Invasive Plants
By Bob Chapman
There are a few landscape plants, shrubs and trees that when planted, or introduced surreptitiously, that tend to "take over," causing grief for the gardener. Some make excellent additions to the landscape, although often belatedly, the gardener finds that the handsome plant is spreading, either by roots, rhizomes, stolons, runners or seeds, into every nook and cranny.
Here's a list of a few of the more common invaders and suggestions on how to control them.
Bermudagrass (Cynodon dactylon).
We're referring to the "wild" Bermudagrass, not the hybrids that are used for lawns in warmer climates. This perennial grass spreads by surface and underground runners, quickly invading flowerbeds, shrubbery borders and lawns. Bermudagrass seeds germinate readily when blown in on breezes and alighting on fertile, moist soil.
Eradicate any found in shrubbery borders or open spaces with the use of glyphosate (Easy Gone Weed and Grass Killer or Roundup).
You can eradicate any Bermudagrass creeping into the flowerbed by aiming the sprayer nozzle through a plastic cola bottle that has the bottom cut off, thus contacting only the Bermudagrass and not allowing the spray to contact desirable plants.
Bermudagrass in lawns can be controlled while it is actively growing and with fully developed blades by spraying the lawn with Turflon ester three times at three-week intervals. Turflon ester is available online at www.monterylawngarden.com
Prevent the seeds from germinating in lawns by spreading a weed preventer/fertilizer combination (sold for controlling crabgrass) over the lawn in February (in Southern California) or March in Northern California.
Baby's Tears (Soleirolia soleirolii).
Baby's Tears serve well as a low-growing ground cover in shady areas or in full sun. It is a cool-looking green mat, growing 2-4 inches high. It spreads indefinitely by creeping stems. It freezes to mush in hard frosts but comes back to life. Only one product kills Baby's Tears, no other sprays will completely kill this plant. "Finale" is the product that does the complete job and it is sold online at www.biconet.com. You must be persistent if wanting to eradicate this weed/plant as even the tiniest piece missed by the spray will come back. See the section on Bermudagrass for killing invaders in flowerbeds.
Mexican Evening Primrose (Oenothera speciosa).
This native to Southwestern United States and Mexico spreads by rhizomes. It produces fragrant 2-inch white to pink flowers in spring to early summer, then dies back. The plant is aggressive and often invasive unless contained by wooden or plastic headers or concrete mow strips. Control the Mexican Evening Primrose by spraying it with glyphosate (Easy Gone Weed and Grass Killer or Roundup) during the active growing season.
Indian Mock Strawberry (Duchesnea indica).
This aggressive ground cover, native to Japan, spreads by trailing stems that root firmly wherever it touches the soil. The flowers and leaves closely resemble the strawberry. Control by spraying with glyphosate and/or containing it with mow strips of plastic, wood or concrete, keeping the runners trimmed as they try to cross over.
Bamboo (Bambusa sp.).
There are two types of bamboo, the running type or the clumping type. The running type causes problems. It spreads by underground rhizomes that grow some distance from the parent plant before sending up shoots. It's difficult to contain the running bamboo. Root barriers tend to deflect the rhizome downwards only to arise much later on the other side of the barrier. To keep running bamboo in its place while maintaining the original planting, dig a trench around the perimeter, severing the woody, tough rhizomes, then spray the unwanted bamboo with glyphosate. It takes a long while to kill the undesired bamboo, so be patient. Remove it after it is dead.
Bermuda Buttercup (Oxalis pes-caprae).
This plant, with its bright green clover-like leaves sports a glorious display of bright yellow flowers in early spring. When warm weather arrives it dies to the ground. This plant was originally imported from South Africa for use as a rock garden plant. It is now an escapee, spreading readily from seeds and indefinitely from underground bulbs. The presence of the underground bulbs makes this plant difficult to kill. The only proven herbicide that kills it is Finale, available online from www.biconet.com.
Sword Ferns (Polystichum minutim).
These graceful plants, with their long deep green sword-shaped fronds spread by aboveground furry roots that root and start a new plant. They are slow spreading but can be invasive, crowding into new areas. To contain these showy ferns, dig up and compost or destroy any new plants found away from the original clump.
Heavenly Bamboo (Nandina domestica).
Well-known by its common name, the plant is not really a bamboo, but does spread by just-under-the-surface underground roots. To contain this handsome shrub and all its many semi-dwarf and dwarf subspecies, dig up any undesirable portions of the clump.
Wandering Jew (Tradescantia zebrina or T. fluminensis).
Normally grown as a houseplant, this trailing vine-like plant and its many species and leaf colors can be grown outdoors and used a decorative ground cover in mild-winter climates. Any piece breaking off or touching the ground will readily form roots, thus spreading out rather quickly. It can be easily hoed off, but make sure you clean up all the severed parts. If you don't you'll be replanting it again. It can be eradicated with a weed killer.
Cinquefoil (Potentilla neumanniana).
This popular persistent ground cover spreads rapidly by creeping, rooting stems. It will easily cross over wooden or plastic headers and concrete mow strips, invading lawns and flowerbeds. If found in the lawn, any broad-leaved lawn weed killer will get rid of it. If found in shrubbery borders and flowerbeds, spot treatments with glyphosate will control it, but you must first sever any runners that cross barriers lest you kill more than wanted. A string trimmer does this job rather quickly and easily.
Hedera helix (English ivy) and Hedera canariensis (Algerian ivy).
Ivy is probably at the top of the list of invasive plants. Ivy spreads quickly, going over other plants, up trees, walls, poles and fences, its holdfasts (aerial rootlets) clinging tightly to almost any surface. Ivy does an excellent job in erosion control and as a hardy ground cover. Their roots grow deeply and the branches root as they grow. It is difficult to kill or contain.
To kill unwanted ivy that is growing out of bounds, dig a 10-12-inch-deep trench, severing all the roots between the wanted planting and the unwanted invader. Then spray the unwanted ivy with a nonselective herbicide, ensuring that the herbicide does not contact desirable foliage.
It is difficult to kill "old" ivy. The leaves develop a waxy surface soon after reaching full size and the waxy coating inhibits the penetration of the herbicide.
If desiring to kill an old, well-established bed of ivy, wait until late winter (if possible) and, using a weed mower, cut off all the foliage. When the new, green, soft leaves emerge and become nearly full size, spray it with glyphosate. The hormone will then be transported to the smallest root, killing the plant completely.
OTHER, LESS WELL-KNOWN INVADERS
The list below is not intended to be complete, but these plants can also invade the landscape.
Eradicate or contain them using the methods stated above.
- Mondo grass (Ophiopogon japonicus)
- Creeping lily turf (Liriope spicata)
- Horsetails (Equisetum hymale)
- Dichondra (Dichondra micrantha or D. repens)
- Aptenia (Aptenia cordifolia or Mesembryanthemum cordifolium)
- Sedum (Several forms root easily from the smallest piece and can spread readily)
- Chlorophytum (Spider plant, often seen as a hanging basket specimen, but grows readily from falling offsets)
- Lippia (Phyla nodiflora or Lippia repens)
- Red Valerian or Jupiter's Beard (Centranthus rubra)
- Creeping St. Johnswort (Hypericum calycinum)
- Wild strawberry (Fragaria chiloensis)
- Cape Weed (Arctotheca calendula)