Weeds can be characterized as selected vegetation that is undesired within other vegetation that is selectively desired. The operative term is selectively desired. Fortunately there are chemical herbicides (weed killers) that are selective in the types of weeds that they kill or affect.
In general, there are four types of selective Herbicides:
1 Grass Killers These chemicals kill grasses and do not harm most foliage with leaves on them. This chemical can be used directly over plants and planter beds to kill grasses in the planter bed that have almost all of the commonly used plants and ground covers with leaves on them. Fortunately this chemical works well on Bermuda Grass.
Read the label carefully to determine which plants it can be used on. If you have a leaf plant that is not listed on the label it is likely because it has merely not been tested on that leafy species. You may wish to try it over one plant before applying it over many of the same that you have in your garden.
These products are primarily commercially available. There are two different ones available: Fuslaid or Post. These are spray-on chemicals that mix with water to apply to grass weeds that are growing in ornamental planter beds.
Follow application directions and rates carefully. Typically they take about 2 weeks (from time of application) to take effect.
2 Broad Leaf Killers These chemicals are the opposite of the Grass Killers. They can be applied directly over most lawns and will selectively kill the broad leafs without harm to the lawn. These chemicals are very volatile and while relatively safe for humans, drift will severely damage adjacent plantings. Spray on calm days with low pressure to reduce spray drift.
Weed B Gone is a very effective broad leaf killer for lawns that is available on the retail market. There is, however, a newer product called Turflon that is a broad leaf killer that is not registered for use on lawns of Bermuda or St. Augustine because it will severely damage or kill those species of grasses. This product is not yet labeled as a Bermuda Grass killer in grass lawns, however, since this particular broad leaf killer will not harm most desirable turf grasses (Blue, Rye and Fescue) it will damage or kill Bermuda, and it has been used to control Bermuda Grass within fine turf grass lawns.
Do not use these products on shrubs or ground covers that have leaves, and that you wish to keep.
Turflon is a spray on chemical that mixes with water to apply to lawns in order to kill weeds. Follow application rates and directions carefully, including ambient temperature restrictions.
3 Broad Spectrum Non-Selective Herbicides Above you read of herbicides that are selective as to what they will have an affect on. The above chemicals will only kill grassy plants or alternate chemicals will kill broad leafed plants. As discussed in the opening paragraph, the selective chemicals are not less dangerous to humans than the ones that kill all foliage.
In some cases the potential danger is even greater. They are all meant to be used in accordance with the label and must be respected. The non selective herbicides are self explanatory in that they are not selective as to what they kill. The generally do damage or kill virtually any plant material they contact. Round-up fits into this category.
4 Contact, Soil Application Chemicals are absorbed by the plants in different ways. Most applications are for contact application. That is to say the chemical must come in contact with the foliage or with some insecticides with the insect itself to be effective. Other, but few, chemicals are absorbed through root absorbsion and are usually applied as a soil drench or root feeder. Note that there are bark injections however this is not recommended for the amateur and other solutions are typically as effective. Translocation is a term that implies the chemical is absorbed into the plant and flows through and with the plant fluids.
5 Chemical Application Chemical labels have the signal word CAUTION, WARNING, or DANGER! indicate the relative hazard of the products. The CAUTION word is the least hazardous to humans, while the DANGER signal word is the most hazardous to humans. This is true for all chemical products, including those around the household.
Whenever applying any of these chemicals, especially as an amateur, you should protect yourself against the concentrate and the mixed product. While these products are safer in their diluted rates after mixing with water and also after drying in the garden, do not underestimate the safety required to apply these commercial chemicals. Their chemicals are concentrated and at their best prior to mixing with water. Therefore, before opening or handling any open container or bag, utilize goggles, rubber gloves, and long sleeves. A dust mask and rubber boots are suggested but not usually necessary.
Pre-read the label as it is and it will dictate the precautions!
Along with the specified safety equipment, the equipment required to apply the granular herbicide and the fertilizer would include a broadcast spreader such as a whirly bird type. The liquid chemicals would be best applied through 1 or 2 gallon pump sprayers. It is recommended to have different sprayers for the different chemicals. Since spot spraying may be necessary from time to time, it may be useful to label the sprayers with a felt pen and store unused diluted portions for subsequent use within the containers.
Do not rinse containers into storm drains or sewers.
Also there are colorants that can be purchased to add to the chemical solutions so that you can easily see the area and amounts that have been sprayed. The spray colorants are inexpensive, and dissipate in the sunlight within a few days.
6 Seed Killers The fourth type of selective herbicide is that of a Pre Emergent Herbicide or a seed killer that will kill seeds but will not harm existing plants or lawns depending on the label.
A product called Ronstar is a granular product that will not harm almost any commonly used plants nor is it harmful to Blue, Rye or Fescue Grasses as are most turfgrass lawns in Northern California. This product is available in 40 or 50 lb. bags for about $60.00. This product is best applied with a whirlybird spreader at the approximate rate of 4 lb. per 1000 square feet. It is effective for about 3 to 4 months.
Upon application of either of the weed killers referenced above it would be prudent to apply this product to minimize reproduction of weeds via seeds that may be left behind or that blow in from neighbors. At such time that your weed population is under control, it would be advisable to regularly treat with Ronstar Pre Emergent (seed killer) as necessary for prevention. At such time that the weed problem is under control you should cut back the use of Ronstar to an as needed basis. (Perhaps in January to control new season seeds through the rain.
Also be very careful about perennials and annuals as this product will damage pansies severely. Spot treating with the other weed killers will be helpful and necessary from time to time. Read the label as some Pre Emergent herbicides may harm some plantings especially herbaceous type plants such as annuals.
If your lawn is dense and has become resistant to weed population, and you're to the point that you can easily control the lawn weeds with spot treatment, then avoid the repetitive applications of Ronstar seed killers as it does have nominal negative impact on the vitality and maturing of a fine lawn; also it will save you costs.
Round Up is a non selective broad spectrum herbicide that is intended to kill all herbaceous plants that it touches. This is especially good for sidewalk cracks and areas with no desired plantings.
7 Translocation Pesticide translocation refers to the plant absorbing the chemical into the tissue and fluids of the plant. Translocating pesticides are preferred as they affect the entire plant from roots to leaf. In the case of herbicides to kill weeds, a Translocating herbicide will translocate to kill beyond what it contacts; it will also kill the roots.
In the case of insecticides, the Translocating insecticide will actually make the plant poisonous to the target insect. As the insect preys on the plant it will ingest the insecticide that has translocated through the plant.