Battling Weeds in the Lawn

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September is a time for the lawn. In my last article, I discussed overseeding and fertilizing the lawn, but weed control is another key to a healthy lawn. Some weeds in the lawn are often tolerated, but when the weed population begins to outweigh the turf population, management should be incorporated.

The Battle with Weeds

Plants are considered weeds because they are adaptive, aggressive and opportunistic. Weeds come into a lawn that is thin or bare. They are often found on the edges of our lawn or places where the grass doesn’t grow as well. Eventually they will work their way into the rest of our lawn. So overseeding a lawn may be the answer to reducing the weed population.

Growing turfgrass in the shade is not always possible. Even the shade mixes are not made to grow in high shade. Dense shade is not the growing condition for turfgrass and it often leads to weeds. In some cases, trees may be pruned to improve the sunlight getting to the turf, but be careful not to ruin the shape or health of a tree or shrub  just to get more sunlight to the turf. If the shade is too dense, a good alternative might be a shade tolerant groundcover or apply mulch where grass won’t grow.

Crabgrass

Crabgrass has been found in high populations this year, even in locations where crabgrass preventer was used. The University of Nebraska-Lincoln Turf department says the weather is to blame. Crabgrass germinates when soil temperatures reach 57-64 degrees at a one-inch depth, with the highest germination when soil temperatures increase to 73 degrees. Crabgrass will continue to germinate through the summer until soil temperatures reach 95 degrees. With the unusual weather we had this spring and summer, the largest amount of crabgrass germinated later than normal when the concentration of our original spring crabgrass pre-emergence herbicide was declining. This is why we saw so much more crabgrass later this summer than we usually do.

To help prevent this problem with later emerging crabgrass in the future, switch to longer lasting chemicals, such as prodiamine. You can also look to a second application of crabgrass preventer in mid-June to stop the germination of the late flush of crabgrass. There are also post-emergent herbicides to use for crabgrass, but I don’t recommend them this late in the season. Remember, crabgrass you are seeing now will die when the first frost hits. Also, it is difficult to kill a large crabgrass plant with a post-emergent herbicide.

Broadleaf Weed Control

As for when to treat for broadleaf weeds that do come into our lawns, fall is the best time to control them. In fall, perennial weeds are moving carbohydrates from the leaves into the roots for winter storage to help get them going again next spring. If you spray them in the fall, the herbicide will also be moved into the roots which makes the herbicide more effective. Also, the weather is more suitable for herbicide use than in the spring when Dicamba and 2,4-D have a potential to drift to non-target plants. Fall is also a great time to apply herbicides to kill the winter annual weeds such as Henbit either with a pre-emergent herbicide such as prodiamine (Barricade) or dithiopyr (Dimension) or with a post emergent after the henbit has germinated.

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Henbit along a sidewalk edge

The best time to apply herbicides in the fall is mid-late September and again in mid-late October. The second application helps to get better control on perennial weeds that may have been missed or were not fully killed. The second application also helps to ensure that winter annuals have germinated to help get control of those with a post-emergent herbicide. Products containing 2,4-D, carfentrazone, sulfentrazone, quinclorac, or triclopyr are all good for controlling perennial weeds in the lawn. Use caution around trees, shrubs, and landscape beds as these products can damage broadleaf plants but they will not harm our turf.

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Fall Lawncare

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Bare lawn in need of overseeding.

There are a lot of activities for us to do in the fall in our lawns to help our plants make it through winter and to improve their growth for next year. I wanted to take time to cover some of those items on your fall lawncare “to do” list.

It is now time to reseed or overseed your lawns for the fall. This is best done in the late summer or early fall, anytime between late August and the end of September. The rule of thumb is that that for each week grasses are seeded before Labor Day, maturation is speeded by two weeks. If you reseed after September 15 you could still be successful, but you increase your risk of dieback on newly emerged young seedlings in the event of an early frost. If you are a homeowner who wants to sod an area of your lawn, you can do that until they can no longer cut it from the fields.

Overseed with the same type of lawn that is already existing or a mix of Kentucky bluegrass and tall fescue. When starting a new lawn, either of these would be good choices. Avoid mixes that contain annual ryegrass, ‘Linn’ perennial ryegrass, or ‘Kenblue’ Kentucky Bluegrass. Make sure that the grass you buy contains less than 0.3 percent weed seed and No noxious weed seeds. If you prefer, you could also plant Buffalograss in your lawn, but this should be done in June and July.

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Lawn Sprinkler Photo Courtesy of Creative Commons License

When establishing new turf, use frequent, light watering practices to get the seed to germinate. Don’t let the seed dry out and don’t let it get too soggy either. As the grass grows, decrease the frequency of watering, but increase the amount of water applied. Don’t drastically change watering practices from newly seeded to established turf watering practices. Don’t apply any herbicides to newly seeded turf until after mowing 3 times on the new turf.

As for fertilizer applications, the fall fertilization is the most important fertilizer application for a lawn. However, fall fertilization recommendations have changed over the past couple of years. For a lawn, a Labor Day to mid-September application of slow release fertilizer is still recommended. Apply a granule with 50% slow release nitrogen or less. If additional nitrogen fertilizer is required later in the fall, use a product with a quick release nitrogen before mid-to late October. We used to recommend Halloween or later for the second fertilizer application and we thought two applications were necessary. New research is showing us that a second application of nitrogen fertilizer may not even be necessary, but if it is, we should move the timing up to more like Columbus Day rather than the typical Halloween time frame. This information is from Bill Kreuser, Assistant Professor and Turfgrass Extension Specialist from the University of Nebraska-Lincoln.

Mowing is always on our minds this time of year as well. Continue mowing through the fall as long as the grass is growing and needs to be mowed. You don’t want to leave long grass blades through the winter as this can smother the grass and lead to snow mold. Also, be sure to rake up leaves that fall on the lawn through the fall as this can also lead to snow mold. You can mow over the leaves with a mulching blade rather than rake them if you prefer.

 

Brown Lawns caused by a leaf blight

Ascochyta, 6-2018

Symptoms from Ascochyta Leaf Blight

This year has been difficult for our lawns. Since our cold April, our temperatures skyrocketed and we haven’t had many rain events throughout this spring and early summer. This has been causing our lawns to look a little ragged and brown.

Ascochyta

Many cool season lawns throughout Eastern Nebraska have begun to look brown due to Ascochyta leaf blight, a widespread disease found throughout the early part of summer this year. Mowing during the hot Memorial Day weekend seemed to have worsened the symptoms of this disease.

Symptoms

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A close-up of the blades of turf infected by Ascochyta

Ascochyta is a diseased that is stress-induced and often shows up in the early summer when the weather shifts from cold and wet to hot and very dry. Ascochyta is a dieback from the tip of the leaf blades of cool season turf. Red-brown spots can also appear lower down on the affected blades. You might also notice a dark brown/black band between green growth and the brown tip of the blade. After the initial disease moves through, a general brown appearance will show up in the lawn, often following lawn mower tire patterns. Mowing worsened the symptoms of this disease, not by spreading it, but by the physical traffic of the equipment  to weaken the turf. This allowed for ease of the fungus to attack the lawn.

Ascochyta affects just the turf blades of the plant, not the roots or crown of the plants. The crown is the growing point of the turf. Because it doesn’t affect the roots and crown of the plant, it is then able to grow out of the disease. Mowing the lawn will remove the infested areas of the plant which will lead to regrowth and regreening of the lawn over time.

Management

There are a few types of fungicides labeled for use on ascochyta, however, research at UNL shows poor results with many different classes of fungicides on this disease. The best management for Ascochyta would be to reduce stress and manage the lawn properly. Provide adequate moisture for the lawn. Remember it is best to provide 1-1.5 inches of water per week to the lawn. If that isn’t provided through rainfall, irrigate two to three times per week with 1/3-1/2 an inch each time to keep the lawn healthy. Also, ensure that your lawn mower blades are sharp to avoid tattering the leaves which can leave more of an opening for diseases to move into grass plants. Finally, early June is a great time for a slow-release fertilizer to help slowly feed your lawn through the summer months and keep it healthy.

For More Information

The information for this news article on Ascochyta came from the UNL Turfgrass Team of Bill Kreuser and Roch Gaussoin. This information came through a Turf iNfo update. You can sign up to be on their listserv to receive these updates automatically as they come out by going to the Nebraska Turfgrass Website and scrolling down to click on the ‘SIGN UP FOR TURF INFO’ tab at the bottom of their page.