Smart gardeners are already thinking about spring and summer weed control. I have had questions about whether it was too early to put down a pre-emergent herbicide "on a graveled area that is always full of weeds," and whether "rain and melting snow will wash it all away with no effect," since the area is slightly sloping. These are excellent questions.
Pre-emergent herbicides prevent weed seeds from developing into plants. They do not work on existing plants. They can be an effective way to control weeds with minimal effort if used correctly. Many factors affect how well and how long an application of pre-emergent herbicides will control weeds.
Not all chemicals work on all weeds, so know which weeds you are trying to control and buy the appropriate product. Also keep in mind that most pre-emergent herbicides do not work against perennial plants. In addition, tall dense stands of old plant residue in the area to be sprayed will greatly reduce weed control. A number of soil characteristics, including pH, texture, moisture, and soil temperature, will also influence the success of pre-emergent herbicide application.
Pre-emergent chemicals require some moisture to activate, whether from rain, snow or irrigation. Too much water can wash the chemical off the treatment area, particularly if it is sloped, as in my caller's case. Too much rain can also move the chemical into the soil below the weed seeds. Some chemicals need to be tilled or raked into the soil and others must be left undisturbed after application. NOTE: Do not apply materials to frozen or snow-covered soils because of the increased potential for run-off mentioned by my astute caller.
The best time to apply pre-emergent herbicides is as soon as the ground thaws and is free of snow. Read product labels thoroughly to know which chemicals will work at this time of year to control the weeds you have identified on your property. University of Nevada Cooperative Extension can help identify weeds. Go to www.unce.unr.edu. Click on "publications," then "natural resources," and then "weeds." Or, bring weeds to our office for identification.
Here's another timely tip from Dave at Greenhouse Garden Center. Apply granular soil sulfur on lawns now on top of the snow. It will help a lawn green up in the spring. It won't burn your lawn now, as it might if you put it down when it's warm.
For information, e-mail skellyj@unce.unr.edu or call me at 887-2252. You can "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu or call your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu.
-- JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.