8 Reasons You Still Have Weeds (and what to do about them)

Spring is coming! The birds are chirping, the grass is growing and the sun is warming the soil. And the weeds are growing. Didn’t you treat them last year? Why are they always a never-ending problem?

You’ll never completely win the battle against weeds, but you can make headway and reduce your weed populations as time goes by by addressing these common reasons that weeds keep appearing in your landscape.


You have perennial weeds not controlled by pre-emergents

Glechoma hedereacea  or ground ivy is a perennial weed that can be difficult to control

Perennials weeds are those that have a multi-year life span, though the leafy growth may or may not die off in winter. The growth of perennial weeds is not controlled by pre-emergents. Some resistant perennial weeds (wild garlic and wild violets for example) may take multiple herbicide applications over several seasons to control even when ideal control measures are used.

What to do: Be sure you choose a post-emergent herbicide to treat perennial weeds, and that the target weed is listed on the label. Make herbicide applications at the recommended intervals. Applying herbicide in fall as perennial weeds head towards dormancy can be particularly effective-the active ingredients are transported to the roots more easily.


Weed seed invaders

Dandelions are spread easily by seed

Maybe you got rid of most of your weeds...but your neighbors didn’t. Wind-blown weed seeds are constantly assaulting your lawn and garden, as well as those brought in and “dropped” by birds. Weed seeds can also arrive via your dog’s fur and even your shoes.

What to do: Use pre-emergent herbicides, particularly in spring and fall, to prevent these seeds from germinating.



Soil disturbance

Turning or tilling soil can bring up weed seeds that can germinate when exposed to sunlight, even after many years. Aerating a weedy lawn without follow-up weed control can make weeds worse. (It shouldn’t have too much impact on a fairly healthy thick lawn, as the grass will block the light needed to germinate)

What to do: Follow up any digging with pre-emergent applications in the disturbed areas.


Changes in environment

Drainage problems can lead to weeds

If an area becomes sunnier/shadier, wetter/drier, or other environmental changes take place, certain types of weeds that thrive in those conditions will have an advantage and may begin to proliferate.

What to do: Try to address changes if possible and use appropriate herbicides for the weeds that grow there.


Poor herbicide choice

No herbicide can control every weed, and the herbicide must be chosen based on the target weeds.  If the wrong herbicide type is used, the weeds just keep growing.

What to do: Every herbicide by law lists weeds controlled, make sure the weeds you want to control are on the list.  Pictures of many weeds can be found on the NC Turf Files site.

Also be sure that you select the right type of herbicide: Selective herbicides are designed to kill weeds and (hopefully) not grass.  Non-selective herbicides will kill anything green. Pre-emergent herbicides prevent the germination of seeds. They are non-selective as they can prevent the germination of all seeds, including desirable grass or flower seeds. Post-emergent herbicides are used on existing weeds. They can be selective or non-selective.


Poor application timing

Related to herbicide choice is timing. Some herbicides work poorly (or not at all) at certain times of the year. This is related to the active growth cycles of the weed in question. Many herbicides, even non-selective types, will have little impact at low temperatures and work best at temperatures above 50℉. Additionally, most liquid herbicides must dry on the plant; rain shortly after application will render the application ineffective.

What to do: Again, the herbicide label is your friend here. Most post-emergents will state to use while the target weed is actively growing. Be sure you know if your target weed is a cool-season weed or a warm season weed. Cool-season weeds can be particularly difficult to control as they are active at lower temperatures. Pre-emergents are a good choice here and should be applied in mid- to late-September.


Herbicide breakdown

If control is poor along walkways and curbs, herbicide breakdown may be at fault. Herbicides are broken down by high temperatures and the expected effective lifespan of an application may be shortened significantly. The heat from stone, pavers and concrete will speed up the breakdown process.

What to do: Shorten reapplication in these areas as much as the product label allows and use spot treatments of other products if needed.


Application technique

AKA "You missed a spot".

What to do: Practice applying herbicides with even coverage. Make sure that you are applying the correct amount since too little product can have little effect, and too much can damage non-target plants like shrubs or your lawn.


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