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Lectures 5 & 6: Preemergence Herbicides

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Preemergence means – before weeds emerge. More importantly, before they germinate.

How do Preemergence herbicides work?
By preventing seedling establishment. Not by killing seeds.

Therefore, timing of applications depends on life cycles of the target weeds.

Preemergence herbicides prevent germinated weed seedlings from becoming established; either by inhibiting the growth of the root, the shoot, or both. The herbicide must be incorporated into the soil via rainfall or irrigation, and be present when the weed seeds are germinating to be effective. Preemergence herbicides have NO EFFECT on weed seeds. Preemergence herbicides say in the soil for a while, preventing weed growth. This is termed — residual. Over time the herbicides are degraded by soil microbes. Reapplication is required before the concentration of the herbicide drops below the critical level required to control weeds. Most preemergence herbicides will last between 8 and 12 weeks; however, some last longer and some shorter. Check the label and your Cooperative Extension agent for re-application guidelines in your area.

Preemergent herbicides which control grasses and a few broadleaf weeds may be used in many different planting situations, but those which are better on broadleaf weeds injure many ornamentals. More caution is needed when dealing with those herbicides or in planting situations (herbaceous perennial beds) where sensitive plants will be planted.

What are the major herbicides used in landscape maintenance and what do they control?


  • Control SA & WA grasses & some broadleaves, henbit, chickweed spurge.
  • Efficacy varies on some broadleaves including ragweed, groundsel, & others.
  • Weak on nightshades, galinsoga, purslane
  • Do not control nutsedges or other perennials
  • Good combination of safety and efficacy

EXAMPLES (dinitroaniline herbicides):

Surflan (oryzalin) — #1 in woody landscape beds. Offers the best combination of efficacy and safety to WOODY ornamentals. A granular formulation is available as XL (oryzalin + benefin). Better than the other dinitroanilines on most broadleaf weeds including ragweed, morning glory, evening primrose, and others.

Barricade (prodiamine) — a close second. Safe on many woody ornamentals but in sprayable formulation is injurious to many herbaceous plants. Granular formulation (Regalkade G) is safer on ornamentals. Can only use twice a year (maximum dose per acre per year).

Pendulum = Corral (pendimethalin) – next best. Controls grasses from seed and small seeded broadleaf weeds. Better than previous two on horseweed but weaker on several other broadleaves.

Treflan / Preen (trifluralin) – the safest on ornamental plants but also the weakest for weed control. Controls winter annual weeds including annual bluegrass, chickweed and henbit well. But tends to be less effective on summer weeds.

Balan — turf only

Team Pro (Balan + Treflan) for turf and some woody landscapes. Generally not widely used in landscapes.

How do Dinitroaniline herbicides differ? Annual broadleaf weed control:

  • Surflan is generally regarded as better
  • On spurge – Barricade is slightly better
  • On horseweed – Pendulum is slightly better

Safety to Ornamentals: Granular formulations are much safer than sprays

  • Pendulum has a broad label on herbaceous ornamentals – that’s the GR
  • Preen / Treflan is the safest on herbaceous ornamentals
  • Regalkade G is also very safe on many herbaceous ornamentals
  • Surflan is more injurious to herbaceous ornamentals and can girdle the stem of young azaleas

Other herbicides represent different classes and modes of action:

Pennant Magnum– primarily used for pre emergent nutsedge control (to be discussed in more detail later) and annual grass control. EC formulation can burn new tender growth.

Tower — New in 2008. Suppression of yellow nutsedge (similar to Pennant Magnum) plus annual grasses, spurge. EC formulation can burn new tender growth.

Freehand is a combination of Tower with pendimethalin but on a granule.

Devrinol – Safe on herbaceous and woody. Long residual grass control. Weak on many broadleaves. Relatively expensive.

Ronstar — expensive, but good on annual broadleaves injures herbaceous ornamentals, particularly spring bulbs like star of Bethlehem

Gallery — pre broadleaf weed control; usually used as Snapshop TG (Gallery + Treflan; Preen Pro is the same as Snapshot TG)

Snapshot TG is a combination of Gallery + Treflan for broadleaf and grass control. Safe on most woody ornamentals and many “blue collar” perennials

Princep is not used in landscape beds due to potential injury to many ornamentals.

With the exception of Devrinol, Each of these is safe on most woody plants but injures many herbaceous ornamentals.

Reminder: if you are working in a woody landscape bed you have more options whereas herbaceous beds will have limited options.

Relative susceptibility of common landscape weeds to preemergence herbicides.

Herbicide Crab-grass Bittercress Horseweed Spurge Oxalis Nutsedge
Surflan G G F G G P
Barricade G F F G G P
Pendulum G G G
Treflan / Preen G F F P F P
Devrinol G F P-F P P P
Pennant G P P F P G

How to choose the right herbicide:

  • Weed control (efficacy)
  • Plant safety and management
  • Weed status (pre, post)
  • Spectrum of weeds controlled, longevity of control, Weeds Not Controlled (AG-427)
  • Available application equipment (Granular only???)
  • Economics (Ronstar @ $200 per acre or Pendimethalin @ ~$40, if they will work)
  • Proximity of Susceptible species — in a diverse bed; adjacent turf
  • Environmental impact and mammalian toxicity
  • Use label info: REI, other warnings about leachability, etc.
  • When you have a choice, choose the safer product.

Landscape Weed Management Options:

Simple approach – simple, effective on most annual grasses and small-seeded annual broadleaf weeds but will leave many broadleaf weeds. However, there is little concern about injury to ornamental plantings.

  1. Mulch
  2. Treflan / Preen
  3. Hand weed and spot spray weeds that escape

Optimized for the type of planting. This approach is more complex but will provide better weed control (and reduced hand weeding).

  1. Mulch
  2. Select broadest spectrum herbicide for the site

Woody beds:

  • GR: OH2, Rout, Snapshot, Regal O-O for woody landscape beds
  • Spray: Gallery + Surflan or Barricade or Pendulum or Pennant

Herbaceous Perennial beds:

  • GR: Treflan / Preen, Pendulum, Snapshot on some species (kills others);
  • NOT: OH2, Rout,Ronstar, Regal O-O
  • Spray: Barricade over dormant perennials but can damage many emerged plants.

Bedding plants:

  • GR: Safer: Treflan / Preen or XL or Devrinol
  • A bit better: Pendulum 2G or Regalkade G on some species
  • Spray: Surflan, Barricade, on some species

Where to get this information?

  • AG-427 – Tables
  • Trade journals
  • Extension fact sheets and Bulletins
  • Your Peers


Weed Control Suggestions in Christmas Trees, Woody Ornamentals and Flowers

N.C. Cooperative Extension Publication AG-427 Includes:

  • General weed biology information
  • Calibration information and examples
  • Herbicide information sheets (active ingredients, uses, cautions, rates)
  • Weeds controlled by pre & post herbicides
  • Ornamentals listed on herbicide labels

How to use AG-427:

  • What herbicide(s) control the weed(s): Tables 1 & 2 (pre & post, respectively)
  • What herbicides are labeled for use in this crop: Tables 3 & 4 for woody ornamentals and Tables 5 & 6 for herbaceous ornamentals.
  • Cross reference herbicides between the tables then read about the herbicides’ strengths and weaknesses in the “Herbicide Label Information” section (page 18 – 58).


How to control bittercress in pansy beds? Look up the herbicide efficacy in Table 1.

Weed Species Barricade Gallery Pendulum Pennant Surflan Treflan XL
Bittercress, hairy p G f P f p f
Chickweed, common G G G f G G G
Dogfennel p F p p f p f
Evening primrose p f G f g
Geranium, Carolina f G f p g p f
Groundsel, common f G p F g p F
Henbit G G G f G g G
Horseweed f G p f f
Lettuce, prickly ? G ? ? g ? f
Pigweed spp. G G G G G G G
Spurge, spotted g f G p f p f
Yellow Woodsorrel (Oxalis) G G G p G f F

In this except from Table 5: find the herbicide(s) that are labeled for use on pansies that will control bittercress.

Common Name


Gallery Pendulum Pennant Surflan Treflan XL
Daffodil R R R R R
Dahlia R R
Lantana x R R x x
Marigold X R* I R R R
Melampodium x x x x x
Pansy x x I R I R*
Yarrow R* R R R R

Key: R = registered on this species; R* = registered on some varieties of this species; I = registered but injury has been reported; X = injured in research or prohibited on the label. Blank indicates no label information. .

Based on this data one would choose XL for to control bittercress in pansy beds.

You may of course choose to go the other way – looking up the herbicides that are labeled for a particular crop species first then finding the appropriate herbicides for weed control.

Study Questions

  1. How do preemergence herbicides control weeds?
  2. When should a preemergence herbicide be applied? And, how does this relate to a weed’s life cycle?
  3. Why are dinitroaniline herbicides the most commonly used herbicides in landscape plantings?
  4. Give 4 examples of dinitroaniline herbicides labeled for use in landscape beds. List the strengths and weaknesses of each.
  5. What publication is the most appropriate source for information on herbicides for use in ornamental plantings?
  6. Given the species of weeds and ornamental plants – be sure you know how to use the tables from that publication to develop a herbicide recommendation.
  7. Which would be safer on herbaceous ornamentals? And Why? Surflan vs XL; Pendulum 2G vs Treflan 5G; Barricade vs. Regakade; Gallery vs. Snapshot TG?
  8. You need to control mostly broadleaf weeds in a woody landscape bed. Which herbicide(s) have the greatest likelihood of controlling those weeds — Gallery, Surflan, Ronstar, Snapshot TG, or Treflan? Explain your answer (s).
  9. Describe what is meant by the KISS method and OPTIMIZED methods of choosing preemergence weed management options. Give examples.
  10. Describe at least 5 factors to be considered when choosing a herbicide.