Dr. Ron Strahan is an Assistant Professor and State Turfgrass Weed Science Specialist at Louisiana State University.

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The Best Defense
by Ron Strahan       #Pests   #Turf Grass   #Vines

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pink oxalis should be spot treated with glyphosate.

Gardeners take pride in the appearance of their landscapes. However, nothing detracts from the beauty of flowerbeds like weeds. Along with being aesthetically displeasing, weeds in flowerbeds compete with desirable plants for water, nutrients and light. If weeds are out of control, expect fewer flowers and more headaches. For most people, backbreaking hand removal is relied upon exclusively to remove weed problems. Hand pulling may be successful for a few weeds, but for most weed problems it is only partially effective. Weeds have very effective defense mechanisms that reduce the effectiveness of hand pulling. Annual weeds often break at the stem when pulled, leaving the root or single stem available for potential reestablishment. Perennial weeds like purple nut sedge (Cyperus rotundus) and common Bermudagrass (Cynodon dactylon) have underground structures that are left in the soil after hand removal. You have probably noticed that these weeds re-infest the beds very quickly. In reality, hand pulling weeds is one of several practices that should be used to optimize weed control in flowerbeds. These additional practices include the use of mulch, preemergence herbicides and, to a limited extent, postemergence herbicides.


Mulch
I am a big fan of using mulch in flowerbeds. Mulch essentially serves two weed-control purposes: It is a physical barrier to the emerging seedling, and it prevents sunlight from reaching the soil surface. Blocking sunlight is important because some weed seeds, such as crabgrass (Digitaria spp.), need light for germination. Also, sunlight is necessary for the new weed seedling to begin photosynthesis for growth and development.

There are several materials available that are suitable for mulch such as compost, leaf litter, pine bark, pine mulch and pine straw. Even newspapers can be used as a barrier to weed emergence. Mulches must be thick enough to block light to be effective. As a rule, mulch trees to a depth of 3 to 4 inches and shrubs to a depth of 2 to 3 inches.

There’s no doubt that mulch is very beneficial, but mulch alone usually will not hold back most weed infestations. It is important to use mulch in conjunction with hand pulling, preemergence herbicides (prevents weed) and postemergence herbicides (kills emerged weeds).
 

Mulch such as pine straw is an important weed-control strategy.

Preemergence Herbicides
Wouldn’t it be great to just be able to spray an herbicide in the flowerbed that cured all of your weed problems and caused no harm to your landscape plants? Unfortunately, postemergence herbicide options used to remove existing weed problems are very limited, since plants in flowerbeds can be very injury prone. Less injurious preemergence herbicides are the backbone of weed control in flowerbeds. You are really missing out if you don’t use them regularly in your flowerbeds. Some preemergence herbicide choices available to homeowners include dithiopyr (Hi-Yield Turf & Ornamental Weed Stopper with Dimension and some formulations of Preen), trifluralin (Treflan, Preen and Miracle-Gro Garden Weed Preventer), oryzalin (Surflan) and benefin plus oryzalin (Amaze Grass & Weed Preventer).

Preemergence herbicides work by forming a barrier in the upper 1⁄2 to 1 inch portion of the mulch or soil where most seeds are germinating. These types of herbicides kill weeds as they attempt to emerge from the soil. Since these herbicides have no effect on existing weeds, applications must occur before the weeds germinate. All existing weeds should be removed by hand or carefully spot treated with a non-selective herbicide, such as glyphosate (Roundup or generics), prior to treatment.

Most consumer preemergence herbicides should be applied directly on top of the mulch and existing landscape plants then watered in soon after application to move the herbicides into the zone where weed seeds are germinating. It is always a good idea to rinse off landscape plants to remove herbicide granules. If you are putting down new mulch in the flowerbed, apply the preemergence herbicide on the old mulch before adding the new mulch layer.

In most cases, preemergence herbicides should be applied every two and a half to three months. Consult product labels concerning desirable plant tolerance and application methods. Preemergence herbicides can be effective on several annual weeds including crabgrass, goosegrass (Eleusine indica) common purslane (Portulaca oleracea) and mulberry weed (Fatoua villosa). Most troublesome perennial weeds, such as purple nut sedge and Bermudagrass, are not controlled with preemergence herbicides.


Postemergence Herbicide Options
It is important to control weeds with mulch and preemergence herbicides, because once they have emerged your options become more limited, since there are very few selective postemergence herbicides available, especially for broadleaf weeds. There is good news when it comes to selectively controlling most summer grasses such as crabgrass and Bermudagrass. Summer grasses are controlled with herbicides containing the active ingredients fluazifop (Ortho Grass-B-Gon) or sethoxydim (Vantage, Fertilome Over the Top II, etc.). These types of herbicides only kill grasses and are usually safe over the top of most non-grass landscape plants including shrubs, perennial ground covers and bedding plants. They are even safe for over-the-top applications in grass-like plants such as daylily (Hemerocallis spp.), iris (Iris spp.), monkey grass (Liriope spp.) and mondo grass (Ophiopogon japonicus).

Sedges, like purple nut sedge, can be controlled by directed sprays of halosulfuron (SedgeHammer, Monterey Nutgrass Killer) or imazaquin (Image Nutsedge Killer). Consult the product labels thoroughly for sedge-killing herbicides before you use them. Additionally, glyphosate (Roundup, etc.) can be carefully spot treated or applied as a wipe (paintbrushes and sponge mops can be used as applicators) for hard to control weeds. Spraying glyphosate is not always safe in landscape plantings due to the potential for drift. I have had some success using a paintbrush or a sponge mop to wipe glyphosate on weeds in my flowerbeds. Glyphosate can be very effective on perennial plants in landscapes, because it is systemic and moves effectively into roots and underground storage organs.


Common Weeds That Infest Flowerbeds

1. Spurge (Chamaesyce spp.)
There are several types of spurges that are common in landscape beds. Spurges are members of the Euphorbiaceae family and are prolific, seed-producing annuals that thrive in hot weather. Under optimum growing conditions, plants can go from a germinating seed to producing their own flowers in only three weeks. Some spurges have a more prostrate growth habit that can form dense mats, whereas many spurge species grow more upright. Spurges emit milky latex from broken stems that can be helpful in distinguishing this plant from other species. The plants are difficult to manage in flowerbeds due to heavy seed production and the inability to be successfully removed by hand. Plants often break at the stem during this process, leaving the root and several buds or a single stem available for potential reestablishment.

Control: Spot-treat existing plants with glyphosate before applying preemergence herbicides. Most preemergence herbicides work well on spurge. However, the problem usually is in the frequency of the application because spurge control starts breaking four to six weeks after the herbicides are applied. Consumer preemergence herbicide options include Preen, Surflan and Amaze.


2. Wood sorrel (Oxalis spp.)
Wood sorrel are members of the Oxalidaceae family and are perennial weeds that produce underground storage organs that make hand removal difficult. The plants are heavy seed producers and possess a very efficient method of seed distribution. Wood sorrel has three heart-shaped leaf components that vary in color from dark green to reddish purple. The plants are often called clovers, but they actually are in a different plant family. There are several species of wood sorrel that are common landscapes. Yellow wood sorrel (O. stricta) grows more upright and produces below-ground storage organs. Yellow wood sorrel produces thousands of seed and has a very effective method of seed dispersal. At maturity, okra-shaped seedpods burst open and expel seed 10 to 12 feet in all directions. Pink wood sorrel (O. crassipes) has very large leaves, pink flowers and commonly infests border “grasses” like mondo and liriope.

Control: Most preemergence herbicides that work on spurge work well on oxalis. Hand removal is difficult because underground storage organs are left in the soil when the top areas are removed. When possible, spot spray or wipe existing plants with glyphosate before applying preemergence herbicides such as Preen, Surflan and Amaze.


3. Mulberry weed (Fatoua villosa)
Native to Asia, mulberry weed is a summer annual that is a member of the Moraceae (mulberry) family. The plant has an upright growth a habit and can grow to a height of 3 to 4 feet. Leaves are triangular, serrated and prominently veined. Plants resemble seedling mulberry. However, mulberry weed has pubescent leaves and stems and is herbaceous. Mulberry weed has unique feathery flowers that first appear purple and then brown as they mature. Plants are prolific seed producers and can forcefully expel seed up to 4 feet. The weed develops quickly — it can go from seed to flower in fewer than two weeks and produce several generations in one growing season.

Control: Hand remove existing plants. Consumer preemergence herbicide options include Preen, Surflan and Amaze.


4. Chamberbitter (Phyllanthus urinaria)
Chamberbitter is an extremely invasive summer annual that is a member of the Euphorbiaceae family. Chamberbitter resembles hemp sesbania (Sesbania exaltata) or mimosa (Acacia baileyana) seedlings. However, the most distinguishing characteristic are the round seed capsules located on the underside of slender branches. Chamberbitter needs temperatures consistently above 75 F, so these plants tend to germinate a little later in the spring than many other flowerbed weeds. Populations of chamberbitter have increased significantly since their introduction from Asia because of their prolific seed production.

Control: Light may be necessary to stimulate chamberbitter germination, so thick mulch is helpful in reducing plant populations. Chamberbitter hand pulls very easily, but frequent germination and high populations will keep you busy. Preemergence herbicides available to homeowners have performed poorly on this weed, so diligent hand removal and mulch will be very important.


5. Florida betony or rattlesnake weed (Stachys floridana)
A square-stemmed perennial weed native to Florida, Florida betony or rattlesnake weed is a serious problem in landscapes during the fall and spring. What makes this weed such a problem is its ability to overtake flowerbeds in a short time and the lack of good control options. There may be more common weed problems, such as nut sedge, but betony is more difficult to remove once it gets established.

Although the plant does produce seed, the weed mainly reproduces by rhizomes and tubers. The tubers resemble the rattle on a rattlesnake’s tail, hence the nickname “rattlesnake weed.” Hand pulling only removes the shoots but leaves the rhizome and tubers. Betony is easily spread from flowerbed to flowerbed when landscape plants are shared or purchased from commercial growers that produce plants in areas where the weed infests. We see this weed most often in the fall and spring. It goes nearly dormant during hot weather and is not noticed as much in the landscape during the summer. I am flooded with calls from landscape maintenance companies and homeowners concerning the control of Florida betony this time of the year. There are no preemergence herbicide options, and weed barrier fabrics have not been effective.

Control: Glyphosate provides control of the weed, so spray or wipe with highly concentrated solutions in sensitive areas. Repeated applications are always needed.


6. Nut sedges (Cyperus spp.)
Purple nut sedge ranks as the number-one weed problem in the world and is the most common weed infesting flowerbeds. Yellow nut sedge (Cyperus esculentus) prefers moist environments and is more common in irrigated beds or during wet growing seasons. Both are grass-like plants with an extensive system of tubers that allow the plants to reproduce rapidly in landscape beds. Homeowners often call nut sedge “coco” or “nut grass,” however, these weeds are sedges and not grasses. In fact, sedges are in a totally different plant family from grasses. Herbicides that kill true grasses, such as Grass-B-Gon, will have no effect on sedges.

Control: Nut sedges are very difficult to manage consistently in landscape beds. Neither purple nor yellow nutsedge can be controlled by hand removal, and mulches are only slightly effective. Preemergence herbicides that are available to homeowners provide no control of nut sedge. However, postemergence herbicides with the active ingredient halosulfuron, such as SedgeHammer, can be an effective option when used as directed in flowerbeds. The herbicide works very slowly and may take as much as a month to kill the sedges. It will not prevent nutsedge re-infestation in the flowerbed. The herbicide will have to be applied periodically as nut sedge plants emerge. Consult product labels for lists of tolerant plants and application techniques.


7. Common Bermudagrass (Cynodon dactylon)
Common Bermudagrass is the most widespread grass problem infesting flowerbeds. This perennial warm-season grass originated in Africa and grows well in Southern climates. The grass is widely used for lawns, athletic fields and golf courses, but it is very invasive in flowerbeds. Common Bermudagrass is characterized by its dark green color, fine texture and the production of rhizomes (below ground stems) and stolons (above ground stems) that allow the plant to establish quickly in the landscape.

Control: Hand removal is not an effective method for controlling common Bermudagrass infestations in landscape beds. Since the weed mainly reproduces by plant parts (not seed) and creeps into flowerbeds, preemergence herbicides have no effect on the weed. Frequent applications of grass-killing herbicides, such as Ortho Grass-B-Gon and Fertilome Over the Top II, can be effective in managing Bermudagrass in landscape beds.


8. Torpedograss (Panicum repens)
Torpedograss is a perennial rhizomatous grass that is considered one of the most invasive grasses in the world. Although the plant does produce seed, the seeds are not viable. The weedy grass solely reproduces vegetatively by robust rhizomes.

The spread of torpedograss can mainly be attributed to the movement of soils infested with torpedograss from one location to another usually during flowerbed construction. The weed is a very common problem in landscape beds all along the Gulf Coast.

Control: Complete control of torpedograss may not be possible. Grass-killing herbicides normally prescribed for flowerbeds, such as sethoxydim and fluazifop, are just not very effective on torpedograss, although fluazifop is a little better than sethoxydim. Glyphosate is the best herbicide on the weed, but high rates and multiple applications are necessary for control.


9. Bushkiller vine (Cayratia japonica)
Bushkiller vine is a perennial herbaceous vine with compound leaves containing five leaflets. It produces salmon flowers, eventually bearing fruit with two to four seeds. Thankfully, the seed are not thought to be viable. The plant solely reproduces vegetatively. Native to Asia, bushkiller vine gets its name because the vine climbs over desirable plants and kills other plants by blocking out sunlight. Few weeds take over areas as fast as bushkiller vine, which rapidly engulfs landscape shrubs and ground covers. I am seeing infestations of this weed all along the Gulf Coast.

Control: Bushkiller vine can be suppressed with repeated applications of two herbicides, glyphosate and triclopyr (Ortho MAX Poison Ivy & Tough Brush Killer, Hi-Yield Brush Killer), applied as directed sprays. Unfortunately, the vine intertwines in the landscape and makes herbicide applications very difficult. Often, it is necessary to treat freshly cut plants or wipe the weed directly when spraying the herbicide is too risky in the landscape. Don’t expect to get rid of it with one application. Start your bush killer management program in the spring as the vine emerges. Be sure to treat properties nearby, because the weeds will rapidly re-infest treated areas again.


Summary
The best defense against weed infestations in flowerbeds is a combination of mulch, periodic hand pulling and an aggressive preemergence herbicide program. When applicable, use postemergence herbicides for emerged sedges and grasses. On really tough existing weed problems, spot apply a non-selective herbicide like glyphosate. In areas where spraying glyphosate is not possible because of drift, wipe the solution on the weeds with a paintbrush or even use a sponge mop as an applicator. The chart on page 29 provides a list of some herbicides available for landscape bed weed control. Consult product labels for tolerant plants, application rates and procedures.

 

 Preemergence Herbicides

 Active ingredients

Weeds Controlled

 Amaze

 Benefin + oryzalin

Annual grasses and small-seeded broadleaves like spurge, purslane, mulberry weed, etc.

 Hi-Yield Turf & Ornamental Weed
 Stopper with Dimension

 Dithiopyr

Annual grasses and small-seeded broadleaves like spurge, purslane, mulberry weed, etc.

 Preen

 Dithiopyr or trifluralin

Annual grasses and small-seeded broadleaveslike spurge, purslane, mulberry weed, etc.

 Miracle-Gro Garden Weed
 Preventer

 Trifluralin

Annual grasses and small-seeded broadleaves like spurge, purslane, etc.

 Surflan

 Oryzalin

Annual grasses and small-seeded broadleaves like spurge, purslane, mulberry weed, etc.

Postemergence Herbicides

Active Ingredients

Weeds Controlled

 SedgeHammer, Monterey
 Nutgrass Killer

 Halosulfuron

 Nut sedge

 Image Nutsedge Killer

 Imazaquin

 Nut sedge

 Ortho Grass-B-Gon

 Fluazifop

 Annual and perennial grasses

 Fertilome Over the Top II,
 Vantage, Poast, Hi-Yield Grass
 Killer

 Sethoxdim

 Annual and perennial grasses

 Bayer Brush Killer, Ortho MAX
 Poison Ivy & Tough Brush Killer,
 Hi-Yield Brush Killer, Green Light
 Cut Vine & Stump Killer

 

 Triclopyr

 Vines and unwanted trees

 Roundup, Eraser, Killzall,
 Razor Pro, etc.

 Glyphosphate

 Most annual and perennial plants

 

 

A version of this article appeared in a May 2012 print edition of State-by-State Gardening.
Photography courtesy of Ron Strahan.
 

 

Posted: 05/11/18   RSS | Print

 

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