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Due to the dry season, the weeds are overtaking our lawn. Besides watering, is there a weed killer I can apply when it is so hot? Is there a fertilizer or food that would help it?

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The most common broadleaf weed herbicide contains 2,4-D, which can volatilize and drift to nearby plants and damage them when temperatures are hot. There are a few other herbicides, but most are effective when temperatures are not so intense. I would recommend that you hand-pull weeds in areas of greatest concern, and try to prevent seed set in the others.  Weeds are often a function of watering practice. If your lawn is in good health and you water properly, weeds are much more easily controlled.  Water lawns only as often as absolutely necessary. By allowing the top one inch of soil to dry between irrigation, you are killing any weed seeds that are sprouting immediately after the irrigation.

Train the Turfgrass to grow deeper roots by wetting the soil at least 8 inches deep every time you irrigate. (Test it by digging a hole and looking at the soil or by poking a long screwdriver down into the lawn - when you meet resistance, that is where dry soil begins).  Thatch buildup creates a good place for weeds to germinate, because this spongy layer between grass blades and the soil retains water longer than soil would. If thatch is thicker than one-half inch, core aerate this fall or late summer (once hot temperatures subside). Too much nitrogen fertilization can lead to thatch buildup, because grass is growing faster than the clippings and dead roots can decompose.

Also, don't forget to raise the mower deck so that the soil surface is more shaded (this will discourage weed growth there). And, taller grass plants grow deeper roots, so you can go even longer between irrigations (thus allowing the soil surface and/or thatch layer to dry out and kill weed seed that may be germinating).  Review the USU Extension publication "Basic Turfgrass Care" and follow the maintenance guidelines therein. You can download that publication at http://extension.usu.edu/files/gardpubs/hg517.pdf

You can shop for a weed control that doesn't have an temperature limits - be sure to identify the weeds you want to control, read the herbicide label carefully and follow all label directions.

Posted on 1 Aug 2007

Maggie Wolf
Horticulture Agent, Salt Lake County

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