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Attack weeds before they overrun gardens

I received a letter from a reader this week lamenting the existence of the weed oxalis.

Let me preface this discussion on weeds by assuring everyone _ you are not alone! We are all engaged in the constant pursuit of trying to rid our garden of these unwanted plants. Barbara and Tom Joyce have been in a long-established, losing battle with oxalis and appear about ready to throw in the towel. Dare I even tell you that some people have been known to grow this as an ornamental? That means they purposely plant and cultivate this little villain.

Oxalis, for those of you who are lucky enough to not have experienced it, is sometimes confused by the novice gardener with clover. The showy flowers and distinctive acid taste of oxalis, however, easily distinguishes the two families. Yellow oxalis is an African native that has taken root all across America. These hardy plants are quite difficult to get rid of once they become established.

You can try cultural practices or use herbicides to rid your yard of oxalis. A combination and lots of patience may lead you to some success.

Cultural Practices: Since seeds may be projected several feet when seedpods mature and burst, it is important to keep oxalis out of adjacent ornamental and flower beds, as well as removing it from turf areas. Dense, vigorous turf will resist infestation. Physical removal of single plants can be easily accomplished. Follow good turf management practices, including disease and insect control and proper fertilization.

Herbicide Use: Properly timed pre-emergent herbicide applications can prevent oxalis germination. Spot treatments with a postemergent broadleaf herbicide will clear out oxalis infestations. For optimum control, make your herbicide application when oxalis is actively growing and in the second trifoliate leaf to flower stage of growth.

What this means is you are going to have to walk around your yard and spray a nonselective herbicide on individual plants. Often one application is not enough. As with most weed control, the key is to keep your turf in excellent health so the weeds have a hard time gaining a foothold.

A simple tilling of the soil in your planting beds will not help and in fact will only encourage a stronger presence of this weed _ or most weeds, for that matter. The tilling gives the seeds a great start. After tilling you will need to use a pre-emergent made especially for flower beds.

Generally weed control consists of one or a combination of tasks. Hand weeding, tillage, mulching and herbicides should be considered when designing a weed control program for ornamentals. The most effective method or combination of these methods will depend on the weed species. Therefore, accurate identification of the weed problems is essential.

Mulches can be very effective against weeds, especially annual weeds. Mulches work primarily by depriving young weed seedlings of vital sunlight. Many materials have been used effectively as mulches. Examples include sheets of plastic, grass clippings, sawdust, straw, wood shavings, bark chips, newspapers, gravel and even rocks. The latest commercial mulch materials are made of a tightly woven black fabric that prevents sunlight from reaching the soil, but allows the passage of water and air.

Several herbicide options are available for residential use. Pre-emergent herbicides kill weeds as they germinate. Pre-emergent herbicides are usually preferred in ornamentals. They must be applied before weeds sprout. Overhead irrigation (or rainfall) is required to move the herbicide down into the top inch or two of soil. Pre-emergent herbicides may be mechanically mixed into the soil, but still need water to be activated. Application is usually made in the fall or early spring.

Spray freshly prepared beds with water and shallowly incorporate the pre-emergent herbicide into the soil using hand implements or sprinkler irrigation. Flowers are transplanted directly into the bed, with their roots placed below the layer of treated soil. Pre-emergent herbicides usually are effective for several weeks; after which time the flowers will provide sufficient competition to prevent new weeds from becoming a serious problem. If you want to plant flower seeds in treated beds, you should wait at least two weeks after application of the chemical. Weeds can also be controlled under ornamentals trees, shrubs and some ground covers using pre-emergent herbicides.

Certain selective postemergent herbicides can be applied over the top of many ornamentals to kill annual and perennial grasses without injury to the desirable plants. You should talk to a landscape professional before randomly applying these chemicals to your yard.

The best offense against the weeds in your yard is early intervention. Don't let them take over an area, or you will be doing battle for months or even years. Every time you walk around your yard, pull up a few weeds. This helps make the task manageable. Oh, and good luck!

_ Mary Collister writes about how to garden successfully in Florida's climate and offers problem-solving tips for your home garden. Mail questions to: Mary Collister, North of Tampa, 14358-B N Dale Mabry Blvd., Tampa, FL 33618.