Husker Hort

A Nebraska View of Horticulture

Preemergents Can Only Do So Much

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henbit2

Applying preemergent now won’t help with henbit. Photo from cropwatch.unl.edu

No matter how a hard we try, we can’t predict Mother Nature, especially in Nebraska.  The weather this year, throws all of my previous application recommendations for crabgrass preemergence herbicides out the window.  On the bright side, we have been enjoying plenty of days with warm temperatures and so have our lawns and the weeds in them. 

 

Warmer than average temperatures have bumped up the timing of many of the tasks on our to-do list.  Soil temperatures are as high as 15 degrees warmer than average this time of the year.  If you wanted to apply a preemergence herbicide to control crabgrass, you might consider putting it down sooner rather than later.  Crabgrass, the main target for early season preemergence herbicides, needs a minimum soil temperature of 50 to 55 degrees to germinate.  Normally, that soil temperature is reached the end of April or the beginning of May.  This year, however, we have reached those optimum germinating temperatures a bit sooner than expected, like now.  I checked the soil temperatures for Grand Island and we were over 54.9 degrees, which is 15 degree above average for this time of year.  To see what the soil temperatures are in your area, you can visit http://cropwatch.unl.edu/cropwatchsoiltemperature.

Since the preemergence herbicides will be put down much earlier than normal, consider split applications.  A split application of preemergence herbicides will allow for a longer control window.  Aim to put down half the highest recommended application rate on the label now and the other half in 6-8 weeks for season long control of weeds.  One application now probably won’t last throughout the entire growing season because of the earlier application window.  Be sure to water in the product after application to keep it from degrading in the sun.  The three most common active ingredients in preemergence herbicides are dithiopyr, pendimethalin, or prodiamine.  When choosing a product to control crabgrass, look for one of those active ingredients.  University of Nebraska-Lincoln research has shown that they are all equally effective in controlling crabgrass.

Once a plant is up and actively growing, preemergence herbicides won’t work.  Some weed are winter annuals and have been up for a while now.  Winter annual weeds bloom in the spring, produce seed, and die all before the temperatures get hot. One of the more common winter annual weeds is henbit.  Henbit has scalloped leaves, a square stem, and little purple flowers at the tip of the stem.  These weed seeds germinated last September or October.  Henbit plants sat dormant throughout the winter just waiting for the right time to jump into flower and seed production.  Post emergence, broadleaf weed, herbicides won’t do much good.  Spraying might make you feel better, but it can cause the plant to produce and drop more seeds.  If the area isn’t too large, these weeds can be hand-pulled.  Increasing the density and health of the lawn in the thin areas can help too.  Improve the lawn either by overseeding or by changing cultural practices to promote grass growth.  If you choose to use preemergence herbicides, aim to apply in early September to control winter annual weeds.  For the best selection of preemergence herbicides, consider purchasing them now and storing until fall.

It might also be time to consider irrigating turf.  Warm temperatures and dry, windy conditions have begun to dry out soils. Before firing up the irrigation system or dragging out the hoses, check to see if your soils have finally dried out.  Dig down 4-6”, if the soil is moist, wait another week before checking again.  Another method is to utilize a 6” screwdriver or soil probe.  If the probe enters the ground easily, the soil is moist and irrigation can be held off a little longer.

This year has started off to be a truly exceptional year, in more way than one.  From the blizzard to the warmer than average temperatures, it really is difficult to predict Mother Nature’s next move.

Elizabeth Killinger is the Horticulture Extension Educator with Nebraska Extension in Hall County. For more information contact Elizabeth at elizabeth.killinger@unl.edu, her blog at https://huskerhort.com/, or HuskerHort on Facebook and Twitter.

Preemergent PDF

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Author: Elizabeth Killinger

A Nebraska Extension Educator out of Hall County with a focus in horticulture and sustainable landscapes.

One thought on “Preemergents Can Only Do So Much

  1. Reblogged this on JenREESources's Extension Blog and commented:
    Elizabeth shares great tips on crabgrass prevention in an unconventional year!

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