When to Apply Herbicides

Gardening experts share tips on when to apply pre-emergent and post-emergent herbicides.
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Corn Gluten Pre-Emergent Weed Control

Corn Gluten Pre-Emergent Weed Control

Photo by: Julie Martens Forney

Julie Martens Forney

Annual weeds like crabgrass, pigweed, lambsquarters and wild mustard sprout from seed as soon as soil warms in spring. Perennial weeds like dandelion, sorrel and thistle can also sprout from seeds. Easy Solution: Short circuit weed seed germination by spreading a pre-emergent herbicide. Corn gluten is an effective, natural control that prevents weed seeds from growing successfully.

Master gardener Paul James' neighbor recently applied a pre-emergent herbicide to his lawn. He used the right product, but his timing was all wrong. He applied a pre-emergent herbicide at least two weeks after the most common lawn weeds had already emerged, which defeats the purpose of applying a pre-emergent.

As the name suggests, pre-emergent herbicides must be applied before weeds emerge to have any real effect, since they work by preventing weed seeds from germinating in the first place. If you wait until after the weeds emerge, the herbicide won't have any effect. Yet many unsuspecting people do just that, which not only results in a waste of money, but in many cases results in pollution of the soil, ground water and nearby rivers, lakes or oceans.

Timing Is Critical

A common misconception about the ideal time for pre-emergent herbicides is that application should coincide with certain events, such as daffodils or forsythias in bloom. In nearly every area of the country, the weeds will have already emerged by that time. To determine the best time of year to apply pre-emergents, note when the weeds begin to sprout this year and count back two or three weeks. That's when you should apply a pre-emergent next year.

If you missed the window of opportunity for applying a pre-emergent herbicide, you can apply a post-emergent product. Post-emergents work by destroying already established weeds. However, take care when applying post-emergent herbicides. Some are selective, meaning they target specific weeds, while others are nonselective, which means they destroy anything and everything green, whether they're weeds, your grass, flowers or shrubs. Therefore read the product label.

Keep in mind that you'll never get rid of all the weeds in your lawn. The wind will blow weed seeds from nearby lawns into your lawn, birds will deposit them and kids running from one lawn to the next will transport weed seeds on their shoes. So do what you feel you must to battle the weeds in your lawn, but do it wisely. Paul recommends using natural pre-emergent herbicides made from corn gluten or natural post-emergents made from vinegar or clove oil. Raise the height of your mower to help prevent weed seeds from germinating in the first place. Fertilize twice a year with a natural product to promote vigorous grass.

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