It is best to get rid of any weeds before planting sod or grass seed, but use a post-emergent herbicide, not a pre-emergent weed killer. Pre-emergent herbicides leave active residues in the soil for several weeks or months, which can damage new grass. Post-emergent herbicides containing glyphosate will kill the weeds and won’t leave residue behind. Weeds springing up is generally a sign that something is wrong with the lawn. A well-maintained lawn generally won’t have problems with weeds. Over-watering, poor drainage, nitrogen deficiency and mowing the lawn too short can all lead to weed growth.
New grass is more susceptible to damage from herbicides than an established lawn. It is weaker, more delicate and cannot tolerate harsh chemicals designed to prevent and kill undesirable plants. Pre-emergent herbicides will interfere with any remaining grass seeds that haven’t germinated yet or have just started sprouting. Post-emergent herbicides can severely damage new grass that isn’t established. Unless the weed killer is designed for use at the time of seeding, do not use the herbicide on new grass.
Herbicides applied before the undesirable plant germinates are called pre-emergent weed killers. These herbicides kill sprouting weed seeds and weeds newly germinated, but won’t kill existing weeds. Pre-emergent herbicides leave a residue in the ground that stays active for an extended period. Post-emergent herbicides kill established weeds. These types of herbicides control various lawn weeds such as dandelions. Herbicides are described as either translocated or contact. Weeds absorb translocated herbicide — when it is applied to their foliage — which interferes with metabolism. This will kill both above-ground and below-ground portions of the weed. Contact herbicides only kill the portions of weed they come in contact with and the weeds will often grow again.
Controlling Weeds in New Grass
Weeds consume the nutrients and moisture that new grass desperately need for proper growth. These undesirable plants will also cause the new grass to appear messy and unruly. Weed killers are readily available to control various species of weeds. Unfortunately, weed killers can cause more harm than good on new grass.
Don’t apply weed killer to new grass until you have mowed it three times, advises North Dakota State University Master Gardener Extension. The University of California Integrated Pest Management Program suggests pulling weeds by hand as they appear. This will help prevent a larger invasion of weeds. In addition to manual removal, proper care will control weeds in both new grass and established lawns. The roots of newly planted grass are short for the first few weeks and require only a light watering to keep the top 2 to 3 inches of soil moist. Once established, water deeply but less frequently to encourage the roots to grow deeper, which will lead to a healthier lawn.
Amanda Flanigan began writing professionally in 2007. Flanigan has written for various publications, including WV Living and American Craft Council, and has published several eBooks on craft and garden-related subjects. Flanigan completed two writing courses at Pierpont Community and Technical College.
Note that it’s most critical to keep weeds away from newly emerging seedlings. Keep your crops weed-free for the first four weeks of their life.
Some types of weeds, especially those with deep roots, grow well because the soil is compacted. The plants roots aren’t getting the air, water, and nutrients they need so the weeds start to take over. If you rent an aerator from your local home improvement store, you’ll be amazed at how providing annual aeration will reduce the amount of deep-rooted weeds.
1. Mulch Over Them
Note: If you use leaf blowers, many come with shredders that can turn yard debris into garden mulch fast, which saves you the costs of making or buying your own mulch. See our mulching guide.
Ever noticed many weeds collect at the edges of your yard or garden? Keep your grass and garden edges trimmed to cut down on invasions of weeds into your fertile garden soil. The places to watch are the not only the edges of your lawn but also around posts and fence lines as well as close to planting beds. Another idea is to grow perennials or ground roses that will shade those edges and make it easier for you!
Make quick work of gliding through and getting too hard to reach spots. It’s especially useful early in the season. Once a week, even if there aren’t many weeds, quickly go over the surface and keep the soil moving. Over time, there won’t be many weeds left.
When used correctly before weeds sprout in early spring, pre-emergent herbicides are one effective way to prevent weeds coming up in mulch. They won’t, however, do anything for weeds that have already sprouted.
To stop weeds in mulch with pre-emergent herbicides, begin by raking mulch off to the side, then hoe or pull any existing weeds. Apply the product, following manufacturer directions to the letter. Pay attention to the label, as some plants don’t tolerate certain types of pre-emergent herbicides.
Manual Mulch Weed Control
Mulch acts as physical barrier against weeds, but it must block sunlight in order to be effective. If you notice weeds coming up in mulch, you may need to thicken the layer as blocking light generally requires at least 2 to 3 inches (5-7.6 cm.). Replenish mulch as it decomposes or blows away.
Weed control is one of the primary reasons for applying mulch, yet pesky weeds may persist, even through a carefully applied layer of bark chips or pine needles. This happens when weed seeds are buried in the soil or are distributed by birds or wind. What should you do if you’ve got weeds coming up in mulch in spite of your best intentions? Keep reading for a few helpful tips.
If you haven’t applied mulch yet, landscape fabric or weed barrier cloth is a safe way to block weeds while still allowing water to pass through to the soil. Unfortunately, landscape fabric isn’t a perfect solution because some determined weeds will push through the fabric, and those weeds will be extremely difficult to pull.