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Fall Brings Opportunities for Perennial Weed Control

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Cooler fall weather is a prime opportunity to control warm-season grassy weeds like nimblewill (Muhlenbergia schreberi). Fall is a particularly good time to control nimblewill because the increased competitiveness of cool-season turfgrass enhances the effectiveness of herbicide programs.

Blueish-green mat appearance of nimblewill in cool-season turfgrass.

Nimblewill is a warm-season perennial grass that thrives in shady, moist environments but is commonly found in sunny areas as well. It spreads through stolons (above-ground runners) and can form dense mats, making it difficult to remove once established. Nimblewill is most obvious during the late fall and winter, as it turns brown and goes dormant at first frost, making it more noticeable in cool-season turf.

Dormant nimblewill in a thin stand of cool-season turfgrass.

Nimblewill resembles both bentgrass and bermudagrass. Bermudagrass is also a perennial warm-season grass that goes dormant after the first frost, but is often found in sunny, drier areas. Bermudagrass is coarser and challenging to pull out of the ground than nimblewill, which has a shallower root system. While all three grasses spread through stolons, bermudagrass also has rhizomes, adding to its resilience. Bentgrass has thin stolons that creep along the ground. Nimblewill stolons appear wiry as they are often stems that fall to the ground and root at the nodes. Nimblewill often has a hairy collar, while the bentgrass collar lacks hairs, and bermudagrass hairs are more conspicuous. Nimblewill has a shorter membranous ligule than bentgrass, and the bermudagrass ligule is hairy.

Nimblewill collar with hairs

Selective herbicide options for nimblewill control include mesotrione (e.g., Tenacity),  topramezone (Pylex) and SpeedZone (2,4-D + MCPP + dicamba + carfentrazone). Although SpeedZone is often considered a broadleaf weed herbicide, we found it provides nimblewill control when applied in September and October. Mesotrione and topramezone are more compatible with seeding of cool-season turfgrasses, but are less effective for broadleaf weed control than SpeedZone. Another option is a single application of the non-selective herbicide glyphosate if you plan to reseed. Regardless of the herbicide selected, a multi-application program over two years will provide more control than single applications, targeting regrowth from stolons and any nimblewill seedlings remaining in the soil seedbank after the perennial nodes are killed. In addition to herbicide programs, enhancing the competitiveness of desirable turfgrass should be part of an integrated approach. While this approach has not been evaluated for nimblewill control, large increases in long-term control of other warm-season perennial weeds including dallisgrass, bermudagrass, and false-green kyllinga have been found when fall seeding and herbicide programs are combined. For nimblewill, this integrated approach might include interseeding, or simply applying more nitrogen fertilizer (up to two applications of 0.9 lbs N/1000 ft2 in the fall) around herbicide applications if existing cool-season grass already mixed into patches of nimblewill. Check herbicide labels for compatibly with seeding.

Good Time to Assess Drainage

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The soil that supports our lawns, sports fields, golf courses, and other land uses begins to recharge (store) with water as the evapotranspiration rate drops off during autumn and continues to decline to a very low rate during winter. Water storage in soil is good since that provides the water needed to support green-up and re-initiation of plant growth in the spring. However, an excessive accumulation of water is an indicator that one or more conditions in the landscape are restricting drainage.

Poor drainage in a softball field
A depression in the grade of an outfield causing ponding of water on a softball field.

Observing the drainage capacity of landscapes during late winter and early spring 2024 should be highly effective at determining whether corrective action is needed to improved drainage. We have experienced above normal precipitation In New Jersey during three of the last six months. September 2023 precipitation averaged 7.55” across the state, which was 3.39” above normal and ranks as the 9th wettest September. December 2023 precipitation was the wettest 12th month on record dating back to 1895. Statewide, precipitation averaged 8.20”, which was 3.93” above the 1991–2020 normal and was 0.33” above the previous wettest December in 1996. January 2024 was 6th on record dating back to 1895. The statewide 6.39” of rain and melted snow was 2.90” above the 1991–2020 normal. See https://www.njweather.org/news for more weather details.

Thus, autumn and winter precipitation has certainly recharged our soils to be at or very near field capacity, Field capacity refers to the water remaining in a soil after it has been thoroughly saturated and allowed to drain freely, usually for one to two days.

With our soils essentially full of water, any additional precipitation during the remainder of the winter and into early spring will provide us a good assessment of drainage. Soil that is freely drained will be wetted after precipitation but will return to a well drained condition (field capacity) within one to two days. Slow to drain areas will become soggy and possibly pond water for more than two days. The longer excess water lingers beyond 2 days within or above the soil, the poorer the drainage.

It is a good practice to scout your landscapes during this time of year and watch for areas that remain soggy or ponded well beyond 2 days after rain. Once you find poorly drained areas the key to improving drainage is to identify where the excess water is coming from and to intercept it before impacting the turf and other landscape plants. The two essential concepts behind good drainage designs are to protect soils from becoming ponded and/or the root zone from becoming waterlogged. Drainage solutions range from re-contouring the landscape (shed excess water through surface drainage) to installing a network of below-ground pipe drainage (intercept excess subsurface water).

Time to Control Yellow Nutsedge

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Now is the ideal time to control yellow nutsedge (Cyperus esculentus). This plant is a perennial that grows best during the heat of summer and survives the winter through underground tubers. While the drought may have limited nutsedge emergence in unirrigated areas, it is visible and rapidly growing in irrigated beds and lawns.

Yellow nutsedge and other sedges look similar to grasses, but sedges have a unique triangular stem. This triangular stem results in a three-ranked leaf arrangement. While many sedge species can be found in the lawn and garden, yellow nutsedge is the only perennial sedge found in New Jersey and is far more difficult to control than annual sedges.

A cross section of a yellow nutsedge stem. This triangular shape is unique to sedges.
A yellow nutsedge plant from above. Notice the leaves at 120-degree angles from one another forming a three-ranked leaf arrangement.

Yellow nutsedge is a perennial through tubers. The aboveground plants and rhizomes are killed each winter and the tubers are the source of the infestation each spring. Initiating nutsedge control measures now will prevent the formation of new tubers and make this weed less problematic in the future.

Yellow nutsedge tubers.
Yellow nutsedge tubers. The brown tubers are mature while the white tuber is immature.

Lying dormant during winter up to several inches below the surface, nutsedge tubers sprout in spring, giving rise to new plants aboveground. These plants then produce rhizomes (underground stems) that form more plants and tubers for next year (and beyond). If left unchecked, a single tuber sprouting in spring can produce thousands of new tubers by the end of the summer.

A severe yellow nutsedge infestation.
A severe yellow nutsedge infestation in a recently mulched landscape bed. The open space between ornamental plants allows ample sunlight and nutsedge thrives.
Yellow nutsedge in a lawn in early June.
Yellow nutsedge in a lawn in early June is relatively inconspicuous. The thin and light green leaves are the nutsedge. The nutsedge will become more obvious as temperatures increase through the summer.

Implementing strategies to control nutsedge now before it forms tubers is critical to get rid of this weed. Right now in early June, yellow nutsedge shoots are producing rhizomes but have probably not yet begun producing tubers. Treating nutsedge with herbicide now and again in 6 to 8 weeks (if green shoots are visible) is an ideal strategy.

Yellow nutsedge rhizomes.
Yellow nutsedge rhizomes

The herbicide halosulfuron can be used for selective sedge control in both lawns and ornamental beds. It is safe to the turfgrass and can be applied around ornamental plants. Numerous formulations are available for professionals. A user-friendly product for non-professionals is Sedgehammer®+, which is widely available and sold in small pouches with the exact amount to mix into a gallon of water for a pump sprayer. This product also contains the surfactant halosulfuron needs for best nutsedge control. Halosulfuron is systemic (moves in the plant), but slow to work, so it will be 2 to 4 weeks before there are noticeable results.

Nutsedge tubers can persist for several years in the soil, so one year of herbicide treatment will not eliminate yellow nutsedge. But treating now and once more later in the season will reduce the severity of the problem in future years. This approach is better than waiting until mid or late summer for the first application.

Combine herbicide programs with non-chemical management to improve control. More than other weeds, nutsedge is poorly competitive in shade and in dry soils. You may notice nutsedge in thinner areas of the lawn. Focusing on turf management practices that increase lawn canopy density during the autumn (e.g. N fertilization and seeding) will result in fewer herbicide applications the future. Do not apply halosulfuron sooner than 4 weeks before seeding turfgrass. Modifying landscape beds to reduce large spaces between plants can be similarly combined with herbicide programs for better long-term control. Fixing poorly drained areas and being careful not to over irrigate may also help manage nutsedge.

It is possible to eliminate yellow nutsedge by hand weeding or by using OMRI-approved herbicides (which are non-systemic). But keep in mind, pulling shoots or killing with non-systemic herbicides usually signals the tubers to produce many new shoots. So, hand-weeding or spraying more frequently than with systemic herbicides like halosulfuron is necessary. Digging the tubers is not practical as they are several inches underground and many are too small to be easily seen.

Weeds of Early Spring

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With warmer weather comes plant growth. The most conspicuous plants in the lawn and landscape right now are winter annuals such as hairy bittercress (Cardamine hirsuta), henbit (Lamium amplexicaule), purple deadnettle (Lamium purpureum), and chickweeds (Stellaria media and Cerastium fontanum ssp. vulgare), to name a few. In lawns, these winter annuals are typically found in areas where the lawn was thin in September and October when these weeds germinated. Most winter annuals do not establish readily where the lawn canopy is dense. In ornamental beds they can be prolific unless the beds were recently mulched (which buries the seed), as there is plenty of open space and little competition from other plants.

Henbit flowering.
Henbit flowering
Purple deadnettle flowering in early spring.
Purple deadnettle flowering in early spring
Common chickweed in an ornamental bed.
Common chickweed in an ornamental bed

In the lawn, these winter annual broadleaf species can be easily controlled by herbicides that contain 2,4-D, MCPA, mecoprop, or dicamba. But keep in mind that these winter annual weeds are near the end of their life cycle and will be dead within 3 to 4 weeks even if no herbicide is applied. Exceptions include mouseear chickweed (Cerastium fontanum ssp. vulgare) and corn speedwell (Veronica arvensis) which can persist farther into the summer.

Corn speedwell in an ornamental bed.
Corn speedwell in an ornamental bed

Perennials such as wild garlic (Allium vineale), star-of-bethlehem (Ornithogalum umbellatum), and lesser celandine (Ranunculus ficaria) are also noticeable this time of year. They are ephemerals that will return to dormancy within the next few weeks where they reside as bulbs for the summer, emerging again in late fall or mid-winter.

Star-of-bethlehem.
Star-of-bethlehem. Notice the white midrib on the leaf.
Star-of-bethlehem in turf.
Star-of-bethlehem in turf
Lesser celendine in a lawn.
Lesser celendine in a lawn

Another ephemeral winter perennial is bulbous bluegrass (Poa bulbosa). This also emerges from bulbs in late fall and is dormant all summer. It is most conspicuous when in mid spring it produces what looks like an inflorescence, but is actually a collection of bulbils (small bulbs) and not flowers. Bulbous bluegrass is occasionally widespread in extremely low input lawns, but is more commonly found in small patches where disturbance and competition from other plants is low. I commonly see this at the base of tree trunks or between the sidewalk and road.

Bulbous bluegrass bulbils. Photo courtesy of Aaron Patton.
Bulbous bluegrass bulbs and leaves.
Bulbous bluegrass bulbs and leaves

Roughstalk bluegrass (Poa trivialis) is a perennial grass that is obvious this time of year due to rapid growth and light green color. There are no selective herbicide options for roughstalk bluegrass control. Except in wet and shaded areas where it is well-suited, roughstalk bluegrass growth tends to slow by late spring and it goes dormant in summer. If it is mixed well with other turfgrass species, this slow decline to dormancy is not noticeable. For more information about roughstalk bluegrass, see this recent blog.

Patches of roughstalk bluegrass in a lawn.

While winter annuals are nearing the end of their life cycle, summer annual weeds are just getting started. Japanese stiltgrass emergence (discussed in a recent blog) has been noted in several locations in central and southern NJ. It may be too late for preemergent herbicides to control stiltgrass. For more information on stiltgrass control, see this RCE factsheet.

Forsythia in full bloom.
Forsythia in full bloom

Soil temperatures and Forsythia full bloom we are now seeing suggests crabgrass (Digitaria spp.) emergence will begin within two to three weeks. Now is a good time to begin making preemergence herbicide applications. Based on what we know about crabgrass and other summer annual species, the mild winter is not likely to accelerate germination, since the seed is not active until soil temperatures are in the 50s. Other than using Forsythia spp. full bloom as an indicator, soil temperatures can be monitored, but the science doesn’t provide precise guidance on this. But as a general rule, once soil temperature consistently exceeds 55 °F at a 2” depth for at least few days, crabgrass seed germinates and emergence is often noted a few days later. Live soil temperatures can be found on the Rutgers NJ Weather Network. The 24-hr rolling average at locations we monitor in North Brunswick, NJ is between 48 and 54 degrees F, depending on sun exposure. See these links for more information on crabgrass control in the lawn for professionals, and homeowners.

Japanese Stiltgrass Control in Turf

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Japanese stiltgrass (Microstegium vimineum) is a C4 summer annual grassy weed commonly invasive to forests. It can be problematic in lawns. Often it is problematic in shaded areas adjacent to an infested forest. But we have observed infestations in full sun turfgrass areas where you would normally expect crabgrass to be the predominant summer annual. It seems this weed is becoming increasingly problematic in lawns. But why I am I writing about a summer weed in February?

Japanese stiltgrass in a lawn during late summer.
Japanese stiltgrass in a lawn during late summer.

Japanese stiltgrass is the first summer annual grassy weed to emerge. Preemergence herbicides commonly used to control the summer annual crabgrass (dithiopyr, prodiamine, and pendimethalin) were effective for Japanese stiltgrass control in our research. However, stiltgrass emerges sooner than crabgrass and these preemergence herbicides are only effective when applied before stiltgrass emergence. In our 2022 research we applied these preemergence herbicides on March 11, 2022 in Pittstown, NJ. Stiltgrass typically emerges in March, often about a month before crabgrass. Germination of the seed and subsequent emergence is probably highly variable from year to year and depends on soil surface temperatures. Do not expect seedlings to be killed by frost. While Forsythia full bloom can be used as an indicator to apply crabgrass preemergence herbicides, there is no information available regarding soil temperatures or phenological indicators for stiltgrass emergence.

Japanese stiltgrass seedlings.
Japanese stiltgrass seedlings emerged in tall fescue turf in March

Our research found that while preemergence herbicides applied before stiltgrass emergence (in early March) were effective, those same treatments applied around crabgrass emergence in early May were not effective. A mixture of the postemergence herbicide Acclaim Extra (9 fl oz/A) and a preemergence herbicide was effective for stiltgrass control when applied in early May at crabgrass emergence. For stiltgrass control in June, July, and August, higher rates of Acclaim herbicide will be necessary for control. Acclaim Extra is the only product registered for postemergence stiltgrass control in turfgrass. Field research found it to be the most effective control option. See this RCE factsheet for more information about Japanese stiltgrass identification and other control options in non-turf areas.

https://njaes.rutgers.edu/fs1237/

In summary, to control Japanese stiltgrass, preemergence herbicides need to be applied a month earlier than is typical. If this is not feasible, tank-mixing preemergence herbicide with low rates of Acclaim Extra is an effective option.

Tropical Storm Brings Rain But What About Gray Leaf Spot?

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The passing of tropical storm, Fay brought some needed precipitation to the region, albeit too much at once in many places. Relatively dry soils (assumes not being over-irrigated) and the long, warm days of summer should allow turfs to dry out reasonably fast.

But what about gray leaf spot? Spores of the pathogen are thought to be blown up from southern regions to the north on the winds of tropical systems. With the recent passing of Fay, Dr. Bruce Clarke recommends being on the watch for early symptoms and signs of gray leaf spot outbreaks. Gray leaf spot can be severe on older, non-tolerant cultivars of perennial ryegrass while new, improved cultivars will be much less sensitive although not immune. Over the last couple years, we have also experienced some strong outbreaks of gray leaf spot on tall fescue across the region. So don’t ignore the tall fescue turfs if you have some.

For more information, see the fact sheet, Integrated Control of Gray Leaf Spot on Perennial Ryegrass.

Gray leaf spot control with fungicides on perennial ryegrass.
Fungicide plots on a perennial ryegrass cultivar that is highly susceptible to gray leaf spot.

Green-up Started

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Soil temperatures have been warming and reached the mid-40s °F last week at Rutgers Hort Farm No. 2 in North Brunswick NJ and are creeping into the upper-40s °F early this week. Dr. Clarke recommends that treatment of turf with a history of take-all patch be initiated when soil temperature averages 40 to 60 °F. Treatment of fairy ring should be initiated when average temperature is in the range of 50 to 55 °F. Waiting to treat beyond these soil temperature thresholds will increase the risk that control strategies are not as effective.

Table 1. Soil Temperature Data under Bentgrass Turf Mowed at 0.375-inch in North Brunswick, NJ on 17 March 2020 at 2:00 PM.

 3-inch DepthThatch-Soil Interface
Current48 °F50 °F
24-hour Average45 °F44 °F
5-day Average47 °F47 °F

Additionally, the Forsythia bloom started last week in central New Jersey, which Dr. Matthew Elmore recommends as a traditional phenological indicator for preemergence herbicide programs on turf.

Initial forsythia bloom in a home lawn.
Initial forsythia bloom in a home lawn in central NJ last week.

Glyphosate Alternatives

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Recent news about glyphosate has many thinking about alternatives.

Joe Neal (Professor of Weed Science, Extension Specialist & Department Extension Leader Horticultural Science) and Andrew Senesac (Extension Weed Scientist Cornell Cooperative Extension, Suffolk Co., NY) have published a thorough summary of alternatives and the associated pros and cons through NC State Extension at https://content.ces.ncsu.edu/are-there-alternatives-to-glyphosate-for-weed-control-in-landscapes

Green-up Transitioning to Growth: Ready to Mow?

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Three weeks ago, many turfgrasses began greening up as surface soil temperatures warmed into the 50s °F during the day. Currently, surface soil temperatures are reaching into the 60s °F during the day, which stimulates vertical leaf growth especially after a rain. Any early spring N fertilization will also encourage vertical leaf growth.

Accordingly, mowing equipment should be reconditioned and ready for routine use. Setup of mowing height should not be overlooked; an incorrect mowing height will lead to problems. Generally, lawn grasses will be easier to maintained at a mowing height of 3-inches or higher.

Grasses will be healthier when there are more leaves (leaf area) to capture sunlight for photosynthesis and shade the soil surface. More photosynthesis helps the grass grow more roots and a shaded soil surface remains cooler. Shade also blocks the exposure of weed seed to light, which is an environmental trigger for weed germination.

Uneven ground is another reason to recommend relatively high mowing. Many lawns are relatively uneven. A lack of smoothness in a lawn contributes to poor mowing.  A mower set 3-inches helps protect high spots in a lawn from being severely scalped. For example, a high spot in a lawn that ends up being cut at 2-inches with a mower set at 3-inches would be cut at 1-inch if the mower were set at 2-inches.

Mower scalp caused by mower deck being set too low on a lawn with uneven ground.

 

Roughstalk bluegrass

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Roughstalk bluegrass (Poa trivialis) is a cool-season perennial grass often considered a weed. Light green in color it is most obvious in the early spring when it grows more rapidly than other turfgrasses. As temperatures rise in the summer its growth will slow and it often goes semi-dormant until cooler temperatures and rainfall return in the autumn.

A roughstalk bluegrass plant in a lawn.
A roughstalk bluegrass plant in a lawn in spring.
A small roughstalk bluegrass plant with purple stems.
A small roughstalk bluegrass plant with purple stems.

Infestations usually begin as small plants, but over time these plants spread via stolons and form patches that don’t tend to mix well with other cool-season grasses. These patches form as small plants expand from stoloniferous growth. These stolons (in addition to the lighter green color) can be used to differentiate roughstalk bluegrass from other bluegrasses such as annual bluegrass (no stolons or rhizomes) and Kentucky bluegrass (rhizomes only). Unlike Kentucky bluegrass, roughstalk bluegrass also has a long membranous ligule with a pointed tip.

Patch of roughstalk bluegrass in a lawn.
Patch of roughstalk bluegrass in a lawn.
Patches of lighter green roughstalk bluegrass amongst Kentucky bluegrass.
Patches of lighter green roughstalk bluegrass amongst Kentucky bluegrass

Currently there are no selective herbicides you can use in a home lawn to control roughstalk bluegrass. For many years Velocity herbicide was an option for professional turf managers, but it is no longer being manufactured and is not registered for use in home lawns (golf courses and sod farms only). Xonerate herbicide can provide some suppression, but should be used carefully to prevent injury to desirable turfgrass.

To control roughstalk bluegrass in cool-season lawns and athletic fields, nonselective control with glyphosate (Roundup and others) followed by reseeding is the best option. Apply glyphosate at this time in early spring, before summer stress, for best control. Glyphosate will kill any plant the spray contacts so apply carefully. If the areas are small, consider removing them with a shovel or a sod cutter. Be sure to remove the patch and at least 12 inches of turf surrounding the patch. Removing soil to a 0.5-inch depth should be sufficient to remove the stolons and all growing points. Reseed or sod the area with a desirable turfgrass species after removal.

These recommendations are based off of the 2019 Turfgrass Weed Control for Professionals guide. I highly recommend this guide for professionals managing cool-season turfgrass.