Dandelion, clover, plantain, and other broadleaf weeds are among the most common and troublesome turf pest problems in lawns. Even though most broadleaf weeds can be easily controlled with herbicides, a completely weed-free lawn is neither practical nor environmentally sensible. A safe and sound approach to lawn weed control is to grow a healthy lawn, spot-treat weeds with the correct weed control product as they appear and avoid the temptation to have a 100% weed-free lawn.
The best way to minimize weed problems in your lawn is through the use of good cultural practices: proper mowing height and frequency, sensible fertilization, and adequate irrigation. On the other hand, lawn weeds are encouraged by mowing your lawn too short or not often enough; fertilizing too much, not enough, or at the wrong time of the year; and over- or under-watering.
Where Do Lawn Weeds Come From?
Weed seeds occur naturally in all soils and can persist for 30 or more years. They will germinate when a lawn is thin and not healthy, when the seeds are brought to the surface by human or pet traffic, or when the turf is damaged or killed by drought, heavy traffic, insect feeding, or disease activity.
Cheap, low-quality grass seed often contain unwanted weed seed. If the seed label lists ANY weed seed as a component, DON’T buy it! The best quality grass seed (sold by professional seed suppliers) will almost always be 100% weed-free and will often cost nearly the same as poor quality products which contain weed seed. READ THE SEED LABEL! The weed content of any grass seed you buy (expressed as a %) should be 0%.
Weed seeds are often brought to a landscape in topsoil or compost. Make sure that all soil or compost comes from a reputable supplier and is guaranteed to be weed-free.
Using Herbicides to Manage Lawn Weeds
The most common herbicide choice is a general- purpose mixture comprised of two or three of the following individual herbicides or active ingredients: 2,4-D, MCPP, and dicamba. A combination active ingredients will control a wider spectrum of broadleaf weeds compared to a single active ingredient. Read and follow all directions on the herbicide label if you choose to apply a herbicide to your lawn.
The best time to apply a general-purpose broadleaf herbicide to control perennial broadleaf weeds such as dandelion, plantain, and clover is early September to early November. As winter approaches, perennial broadleaf weeds are storing energy reserves in stems and roots. A fall-applied herbicide will enter the plant and travel to these plant parts with the food reserves. The second best time is in the late spring or early summer period after the weeds have flowered. If applying in the late spring, be extremely cautious with these herbicides near ornamentals, trees, flowers, and vegetable gardens because these plants can be damaged by these herbicides through direct application, drift, and/or volatilization (the herbicide turns into a vapor). This is another reason fall herbicide applications are preferred.
If you only have a few weeds in your lawn, simply spot-apply a herbicide rather than applying it to the entire lawn. Apply just enough to wet the leaf and do not apply to the point that the herbicide is dripping off the leaf.
Herbicide Application Tips for Success
- Apply to actively growing, preferably young weeds.
- Do not apply herbicides when the soil moisture is low and weeds are drought-stressed; an actively growing, healthy, non-stressed weed is the easiest one to control.
- Apply herbicides on a calm, clear day when the air temperature is between 50° and 85° F; applying them when temperatures exceed 90° F increases the potential for volatilization injury to other plants in the landscape.
- Do not apply if rainfall will occur within twelve hours; avoid applying irrigation for at least twelve hours following a herbicide application.
- Do not mow the lawn for two days before and after the herbicide application.
- Do not apply to new turfgrass seedlings until the grass has been mowed at least three times.
- Delay applying a broadleaf herbicide to new sod for four to five weeks after planting.
Summer Broadleaf Weed Management
Summer annual broadleaf weeds (e.g., spurge, knotweed, purslane, etc.) are very difficult to control for a number of reasons. Depending on the species, these weeds germinate at different times during the summer and mature in a very short period of time. Thus, a single application of herbicide might only control a single weed species because other species have not germinated or have grown too large to be controlled. Summer annual weeds often have a thick, waxy cuticle layer on their leaf surface to prevent water loss; this layer may also make it more difficult to get herbicide into the weed.
Some annual broadleaf weeds can be effectively controlled by preemergence herbicides. For example, summer annuals like spurge, knotweed, purslane, and puncturevine can be controlled with products containing Prodiamine, pendimethalin, or Isoxaben.
Difficult-to-Control Weeds
Weeds such as bindweed, thistles, and wild violets are difficult to control because they spread by underground stems. Multiple herbicide applications may be necessary to completely control difficult perennial weeds, including dandelions. Post-emergence broadleaf herbicides containing 2,4-D, MCPP, dicamba, triclopyr, or Sulfentrazone can be used.
This publication, reference GardenNotes #552, is developed as part of the Colorado State University Extension Master Gardener Program.