Dealing with Lawn Weeds

April 5th, 2007

   Spring has officially sprung, but it’s not just bulbs and trees doing the springing.

   Crabgrass, chickweed, ground ivy and assorted other lawn weeds also are gearing up now for another season of annoying the heck out of those who lust after pristine green carpets in front of the house.

   Before automatically using the old apply-all-kinds-of-stuff-to-head-off-every-possible-problem approach, consider rethinking your options.

   For starters, how about easing off the zero-tolerance weed policy? An occasional dandelion or patch of clover won’t get you kicked out of the neighborhood (probably). Look for other ways to validate your suburban lordship and masculinity.

   Lawn perfection can be had, but it comes with a price – not only in lawn-care bills but also in its effect on soil organisms, groundwater and runoff into waterways.

   A lawn can be nice without being perfect. That you can achieve without chemistry degrees and 12-syllable herbicides.

Crabgrass.

   If you haven’t had a problem with crabgrass, why put down a crabgrass preventer every year at this time?

   If you only get a few patches of weeds here and there, why drop-spread a weed-and-feed product all over the whole lawn every May?

   But if you DO have substantial lawn-weed problems, a better approach is to shore up your weaknesses by adding more grass seed instead of more weed-killers.

   Weeds are opportunists. Bare dirt is their ally. If you beat them to the punch by getting grass out there first, you win.

   Early to mid-spring and early fall are the year’s two best windows to thicken up thin lawns with additional grass seed.

   You’ll have to decide which way to go because you can’t prevent weeds and start new grass seed at the same time. Most products that prevent crabgrass and other weeds also prevent grass seed from germinating. An exception is siduron (Tupersan). Check labels for when it’s safe to seed after using other products.

   Go with a quality grass seed that’s naturally resistant to bugs and disease (variety ratings are available at www.ntep.org), and remember that grass seed germinates best when it’s raked lightly into the soil surface and kept consistently moist until sprouting.

   What do you do about weeds that come up after you scrap the chemical four-step plan?

   Before taking action, know the enemy. The type of weeds you’ve got will determine what you use and when you use it.

   Weeds fall into two main camps: annuals and perennials.

   Annual weeds are ones that sprout anew each year, live their entire life cycle in one year, and then produce seed for the next generation. Some common annual lawn weeds are crabgrass, goosegrass, barnyard grass, foxtail, black medic, prostrate knotweed, prostrate spurge, common chickweed, corn speedwell and henbit.

Closeup of nutsedge (a.k.a. nutgrass).

   Perennial weeds are those that come back year after year. They also may set seed (or send out runners), but unlike annuals, they don’t die out with the season. Some common perennial lawn weeds are dandelions, orchard grass, quackgrass, yellow nutsedge, wild garlic, plantain, creeping speedwell, ground ivy, clover and wild violets.

   Annual weeds are best controlled by stopping them from sprouting – i.e. by using those weed preventers now. If most of your weed trouble is from annuals, a reasonable strategy might be to go ahead and use a preventer now and then focus on adding new grass seed in the fall.

   Weed preventers include products such as benefin (Balan), benefin and trifluralin (Team), pendimethalin (Pre-M and Halts) and prodiamine (Barricade) as well as corn gluten meal, a non-chemical alternative that’s a byproduct of corn.

   What few people realize is that these products don’t prevent weeds all season. They generally work for about 8 weeks – even less in rainy Junes like we had last year.

   That means timing is critical for applying them. Put them down too soon, and they run out of steam while annual weeds are still capable of sprouting, which can run into August. Put them down too late, and they’re harmless against weeds that already have sprouted.

   In central Pennsylvania, the usual application window is late March to early April – roughly around the time that forsythias flower. To ensure season-long control, a second application should go down in early June.

   What you don’t know is whether early summer will bring brutal heat and drought. That alone will stop weeds from sprouting, unless you help them out by irrigating the lawn.

   One option to spread out your weed-preventing window and maybe get by with one application is a relatively new product called Dimension. This both prevents new seeds from germinating and kills off young crabgrass and other weeds for the first few weeks after they’ve sprouted. That means you can put it down a few weeks later – about the time dandelion flowers are opening.

   For perennial weeds and newly sprouted annuals, the idea is to zap them without harming the grass around them.

   A good front-line approach: simply pull them or dig them. When my kids were little, I gave them screwdrivers and paid a nickel a weed, which cost less than a bottle of Weed-B-Gon and kept them busy at the same time.

   If you go with a weed-killer, consider a liquid one that lets you spot-spray just the weeds.

   Which product you use will depend on whether the weed is a grassy one like crabgrass or goosegrass or a broad-leafed one like plantain or dandelion.

   Broad-leafed ones are easier to control, and products are readily available at garden centers. Just be sure to choose a product that’s labeled for broad-leaf weed control in lawns! You may need two treatments to kill some tougher weeds.

   Grassy weeds are a little harder to kill because they’re similar botanically to lawn grass.

   Products such as MSMA, DSMA and Acclaim are labeled for control of grassy weeds such as crabgrass, goosegrass, yellow nutsedge and the like, but they work best when applied early in these plants’ growth stage. Unfortunately, most people don’t distinguish grassy-weed outbreaks until those weeds are too far along. Even under ideal conditions, it may take two or three applications.

   The saving grace is that annual grassy weeds (like the aforementioned crabgrass and goosegrass) die off when frost arrives. But for perennial grassy weeds like orchard grass and quackgrass, you’re stuck with digging out these patches or spraying the infested area with a kill-everything-green weed-killer such as glyphosate (i.e. Roundup).

   Then you’ll have to reseed the bare patch. Or else let weeds grow everywhere and tell the neighbors you’re experimenting with wildflower meadows.

    More ways to discourage lawn weeds by encouraging good grass growth:    * Test the soil to be sure it’s getting the right mix of nutrients at the right levels. Do-it-yourself Penn State test kits – available at county Extension offices and most garden centers for $9-$10 – also give a reading of the soil’s important acidity (pH) level.

   * Aid grass root growth by removing soil cores with an aerator each fall to reduce compacted soil. Most lawn weeds tolerate compacted soil much better than grass.

   * Cut your grass high – 3 inches is good. Taller grass blades shade out baby “weedlings” and provide a greater chlorophyll supply to maximize the production of growth-generating sugars for grass roots.

   * Top-dress the lawn each fall with a quarter-inch layer of compost. This adds nutrition, organic matter and microbes that all aid grass growth.

   * Use insecticides and fungicides only if needed, and target them to specific problems.

   * Let lawns go dormant in summer droughts. If you do water, do it deeply less often instead of shallow and frequently. Put on enough water that the soil is damp to a depth of 4 to 6 inches so the roots are encouraged to go down after it.

    Good resources for identifying lawn weeds:

   * County Extension offices, their Master Gardeners and garden centers.

   * The book “Weeds of the Northeast” by Richard Uva, Joseph C. Neal and Joseph M. DiTomaso (Cornell University Press, 1997).

   * Rutgers University’s New Jersey Weed Gallery at www.rce.rutgers.edu/weeds/default.asp.

   * Purdue University’s weed-identification pages at www.purdue.edu/dp/envirosoft/lawn/src/pest/classification2.htm.

A Penn State do-it-yourself soil-test kit.