Q. Does the crazy warm winter weather we've been experiencing affect the long-standing advice to watch for local forsythia to bloom before applying corn gluten meal to prevent crabgrass? Or can I apply it now? (Email dated February 20th).
- ---Chris in Cherry Hill, NJ
- ---Mary in Fairfax, Virginia
- ---Jeff in Chevy Chase, DC
Fueled by a plethora of emails with 'crabgrass' or 'corn gluten' in the subject line, I decided to ask the man who pioneered the use of corn gluten as a natural pre-emergent herbicide, Iowa State University turfgrass expert Dr. Nick Christians, exactly how a warm winter might affect the timing of this natural pre-emergent weed and feed, that—like all pre-emergents, chemical or organic—must be applied before weed seeds sprout to have the desired effect.
Nick was thrilled to have the chance to modify and expand upon his original 'apply corn gluten when the forsythia blooms' advice, because he explains, University horticulturists have since told him that different named varieties of forsythia can have very different bloom times, with the earliest flowering varieties popping close to six weeks before a late bloomer!
So that makes the Eastern redbud—a much less diverse specimen—a better visual cue. But an even better tactic, stresses Nick, is to move from potentially fickle flowers to the world of measurable science: Insert the probe of a simple soil thermometer four inches below the soil line and apply your crabgrass-defeating corn when that thermometer reads 55 degrees. Because, while the ideal time to apply corn gluten may 'generally' be when the local forsythia and redbuds bloom, it is always when the soil temperature reaches 55 degrees F. four inches down. And, although he hates to try and guess at exact dates, Nick was coaxed into estimating that even with our warm winter, the soil temperature should begin to reach that number around April 1st in the DC area and around Tax Time in the greater Philadelphia area.
But, as we always stress, inner-city soils warm up much earlier than out in the suburbs—and elevation and micro-climates can make for a high degree of variability inside the boundaries of the same general neighborhood. So buy a soil thermometer and stop guessing! You can get a simple mechanical one (that looks a lot like a meat thermometer) for about twenty bucks; or spend a little more and get one that will also measure the heat of your compost pile.
(Note: No matter what, it's always better to apply a pre-emergent a little early than a little late; corn gluten has about a six week window of effectiveness, and all pre-emergents are useless on seeds that have already germinated.)
Oh, and to follow the new legal guidelines designed to protect the Chesapeake Bay from Nitrogen pollution, folks near that national treasure (and other fragile waterways) should apply no more than ten pounds of corn gluten per thousand square feet of turf. Yes, I have advised using up to twenty pounds in the past, but as Maryland Turfgrass Council President Vernon Cooper correctly explains in an open letter he posted to me recently, that amount is now technically illegal, as it would exceed the new limits of nine pounds of nitrogen per single application.
I am happy to so modify my advice. (I know you'd all come visit me on Sundays, but I look terrible in stripes!) And I'd be remiss if I didn't also thank the Council for calling me a "Relentless Radical Media Pundit" on their website. My new title! (It's going to look great on business cards!)
Oh, and one final note from Nick. People who report crabgrass appearing very early in the season are probably looking at knotweed, which, he explains, is common in lawns whose soil has become heavily compacted over time. Pulling plugs out of that overly-dense soil with a machine called a 'core aerator' in the early Fall will open things up and allow the grass to outcompete this weed.