By Jeanne Lawrence, Butte County Master Gardener, August 9, 2013

Given the right conditions, some seemingly mild-mannered plants can become garden thugs, invading entire yards and even moving on to the rest of the neighborhood.  We may all have heard stories about the bamboo or mint that threatened to take over a garden, but there are other potentially nasty plants lurking out there. In the interest of saving others from the anguish of dealing with unexpectedly invasive plants, our local Master Gardeners (MGs) volunteer the following tales of their own (often on-going) battles with particular members of the plant kingdom.  

Violets! Their flowers are lovely, and they create a useful ground cover….and then some. One MG reports with exasperation that they “are truly a pest;” another says that they “will never be gone.” Plus they provide a handy hide-out for leafrollers, the larva of the Geshna Cannalis moth. Leafrollers literally roll themselves up in the leaves. They deform the leaves and will spread to other plants.

Morning glories have proven to be tenacious fighters in many gardens. In particular, the perennial morning glory (often labeled “Heavenly Blue” or “Blue Dawn Flower”) can cover entire buildings, and grows so fast you'd swear you could see it growing while you watched it. One Chico MG reports that it took years to eradicate it from her yard – vines were still emerging in her shady side yard more than five years after she had dug it up from the roots in an attempt to permanently dispose of it. A Durham MG had the same experience.

More than one MG in Chico has engaged in battle with Mexican feather grass Stipa tenuissima (formerly known as Nasella tenuissima). They were attracted to the way its flower stalks “shimmy” so charmingly in the breeze. But that lovely movement with the wind aids the plant's dissemination. One feather grass victim notes that “after a couple of years I noticed small seedlings in many places around my lot. As the plant is drought tolerant, the seedlings did well even if they didn't receive any supplemental irrigation. Where there was supplemental irrigation, they were especially plentiful. After about a year of constantly pulling unwanted seedlings, I finally removed the mother plants. Three years later, I'm still pulling unwanted seedlings.” Another MG notes that “in Chico, it can take over the neighborhood, especially where bare ground is showing.”  

Another ultra-hardy self-seeder is Scabiosa (pincushion flower).  One Chico MG, who has planted two types (atropurpurea and ochroleuca) says that they reseed “almost beyond control.”  Because they are so attractive, she feels guilty pulling them up, but knows that, regardless, she can't really get rid of them.

Russian Sage has proven to be very invasive in the Chapmantown yard of a Chico MG, who reports that “it sends up shoots the full length of my house.” Trying to keep it contained by cutting it just encourages the plant to send up more shoots.

Shady Chico yards provide the ideal conditions for the spread of a nearly evergreen species of alstroemeria, the Peruvian Lily A. psittacina. Its dark red flowers marked with green and blotched with dark purple appear in early summer and early fall; the two-foot-tall foliage provides useful ground cover during the winter and dies back in mid-summer. But don't be fooled! Over time this alstroemeria will move into every shady area in a yard and even colonize sunnier locations. It travels by tuberous rhizomes which develop in clusters. Because it is impossible to dig up every single tuber in these clusters, the plant is impossible to eradicate. A Chico MG with a very shady yard knows she will be battling this ultra-vigorous plant for the rest of her life.

A similar story is told by a Chico MG who moved into a house whose previous owner had planted creeping fig vine all along the backyard fence. She reports that the vine has grown in between the fence boards into the yards of the neighbors on all three sides of her lot. She is in the process of removing it by cutting it down and treating the roots with herbicide, and laments that even with this on-going vigilant approach, it will take several years to get rid of it.

Up in Paradise, an MG planted one yellow flag iris (pseudacrus) ten years ago in a semi-shady location with spring water available all year. She now has an area eight feet long and four feet wide filled with these plants, which die to the ground in cold weather. “The roots are enormous,” she says, adding that “It would require a back hoe to dig them out.”

Finally, a Durham MG has been dealing with the Miner's lettuce planted by his home's previous occupant for twenty years. Initially, he says “it was so thick I couldn't mow it and had to use a weed eater,” and it still sends up a few shoots each year. He has seen this plant advertised in a seed catalog and warns “someone would have to be insane to sow it without putting it in a container.”

The bottom line: Think twice (or three times) before planting these potential garden invaders!

http://www.plantright.org/map

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74139.html