Preventing Weeds

By Jim Janke | Apr 01, 2014

My worst garden headache is weeding the top quarter acre of our lot. This slope is covered with ornamental plants and ground covers, but is too steep for me to climb. So I have to pay a couple of folks more agile than myself to pull weeds. Here’s how I minimize the number of weeds they have to deal with.

Remember that weeds are plants that really like it here; that’s why they do so well. So the best weed control strategy is to prevent them from growing in the first place. Most weed seeds need light to germinate, so applying mulch will minimize the seeds that sprout (at least until additional dirt and seeds blow in on top of the mulch.)

But on our slope mulch doesn’t work well. Distributing it around low growing ground covers must be done carefully to avoid smothering a lot of plants. Double-ground hardwood mulch won’t even stick on the steepest areas, so individual handfuls of pine straw have to be anchored to the soil with sod staples. Moreover, mulch has to be reapplied every couple of years (at well over $1000 each time) to be effective.

I’m a chemical engineer with a healthy respect for chemical herbicides, and minimize their use as much as possible. But for this difficult area a germination inhibitor is a good solution. I use a concentrate containing oryzalin (like Surflan® or Oryzalin 4 PRO®) applied with a hose-end sprayer. A quart of concentrate is enough for two applications per year. The result is astonishing: the number of weeds is reduced by about 90 percent. And considering what I have to pay to have the weeds removed, the cost of the chemical ($40) is easily justified, as is the effort to drag 300 feet of garden hose up the driveway to spray the slope from above.

Read the label carefully before applying any chemical. A list of plants not harmed by the pre-emergent will be on the label, along with plants that it shouldn’t contact. Any spray should only be used on absolutely calm days so it goes only where you want it. Use the personal protective equipment and sprayer setting recommended on the label. These products often contain powerful dyes (for example, Surflan® is extremely orange), so they may discolor any plants that get sprayed until washed off by the first rain.

This should be obvious, but don’t use a pre-emergent herbicide where you are trying to get seeds to germinate. A classic personal example was the time I used crabgrass preventer in an area of the lawn I had just seeded. (Nothing grew for some reason. Duh!!)

Because my Wave® petunias produce so many flowers (and seeds) I also use a pre-emergent in those beds and containers. A granular product containing trifluralin or dithiopyr (like Preen® or Treflan®) is easy to apply and requires a minimum of protective gear. One application lasts up to 3 months.

Weeds in all our food-related beds (veggies, herbs and fruits) are still controlled by pulling them manually, however. No chemicals.

Jim Janke is an Extension Master Gardener Volunteer in Haywood County. For more information call the Haywood County Extension Center at 828-456-3575. © 2014 NC State University.

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