Time for pre-emergent herbicide
Saturday, March 07, 2009
Huntsville Times
Q: When is the correct time to put out pre-emergent herbicide for controlling summer weeds? Will this help with the purple flowering weeds all over my yard? Thank you. - Terry S.
A: Spring is almost here. It was very close to beginning last week and then a bit of winter weather slammed on the brakes for most plants. While I did not enjoy the low 20 degree F temperatures, it did serve a great purpose in slowing things down and not letting our plants get ahead of schedule; this is true of weeds as well.
Most of the summer weeds, especially crabgrass, germinate in early spring as soil temperatures begin to warm. One of our best methods of controlling these weeds is to keep them from sprouting and that is why we use pre-emergent herbicides. These products inhibit the germination of the crabgrass seeds that have lain dormant since last fall. The key to great control is timing - we have to get the product out before the crabgrass has begun to sprout to be the most effective
In our area (zone 7a) you will read in gardening books or publications that pre-emergent herbicide should be applied between March 1-15. Another guide you often see here is to put out pre-emergent when the forsythia is in bloom (generally this will be between March 1-15).
However, at the end of last week I saw several forsythia in bloom in southeast Huntsville. The ones at the Huntsville Botanical Garden had not opened yet, so they did not get zapped by the low temperatures of the weekend.
The only true guide is soil temperature - crabgrass seeds germinate when the soil temperature (one-half inch below ground) is between 50-55 degrees F. On Feb. 27, the soil temperature was 48 degrees F, creeping up to that magic number. The wind and snow came over the weekend, bringing cold air that definitely lowered the soil temp at the surface.
On March 3, it had dropped to 41 degrees F just below the surface. To contrast, the soil temperature at 6 inches deep has stayed between 40-42 degrees throughout the last two weeks - it will take longer for the soil to warm up at this depth.
With the temperatures predicted to be in the 70s F this weekend, I would say your window is now open for applying pre-emergents to control summer weeds.
You have numerous products at your disposal to try - names like Crabgrass Preventer, Halts, Crab-Ex and Dimension are sold at local garden centers. The main active ingredients include pendimethalin, trifluralin and benefin - all have shown good results if applied properly.
Personally, I like Dimension for several reasons. It can be applied to turf areas as well as landscape beds, and it will even work on young crabgrass sprouts. This takes a bit of the guesswork out knowing that if you were a week or so late in your application you will still get good results.
This year I am excited about trying a new product on my lawn. The product is corn gluten meal, a byproduct of the wet-milling process of corn. Researchers at Iowa State University discovered this byproduct exhibited pre-emergent herbicidal qualities back in the 1990s - extensive studies have been conducted, patents issued, and, abracadabra, we have our first organic pre-emergent herbicide that will work on many annual grasses like crabgrass plus dandelions and pigweed.
Research has shown that it may take several years to totally eliminate the weed infestations, but I am thrilled to see an organic product available for use that has potential.
Another important aspect of this formulation is that it is also an organic fertilizer, containing 10 percent nitrogen. This nitrogen is in an organic state so it must be changed to a form of nitrogen that plants can utilize (either NO3, which is nitrate, or NH4, which is ammonia). That is why organic fertilizers are considered slow release; they must be transformed by biological activities so that the plants can utilize the nutrients
Whichever product you choose, follow the application rates listed on the bag. Consider using a drop-type spreader to control the pattern of distribution. If you are using the rotary type, please make sure that you clean up any spills on driveways and sidewalks so that it does not become part of the storm water run-off. This is true of fertilizer as well - both of these products are harmful if they get in our storm water.
Your window for applying is now open, so take advantage of the nice temperatures and get it out for summer weed control.
This will not help the henbit problem you have now - apply a pre-emergent around Labor Day for control of these winter weeds.
Things to do
Rejuvenation pruning (severe cutting back) of holly and crape myrtle needs to be done before new growth begins.
Cut back ornamental grasses to 12 inches tall; new grass blades will come out from the older shoots. You can also divide the grasses at this time.
E-mail Harvey Cotten at harvey.cotten@htimes.com, or write to Harvey Cotten c/o The Huntsville Times, P.O. Box 1487 West Station, Huntsville 35807.
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