Weed control is as much a contest of knowledge and planning as it is reacting to "plants out of place." Many weed problems can be prevented to a great extent by putting desirable plants in the right place. Like desirable plant species, weeds are adapted to specific regions and conditions. If they are better adapted than the desired plants, they will require regular attention.
Always make sure that your desired species is suited to the site. For example, bermudagrass in a heavily shaded site will always struggle and be susceptible to weed invasion. A fescue or St. Augustine will do better in the shade. Likewise, a Nandina hedge might survive in full sun, but a spot that offers a little shade in the hottest part of the day would make its life a whole lot easier. The same goes for soil, pH and drainage preferences.
Don't Plant Problems
While most nurseries go to great lengths to offer clean stock, it pays to examine the rootballs of new plants for signs of trouble. A little nutsedge or tiny bermudagrass rhizome can spread and multiply throughout a carefully prepared planting bed with annoying rapidity. A single spurge plant can drop literally thousands of seeds.
While you're researching the plant's site preferences, pay attention to cultural requirements. Over-irrigating can cause root loss, especially during periods of cooler soil temperatures. Soggy root environments are a wide open invitation for rapid weed seed germination.
Mulching is an important tool in the Southwest. Planting beds and trees benefit greatly from a layer of decomposed or composted organic material, such as bark, or inert materials such as gravel or rock. For extra protection, lay down a permeable weed control fabric before applying the mulch.
Indiscriminate use of herbicides is not a "professional" solution to weed problems. However, few landscapes can remain as designed without some use of herbicides. Manufacturers have developed a variety of products to help landscape managers selectively control weeds when it becomes necessary. These products are carefully formulated to take advantage of the vulnerability of specific weed species at certain times. That is why it is very important to read the product label for directions. The key to using herbicides is to know when they are most effective so the least amount is needed to achieve control. It helps to know the life cycle of the primary weeds in your region because timing varies by region.
For example, timing is very important with pre-emergent herbicides. These products do not prevent weed seeds from germinating. They disrupt growth immediately following germination. Only weeds that reproduce primarily by seed can be controlled with pre-emergent herbicides. Weeds that spread by other means, such as bermudagrass and nutsedge, will not be impacted by pre-emergent herbicides.
A few pre-emergents, such as Goal or Ronstar, have some activity after weeds have emerged. But generally, if you are late with your application, the effectiveness of the herbicide will not be optimum. Of course, not all weed seeds germinate simultaneously, so you need to allow for a period of a few weeks to gain maximum effectiveness. Check the label because pre-emergent herbicides vary in their residual period.
Be sure to water in your pre-emergent herbicides. Products applied in dry or granular form need water to activate a chemical barrier between the weed seed and the surface. For best results, irrigate immediately following application. These products attach themselves to particles in the soil, especially clay, and will not leach through the soil profile. So, a spring rain after making an application will not usually hamper control.
Some pre-emergent herbicides are sensitive to temperatures. Warm temperatures will speed up the volatilization of trifluralin (Treflan), so you might not get the residual activity you expect when spring or fall temperatures are unusually high. Read the product label also for any adjustments recommended for the specific weed, soil type, and the pH or alkalinity of your irrigation water. Goal and Ronstar are activated by light so do not mulch over them.
If you think you missed part of the main window for pre-emergent herbicides, add a post-emergent selective herbicide to your tank when applying the pre-emergents.
Established Weed Control
Control of weeds once they are established depends greatly on getting the herbicide onto the target weed and keeping it there long enough so it is absorbed by the plant. Irrigation or rain can wash foliar-applied herbicides off and reduce their effectiveness. Glyphosate (Roundup) requires a minimum of six undisturbed hours. Fusilade and Vantage need at least an hour. Do not mow before treatment to allow as much foliage surface as possible. Dust covering weed foliage can also reduce control, especially with glyphosate. Irrigation prior to application can improve effectiveness. Some products include materials (surfactants) to help the active ingredient stick to the leaf surface. You can also add these to your spray tank if desired.
You want weeds to be actively up and growing so the herbicide can be absorbed and translocated vigorously. Cloudy, cool weather can slow down the activity of herbicides. Some landscape managers give the area to be treated a shot of fertilizer and irrigation a couple of days prior to applying contact herbicides.
Because they are contact products, application rate and uniformity are critical for effectiveness. Calibrate sprayers and test all nozzles beforehand. Some dry products should be applied to wet foliage. Depending upon the herbicide, expect results over a period of weeks, not days. In tough situations, a second application might be needed. If a few weeds escape your treatments, don't be too proud to pull them by hand or hit them with targeted applications of glyphosate. Allow a few minutes before mowing or trimming to evaluate weed conditions and make minor adjustments. Keep thorough records of all sites and treatments made. You will not eliminate the threat of weeds in one season.
If the weeds win every year, the best solution might be to replace poorly-adapted material with more aggressive plants that resist weeds. You might be irrigating too much or fertilizing too much for the desired plants and encouraging weeds as a result. Plants are on a clock. You should be too. Tell clients successful weed control depends on year-round planning and maintenance. Ultimately, the objective is to decrease the stress on the desired plants and increase the stress on the undesired ones.