Q: What are the advantages and disadvantages of preemergent herbicides?

A: In most cases, herbicide application should not be considered as a first option for weed control. Hand weeding, hoeing and mulching are tedious but effective methods of weed control. Sometimes, however, you may have to get out the big guns.

Preemergent herbicides prevent weed seeds from germinating. They are relatively safe to use around existing plantings, such as turfgrass. They must be applied before the weed seedlings emerge to be effective. To prevent winter annuals (chickweed, annual bluegrass, purslane, dead nettle, etc.) apply in late summer or early fall. To prevent summer annuals and perennial weeds (dandelion, crabgrass, spurge, poa annua, oxalis, etc.) apply in late winter or early spring. Preemergents are most effective when applied in cooler, dry weather. If applying over mulch, the depth of mulch and the size of the mulch particles can impact effectiveness because organic matter can bind to the active ingredient of the herbicide.

Most types of preemergent herbicides can only be applied by licensed professionals, who should be familiar with which type of herbicide should be used against which type of weeds. Several products are available for homeowner use, such as Preen and some types of crabgrass killers.

Preemergent herbicides can be effective against difficult-to-control weeds like oxalis and crabgrass. Unfortunately, they must be reapplied to remain effective. I have seen cases where homeowners have discontinued their lawn service after multiple years, only to see many years of accumulated weed seeds germinate all at once.

Q: Is weed cloth effective at controlling weeds?

A: Weed cloth may seem like a good idea, but it causes more problems than it solves. Many people simply cut existing weeds down to the ground, then lay weed cloth (or, worse yet, plastic sheeting) over the ground and throw mulch on top. This fix is very short-lived since weed seeds that are on top of the weed cloth can germinate and send roots through the weed cloth into the ground. If you try to pull that weed, you’ll pull up the weed cloth over the mulch, tearing it in the process. Soon you’ll have mulch with unsightly chunks of torn weed cloth poking out of the ground. I have also seen weed cloth applied over existing drip lines or emitters. This makes it impossible to monitor the irrigation system because the lines and emitters are hidden. In this case, if the irrigation is not working, you may not know anything is amiss until the affected plants are in great distress or dead.

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