Teague: Good to apply pre-emergent for weeds to come

January 31, 2014 

Winter weeds: We turned off our irrigation system in mid-December, expecting that winter rainfall would be sufficient to keep the grass and the landscape alive during the cold months. All the garden was deeply irrigated during the early December freezes. One of the effects of this winter's drought is already evident — our beige bermuda lawn (completely dormant) is nearly weed-free.

UC Davis' IPM website recommends applying a pre-emergent to treat for annual bluegrass at the end of January or first week of February in the Central Valley. Annual bluegrass (small dark green tufts, fluffy white taller seed heads) is the first weed to sprout when soil temperatures warm to 50-55 degrees for more than three days in a row.

Well, soil temps in January have been more than warm enough for the seeds to germinate, but the lack of water has prevented the annual bluegrass from sprouting in our lawn; annual bluegrass, unlike other weeds, needs ample moisture to germinate — it thrives in overwatered lawns. So, is it a good idea to apply pre-emergents to control for winter and early spring weeds (annual bluegrass, crabgrass) and summer weeds (spurge, oxalis, dandelions) even though we are not watering the lawn? Yes, I think so. Especially on bare or thin areas in the lawn where weeds will proliferate. Any rainfall during the spring months will start germination and it will be difficult to eradicate aggressive, tough survivor weeds in a heat and drought-stress lawn.

To minimize weeds, set mower blades low enough to cut off weed seed heads before they set seed, hand pull and cultivate tiny weeds and use herbicides to spot-treat more mature annual bluegrass and spurge that disperse thousands of seeds when disturbed.

Horticultural oil and sulfur spray: In the Dec. 28 column, I recommended applying a pre-mixed solution of horticultural oil and lime sulfur to pruned roses to smother overwintering pest insect eggs and to kill fungal spores, citing Lily Miller's PolySul as one brand.

Several readers emailed that they were unable to find such a product locally. I can't find it, either. Neither the two garden centers nor the local nursery in my neighborhood stocked a comparable pre-mix of oil and lime sulfur. An online search showed that Lily Miller still manufactures PolySul and it is registered with the EPA through 2014.

Instead, spray pruned roses with neem oil. Green Light's Rose Defense is a widely available brand. Neem oil is plant-based and acts as both a fungicide and a pesticide. Drench the bare rose canes and spray the soil surface around each bush. Or, apply a horticultural oil and dig in a half cup of sulfur granules or epsom salts (magnesium sulfate) around each bush. Sulfur powders also treat for fungal problems, but they are caustic. Copper fungicides can cause discoloration of flowers and foliage on roses. Read labels carefully before applying fungicides.

 

Elinor Teague is a Fresno County master gardener. Send her plant questions at etgrow@comcast.net or features@fresnobee.com ("plants" in the subject line).

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