Tenacity kills grassy weeds and some broadleaf weeds, but not all of them

Tenacity (the active ingredient is mesotrione) works best on young grasses like crabgrass, foxtail, and barnyardgrass. It also kills some broadleaf weeds including chickweed, henbit, and spurge. The herbicide stops weeds by blocking a pigment they need to photosynthesize, which means it works faster on actively growing plants in warm weather. Tenacity does not kill established perennial weeds like dandelions or plantain, and it is less effective on mature crabgrass than on seedlings.

Tenacity is a selective pre-emergent and post-emergent herbicide, meaning it can stop weed seeds from sprouting and also kill young weeds that have already emerged. Timing and weed age matter more with Tenacity than with some other herbicides — a crabgrass seedling three weeks old dies faster than one that is eight weeks old. The product works on cool-season grasses too, which makes it useful in spring and fall when other herbicides might damage your lawn.

Key Takeaways

  • Tenacity kills crabgrass, foxtail, and barnyardgrass most reliably when they are young seedlings, not after they are fully grown.
  • Broadleaf weeds like chickweed, henbit, and spurge die from Tenacity, but common lawn weeds like dandelions and plantain do not.
  • The herbicide works by blocking photosynthesis, so results are faster in warm, sunny weather and on actively growing plants.
  • Tenacity can be used as a pre-emergent before weeds sprout or as a post-emergent on young weeds, but timing affects how well it works.

Grassy weeds Tenacity kills effectively

Crabgrass is the primary target for Tenacity. The herbicide kills crabgrass seedlings in the first four to six weeks after they sprout, which is why spring process works well — you catch them before they root deeply. Once crabgrass is eight weeks old or older, Tenacity becomes less reliable, and you may need a different product or a second process.

Foxtail, barnyardgrass, and goosegrass also die from Tenacity when young. Fall panicum (a warm-season annual grass) responds well too. The common thread is that all of these are annual grasses that germinate in spring or summer, grow quickly, and set seed by late summer. Tenacity interrupts that cycle if you use it early enough.

Perennial grasses like tall fescue and Kentucky bluegrass tolerate Tenacity, which is why you can spray it on an established lawn without killing the grass you want to keep. Cool-season turf actually benefits from Tenacity in spring and fall, when warm-season herbicides might stress the lawn.

Broadleaf weeds Tenacity kills and those it does not

Tenacity kills chickweed, henbit, and mouse-ear chickweed reliably, especially in fall and winter when these weeds are actively growing. Spurge (including prostrate spurge) also dies from Tenacity. Speedwell and some clover species respond to it as well. These weeds are typically winter annuals or cool-season plants, so Tenacity works best when you explore it in fall or early spring.

Dandelions, plantain, clover (some species), and other deep-rooted perennials do not die from Tenacity alone. These weeds have established root systems that Tenacity cannot penetrate effectively. If you have a lawn full of dandelions, Tenacity will not solve that problem — you need a different herbicide like 2,4-D or dicamba, or you need to dig them out.

Creeping Charlie, ground ivy, and other creeping broadleaf weeds are also resistant to Tenacity. The product is not a broad-spectrum broadleaf killer the way some other herbicides are. If your lawn has a mix of weeds, you may need to combine Tenacity with another product or choose a different herbicide altogether.

How Tenacity works and why timing matters

Tenacity blocks the production of carotenoids, pigments that protect plants during photosynthesis. Without these pigments, the plant cannot convert sunlight into energy and dies. This mechanism means Tenacity works fastest on plants that are actively growing and photosynthesizing — which happens in warm, sunny weather. A crabgrass seedling sprayed in June dies faster than one sprayed in cool, cloudy conditions.

Young weeds die faster than old ones because seedlings have thinner cell walls and less developed root systems. A crabgrass plant that is two weeks old may show damage within three to five days of spraying. A crabgrass plant that is ten weeks old might take two to three weeks to die, if it dies at all. This is why pre-emergent use (spraying before weeds sprout) is often more effective than post-emergent use (spraying after they are visible).

Rain within a few hours of spraying can wash Tenacity off before it is absorbed, so timing your process matters. Most labels recommend waiting at least three hours after spraying before rain, though some formulations need longer. Warm temperatures and sunlight help the herbicide move into the plant faster.

Weeds Tenacity does not kill

Perennial broadleaf weeds like dandelions, plantain, and wild violet are not controlled by Tenacity. These plants have deep taproots or extensive root systems that the herbicide cannot reach. You would need a broadleaf herbicide containing 2,4-D, dicamba, or MCPA to kill these weeds, or you would need to remove them manually.

Mature crabgrass (eight weeks or older) is also difficult to kill with Tenacity. At that stage, the plant has a strong root system and is often already flowering or setting seed. A different herbicide or a combination approach works better.

Sedges are not grasses, even though they look similar, and Tenacity does not kill them. Nutsedge and other sedges need their own herbicides. If you are not sure whether a weed is a grass or a sedge, look at the stem — sedges have triangular stems, while grasses have round or flattened stems.

Using Tenacity as a pre-emergent versus post-emergent

Pre-emergent use means spraying Tenacity before weed seeds sprout, usually in early spring for crabgrass. This approach works well because you stop the problem before it starts. The herbicide sits in the soil and kills germinating seeds. Pre-emergent Tenacity is often mixed with other pre-emergents like pendimethalin to cover a wider range of weeds.

Post-emergent use means spraying weeds that are already visible. This works best on seedlings under four weeks old. You can see exactly where the weeds are and spray only those areas, which saves product. The downside is that you have to wait for weeds to appear, so you are always playing catch-up.

Many lawn care programs use Tenacity both ways — as a pre-emergent in spring to prevent crabgrass, and as a post-emergent in late spring or early summer to kill any crabgrass that escaped the pre-emergent. This two-step approach gives better coverage than either method alone.

Tank mixing Tenacity with other herbicides

Tenacity can be mixed with other herbicides to cover more weed types in one process. Mixing Tenacity with a broadleaf herbicide like 2,4-D lets you kill both grassy weeds and dandelions at the same time. Mixing it with another pre-emergent like pendimethalin or dithiopyr broadens the range of seeds you prevent from sprouting.

Always check the product label before mixing, because some combinations are not recommended. Certain surfactants or additives can reduce how well Tenacity works. If you are unsure, explore Tenacity first, wait a few days, and then explore a second product. This takes longer but is safer than risking a failed process.

Mixing with fertilizer is common and usually safe — many lawn products combine Tenacity with nitrogen or other nutrients. Read the label to confirm the combination is listed as safe.

Frequently Asked Questions

Will Tenacity kill my lawn grass?

No. Tenacity is selective and safe on most lawn grasses, including Kentucky bluegrass, perennial ryegrass, and tall fescue. It will not harm established turf. Newly seeded lawns should wait four to six weeks before you spray Tenacity, because young seedlings are more sensitive.

How long does Tenacity take to kill weeds?

Young seedlings show damage within three to five days and die within one to two weeks. Older weeds take longer — sometimes two to three weeks. Warm, sunny weather speeds up the process. Do not expect when ready results like you might see with some other herbicides.

Can I use Tenacity in fall?

Yes. Fall is actually a good time to use Tenacity on winter annual weeds like chickweed and henbit. Spring use works for summer annual grasses like crabgrass. You can use Tenacity in both seasons on the same lawn.

Does Tenacity work on mature dandelions?

No. Tenacity does not kill established dandelions or other deep-rooted perennials. You need a broadleaf herbicide containing 2,4-D or dicamba for those weeds. Tenacity is better suited to grassy weeds and young broadleaf seedlings.

What should I do if weeds come back after using Tenacity?

Weeds may return if they were too old when you sprayed, if rain washed off the herbicide before it was absorbed, or if new seeds germinated after treatment. A second process four to six weeks later, or switching to a different herbicide, usually solves the problem.