Marianne Ophardt: Check soil temperature before applying herbicides to crabgass

For the Tri-City HeraldMarch 21, 2013 

At this time of year, I’m often asked about when to apply crabgrass preventers or pre-emergent herbicides. These herbicides are applied to a lawn before the majority of crabgrass seeds germinate.

Is it time yet? Based on phenology, the right time for applying crabgrass preventer is after forsythia (the bright yellow early spring flowering shrub) is in full bloom and before the flowers start to wither.

Using forsythia as a cue to apply crabgrass preventers is not always reliable, however, especially since there are different varieties of forsythia, some blooming earlier than others. A more accurate way to determine crabgrass germination is using soil temperature, or accumulated growing degree days.

Growing degree days (GDDs) are a measurement of the accumulation of heat over time. They are calculated by taking the average daily temperature minus a base temperature.

Models for bloom time, germination or insect emergence often use a base temperature of 50 degrees, but some use 40 or 60 degree bases. For example, on a day when the maximum temperature is 70 degrees and the minimum temperature is 40 degrees, the average temperature is 55 degrees, subtract 50 degrees and you have 5 GDDs.

Now don’t groan about me getting too technical. WSU has you covered with an app that does all this for you and provides you with the minimum and maximum temperatures for each day, plus totals the accumulated GDDs. Just go to http://weather.wsu.edu/mobile on your phone or computer and pick the WSU weather station site closest to you by entering your zip code. Once you select your site, you can retrieve weather-related information, including current weather conditions, soil temperature and GDDs.

Using this, I can tell you that as of Wednesday at the CBC weather station, the accumulated GDDs (using a 50 degree base) is 38.62 and the soil temperature is 49 degrees. Research indicates that the majority of crabgrass germinates when the accumulated GDDs reach 200, or when the soil temperature reaches 73 degrees, although some of the seeds will start to germinate at 57 to 64 degrees. The best time to apply the crabgrass preventers is before these key times.

The other thing you need to know is that the pre-emergent herbicides contained in many crabgrass preventer products has changed over time, even when the label name has stayed the same. Many of the newer materials provide control during a longer period of time, so that applying them too early is not as great of a concern as it once was. One of these ingredients, dithiopyr, provides pre-emergent and post-emergent control up to four weeks after crabgrass germinates. The label recommends applying dithiopyr when soil temperatures reach 50 degrees and that’s right about now so add this to your chores for this weekend.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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