The cool, wet weather in July and so far inAugust has been a curse for the warm-season garden crops and ablessing for anyone with a cool-season lawn. The lush green lawnsare obvious everywhere. However, a closer look identifies manyother species besides turfgrass. Crabgrass has invaded many areaseven though a preemergence herbicide was used this spring and thebroadleaf weeds such as white clover, and broadleaf plantain aretoo numerous to even attempt to count. One always hears that fallis the best time to sow turfgrass seed and that it is also a goodtime to control broadleaf weeds, but which should be done first?
Unfortunately, there is no easy answer to the question becauseeach lawn has a specific set of circumstances that must beconsidered. For those lawns that are mainly weeds (annual andperennial), the time for complete renovation is here, because thebest time to establish a lawn from seed is between August 15 andSeptember 15. Renovation means killing all existing vegetation andstarting over. The extension publication Pm-1055, TurfgrassRenovation has a lot of helpful information for anyone consideringstarting all over with their lawn.
For those lawns that are mostly turfgrass, the time to starteliminating those broad-leaves is a few weeks away. Mid Septemberto early November is generally the best time to selectively controlthese weeds with a herbicide. Controlling broadleaf weeds in thefall gives the turfgrass an opportunity to fill in those open areasthat appear after weed control. This results in a denser turfwhich culturally helps prevent weed establishment. Fall is also agood time for other cultural practices such as aeration ofcompacted areas. Fertilizing later in the fall, when the topgrowth of the cool-season turfgrasses has nearly stopped, willfavor rhizome production and growth which helps fill in thin areassuch as those spots where crabgrass was growing prior to thatkilling frost. Anything that will promote proper turfgrass growthwill ultimately help minimize weed populations.
There are a number of postemergence herbicides available forselective control of broadleaf weeds. Most of these products aretwo-, three-, and even four-way combinations of 2,4-D, and either2,4-DP, dicamba, MCPP, or triclopyr. These combinations have takenmost of the guessing out of broadleaf control because broadleavesnot controlled by the first chemical are usually controlled by oneof the other herbicides in the mixture. For example, 2,4-Dprovides excellent control of dandelion and plantain, but providespoor control of white clover or red sorrel. Dicamba, on the otherhand, provides excellent control of white clover and red sorrel andonly fair control of dandelion or plantain. By applying acombination of these two, all four weeds will be controlled. Thisdoesn't mean that a combination product will successfully controlall of those unwanted broadleaf weeds in your turf. A fewbroadleaf weeds are just naturally hard to control with the currentherbicide choices, and secondly, the wrong combination product mayhave been chosen (i.e., 2,4-D + MCPP instead of 2,4-D + MCPP +dicamba). So it is still important to identify your problem weedsbefore you try to chemically control them.
Finally, one needs to consider seeding restrictions for thosethat want to overseed thin spots or areas that were killed fromstanding water. Postemergence broadleaf herbicides are selectiveonly in established lawns. Do not apply broadleaf herbicideswithin 4 to 6 weeks before seeding and not until the new grass hasbeen mowed several times. There-fore, if overseeding is currentlybeing considered or has just been done, do not use these productsthis year.
This article originally appeared in the August 25, 1993 issue, p. 144.