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Lecture 4: Preemergence Weed Control in Landscape Beds

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Preemergence – what does that mean?

Before weeds emerge, but more importantly – before weed seeds germinate. There are two methods for preemergence weed control – mulching and preemergence herbicides. Mulches prevent light from reaching seeds and newly germinated seedlings. So, seeds that are not sensitive to light will germinate. Mulch must be thick enough to smother young seedlings before they reach the surface and sunlight. How thick – 3 to 4 inches; Deeper and weeds grow in the mulch. Some large-seeded weeds will grow through the mulch.


  • Weed management options: the first and best for annual weeds – mulches
  • What do mulches do: exclude light
  • What is a good mulch: coarse to dry out
  • Does not slip & wash

How much mulch is necessary?

In woody landscape beds:

  • ~4 inches organic mulch
  • ~3 inches inorganic
  • ~ 2 inches organic if used with a geotextile

In bedding plants:

  • ~1 to 2 inches organic mulch
  • ~Inorganic mulches are not recommended

How much mulch?

  • Bark mulch: 1 cu yd covers ~ 100 sq ft to a depth of 3 inches
  • Pine Straw: 1 bale covers about 35 sq ft.



  • long term weed control (annual weeds only)
  • soil stabilization (prevents erosion)
  • Possible add-on service ($)
  • Holds soil moisture


  • Cost — material and installation
  • Site preparation – to eliminate perennial weeds
  • Cover with mulch — must be long term commitment
  • Weed grow through (perennials)
  • Annual weeds germinate on top
  • Mulch slides off on slopes
  • Water retention (on heavy soils, it may hold too much moisture)
  • Decomposition of organic mulches creates a humus layer which allows weed germination and encourages

Biggest problem with geotextiles is improper installation. How to do it properly:

  • dig a trench
  • anchor edge of geotextile in trench
  • cover with soil (not mulch!)
  • Pull geotextile over the plants, cut and push down. Do not try planting through.

Are there differences in types of geotextile fabrics?

Yes, between commercial & consumer grades. You want the commercial grades
Fine weave (small holes) are better.

Landscape weed control Economics:

hand weeding is expensive but,
But the cost of bark mulch is more (don’t forget this expense when figuring costs)
Geotextiles amortized over 5 years – the cost is similar to preemergence herbicide + bark

Advice on the use of Geotextiles:

Eliminate perennial weeds before installation: Site preparation Proper installation
Keep covered — annual maintenance, replenish mulch annually
Remove escaped weeds when small
Consider using inorganic mulches
Consider using a preemergent herbicide on top

Why use a preemergent herbicide?

Over time the organic mulch breaks down to form a layer of “humus”. This layer on top of the geotextile fabric creates a perched water table and a moist zone which promotes weed seed germination. When annual weeds are a constant problem, this could be the cause. The long-term solution is to remove the mulch and replace with fresh material. Short term, preemergent herbicides may be needed.

My question: if you are going to need a preemergence herbicide then why bother with a geotextile at all?

Biobarrier: A Specialized Geotextile

Trifluralin (Treflan) pellets are bound to the fabric. Trifluralin vaporizes out of the pellets preventing root and shoot growth through the fabric. Useful to containment of roots when installed vertically in the soil. Also, when installed on the soil surface it can control rhizomatous and tuberous perennial weeds for a long time.

Study Questions:

  1. How do mulches control weeds?
  2. How deep should the mulch layer be for:
  3. organic mulches?
  4. Inorgainic mulches?
  5. Organic mulch with a geotextile fabric?
  6. What are some advantages and disadvantages of using a geotextile fabric?
  7. What kinds of weeds are controlled by mulches? And, what kinds are not?
  8. What is Biobarrier and what does it do that other geotextiles do not?
  9. Describe the procedures for proper installation of a geotextile fabric.