Last week’s column was about weeds that grow, produce seed and die in one year — annual weeds. Control measures include prevention of seed production and removal of plants, usually by hand or hoe. A thick layer of mulch is another control tool, limiting the amount of light available to the seed and suppressing weed seed germination.
Perennial weeds — plants that die back in winter then re-sprout in the spring — are harder to control. Prevention of seed production is vital with these weeds as well, but thanks to their extensive root systems, hand removal is ineffective. Hoeing or digging might get the leafy top, but the part of the root that remains in the soil simply regenerates new stems and leaves and continues to grow.
For gardeners, one of the most familiar and frustrating perennial weeds is bindweed. It can sprawl across the ground or twine its way to the top of a lilac bush. It’s hard to imagine that this little vine with its cute pink or white bell-shaped flowers can have a root system that extends to a depth of 20 feet. With all the reserves in a root system that size, it’s no wonder tugging at it has virtually no affect. To make matters worse, the seeds can remain viable for up to 50 years.
Canada thistle is another perennial with impressive roots. Thanks to a sprawling root system that can grow 18 feet in a single season, this perennial forms colonies of spiny weeds. As with bindweed, the shoots and leaves generate food that goes into reserve in the root system.
For both of these plants and other perennial weeds, persistent removal of any shoots or leaves might eventually tax the root system into starvation. It is a long, tedious process and requires vigilance on the part of gardeners. When a quicker fix is needed, post-emergent herbicides are often used for control.
Choosing the right herbicide for the job and applying it at a time when it will be most effective is determined by the life cycle of the plant. A contact herbicide that only kills the part of the plant that it touches is useful for annual weeds if applied before they go to seed. To kill the root system of a perennial plant, a systemic herbicide that moves from the leaves down to the roots is a better choice. Systemic herbicides often contain glyphosate (sold under the name Round-Up and others) or 2,4-D. To be effective they must be applied when the plant is actively growing and has adequate water to move the chemicals within the tissues.
No matter what herbicide you choose, always be sure to read and follow directions.
Colorado State University Extension in the City and County of Broomfield provides unbiased, research-based information about 4-H youth development, family and consumer issues, gardening, horticulture and natural resources. As part of a nationwide system, Extension brings the research and resources of the university to the community. The Broomfield County Extension office is at 1 DesCombes Drive, Broomfield, 80020. For information, call 720-887-2286.
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