I am still plucking out seedlings after planting it 15 years ago.
Next week's column I will share my hit list of plants behaving badly.
A handy fact sheet from Purdue "Spreading Ornamental Plants: Virtues & Vices" http://www.hort.purdue.edu/hort/ext/pubs/hla/hla_001.pdf or stop by our office or call (217)333-7672 for a copy.