Crabgrass, Crabgrass and more Crabgrass

Understanding Crabgrass “Breakthroughs”

How Preemergent Herbicides Work: Preemergent herbicides are so named because they are applied prior to weed seed germination, as opposed to postemergent herbicides that are applied to actively growing weeds after weed emergence. Most preemergent herbicides are characterized by very low water solubility. When properly applied the preemergent herbicide forms a herbicide barrier within the thatch/soil interface in the upper inch of soil. Once the emerging seedling tap root hits the barrier, the plant dies. The chemical barrier does not usually cause any injury to existing plants.
Despite having very low water solubility, granular formulations of certain preemergent herbicides (i.e. PRE-M) are able to effectively control weeds through a process know as volatilization. The active ingredient releases in the gaseous state and attaches to soil particles to fill in the voids between carrier granules to complete the barrier. Herbicides having lower solubility and volatilization (vapor pressure) characteristics would be slower and may be less complete in forming a chemical barrier increasing weed break through potential.
Preemergent herbicide control “failures” occur for a number of reasons. Following is an itemization and brief explanation of these reasons:
• Heavy Spring Rains-Very wet conditions deplete the oxygen in the soil. When this happens, anaerobic bacteria proliferate and can prematurely break down the herbicide.
• Warm Soil temperatures-Higher soil temperatures stimulate soil microorganism activity which speeds herbicide breakdown.
• Cool Spring Soil Temperatures-Cool soil temperatures can delay the germination of crabgrass. Unseasonably cool Spring temperatures may warrant an additional preemergent (late Spring) application to ensure acceptable control.
• Thatch Build Up-An excessive thatch layer (over 1/2”) will prevent proper incorporation of the herbicide barrier.
• Poor Turf Density-Optimum crabgrass management occurs in dense, healthy turf stands which are capable of out-competing weeds for space, moisture, nutrients etc. Thin turf stands allow weeds to get a foothold. This is one of the reasons we often find the first breaks in high traffic areas or “hot spots” such as along side driveways, sidewalks, curbs etc. Heat build up in these areas can also break down the herbicide barrier. It is extremely important culturally manage the turf (proper fertilization, irrigation, proper mowing height, sharp mower blades, aeration etc.) to maximize turf health and vigor.
• Delayed Germination Flushes-Environmental conditions such as drought or cool soil temperatures can delay seed germinations beyond the residual of the herbicide, resulting in mid/late season crabgrass emergence. Weed Pressure-Considering that one plant is capable of producing more that 1,000 seeds, even 95% control will leave 50 seeds to germinate. Additionally, the more seeds present, the quicker the depletion of the herbicide, since the weed uptake is the primary reason for chemical loss under normal conditions.
• Herbicide Barrier Breaks-Any factor which breaks the herbicide barrier in the soil, such as aeration, animal activity, construction, drought cracks or post treatment plantings, allows avenues for crabgrass seedlings to emerge without contacting the herbicide.
• Improper Chemical Incorporation-To be effective, the herbicide needs to be in the area (zone) where seeds are germinating. Preemergent herbicides require rainfall or irrigation to properly incorporate the chemical into the soil. Without the required moisture, the active ingredient will stay on the granular carrier and/or the turf canopy. Since most preemergent herbicides are subject to breakdown by ultraviolet light from the sun, failure to incorporate the chemical, weather sprayable or granular, can lead to premature breakdown and shortened residual.
Preemergent herbicides have much longer residual properties than Post-emergent herbicides providing control for months after application. Residual may vary due to factors such as rates used, weather conditions, soil types, weed pressure and soil pH.