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Different weeds take distinct measures to control them

Published: Wednesday, Oct. 12 2011 10:18 a.m. MDT

Dandelions are a perennial weed with a large taproot. Dig them out or spray them in the fall.

Larry Sagers,

RLast week's column covered what makes a plant a weed and one way that weeds are classified. This week the focus is on specific kinds of weeds and how to best control them.

I classify weeds into five convenient groups to determine their control method. These are annual grasses, perennial grasses, annual broadleaves and perennial broadleaves, including plants with woody stems.

Weeds have specific times in their life cycles when they are the easiest to control. The stages are seed, seedling, vegetative growth, flowering, seed formation and finally death of the plant.

Determine the plant growth stage when they are most susceptible. It is almost impossible to control seeds once the plant forms them. Dormant seeds are highly resistant to most controls. By contrast, the germinated seedlings are very vulnerable. They are shallow rooted and even perennial plants need 30 days of growth before they become perennial.

Seedling control is the preferred option. It controls annuals and perennials and grasses and broadleaves. Hoeing, tilling and other cultivation methods offer excellent control.

Another option is a pre-emergent herbicide. These products do not kill seeds but they form a barrier that kills the weed's seedling after it emerges. Some products work best on grasses while others are more effective on broadleaf weeds.

They are often sold as "weed preventers" or "crabgrass preventers." These products do not control established weeds. Read and follow all label directions to protect pets, plants and people.

Annual weeds are easier to control because they have shallow root systems and nature kills them off for you each season. They are problems because they are among the most prolific seed producers. Remove them before they drop their seeds to reduce the seeds for future years.

Biennial plants typically grow a rosette or a vegetative stage the first year and produce flowers and seed the second season. While the seedling stage is the most susceptible, the next best time to control biennials is in the fall of their first year of growth.

Many biennials have large taproots they use as storage organs to get the energy to grow next year. Digging or spot spraying right now is an effective control for most biennial weeds.

The most difficult weeds to control are the perennials. Finding the right way to control these depends on what type of weed you are battling. Simple perennials have a root system that does not spread. Examples are common mallow and dandelion.

Creeping perennials spread by stolons, rhizomes or tubers. Bentgrass spreads by above-ground stolons, quackgrass spreads by underground rhizomes and Bermudagrass spreads by both. Canada thistle, field bindweed or wild morning glory are in this group. Yellow nutsedge produces small tubers and other weeds produce bulbs or other vegetative propagules.

Because these weeds propagate vegetatively, cultivation is not effective because tilling the plant cuts the stolons or rhizomes into sections. Each section can grow into new plants.

Fall is the best time to spray these plants because the plant will move the herbicide to the root system. Until you kill the roots and rhizomes, you cannot get any effective control.

Now that temperatures are dropping, it is time to spray them. Applying the herbicides now means they will absorb into the leaves and translocate to the roots. If you can kill the roots, you can get these weeds under control.

In the lawn areas use a selective product containing a broadleaf weed killer. These are available under many brand names from local nurseries. They will control the broadleaf weeds while not damaging the grasses.

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