Manage grassy in landscape beds
Grasses include several valuable ornamental species for landscapes. However, they also include many important weeds that infest landscape beds. Like all weeds, grasses detract from the appearance of landscape ornamentals and compete with them for water, nutrients and light. However, because you may need different control measures for grasses than for broadleaves, you need to develop a year-round program for managing this group of weeds. An understanding of grass life cycles will help you develop an effective control program.
Grass life cycles * Annual grasses. Many grassy weeds are annuals-they must grow from seed each season. After germination, annual grasses grow vegetatively for a time, then flower and set seed. As a group, annuals are easier to control than perennials.
Annual grasses are classified as summer or winter weeds based on their emergence pattern. Summer annuals begin germinating in spring, grow during the summer months and then flower in late summer or early fall. Summer annuals generally die off with the first hard frost. A major summer-annual grass in landscape beds is large crabgrass. Smooth crabgrass is primarily a turf weed. Other common summer-annual grasses include yellow and giant foxtail, goosegrass, barnyardgrass and fall panicum.
Winter annuals emerge in fall or early spring. After germination in September or October, winter annuals survive through the winter and resume growth in early spring or grow continuously through winter in milder climates. These species then flower in mid to late spring. When hot, dry weather arrives in spring or early summer, winter annuals die off. However, cool, wet weather in late spring and early summer may allow these species to persist well into summer. Annual bluegrass is a common winter-annual weed. Perennial biotypes of annual bluegrass do exist, but these appear to be primarily a golf-course problem.
* Perennial grasses. Perennials live for many years. They can spread by seed like annuals, but creeping perennials also spread by rhizomes or stolons. Thus, they are especially hard to control due to the difficulty in completely killing all the rhizomes and stolons. Quackgrass and bermudagrass are two creeping perennials that commonly infest landscape beds. Quackgrass is a cool-season grass, meaning it grows primarily in spring and fall. Bermudagrass is a warm-season grass, actively growing in summer, while remaining dormant throughout the winter.
If turf areas consist of a creeping perennial, such as bermudagrass or zoysiagrass, the grass can continually invade adjacent ornamental beds. Repeated treatments may be necessary to maintain a killed strip between landscape beds and turf. Clumping turfgrasses such as tall fescue also can be a weed problem in beds if you don't completely kill the grass before installing a new bed or if you inadvertently spread seed to an adjacent ornamental planting.
A number of non-chemical and chemical options exist for grassy-weed management in landscape beds. The important considerations are the predominant grass species at each site and the type of ornamentals there.
Non-chemical control People commonly use mulches in landscape beds to improve appearance, moisture conservation and weed control. Mulches can control annuals but will generally not control perennials. In my research, I found that rock mulches, such as lava rock, controlled weeds better than organic mulch such as shredded pine bark. As organic mulches break down into finer particles, the mulch layer becomes a good growing medium for weeds. Pine- and hardwood-bark materials are a good growing medium in container nursery production, where they support weed growth, analogous to the landscape situation. Use no more than 2 to 4 inches of mulch in a landscape to avoid over-mulching. Organic and rock mulches do not control creeping perennials such as bermudagrass. Not only can these weed species emerge through a mulch layer, they also can creep into mulched areas from adjacent turf areas.
Various ways exist to improve the weed control you obtain with mulches. Solid black plastic (polyethylene) under mulch dramatically improves weed control compared to mulch alone. However, solid plastic lacks porosity, limiting movement of water and gases. For this reason, I do not recommend black plastic for long-term weed control in landscape beds.
Landscape fabrics do not have the porosity problem inherent to solid black plastic. In my research, however, the control I obtained from different fabrics varied. Weed shoots can penetrate up though any openings in the fabric. More important, grass roots can penetrate down through openings in a fabric when weeds germinate in the mulch layer above. Thus, using a fabric with limited open space results in the best weed control. A key to using landscape fabrics is maintaining the mulch layer free of weeds, either through hand weeding or herbicide application. Weed-X (Dalen Products) and the commercial grade of Typar (Reemay) are two fabrics I've tested that provide good annual-weed control, as well as some suppression of perennial weeds.
Landscape fabrics fit best into woody-ornamental plantings. Fabrics inhibit pegging down of ground covers and prevent emergence of herbaceous perennials, so you shouldn't use fabrics in areas where you've planted these species. When covered with mulch, a landscape fabric should last for many years.
Landscape fabrics improve weed control over mulch alone, although the improvement is not as great as you might see with solid black plastic. Weeds can germinate and develop in mulch above a fabric. Weeds also can penetrate through any opening in the fabric, such as gaps between sections of material. In addition, perennial weeds can push through landscape fabrics. Applying a pre-emergence herbicide in conjunction with a landscape fabric/mulch strategy will improve weed control compared to a fabric/mulch combination alone. Use shallow layers (about 1 inch deep) when placing an organic mulch above a fabric so as not to encourage weed growth in the mulch layer. Using a landscape fabric that contains a pre-emergence herbicide, such as Biobarrier II (Typar fabric plus Treflan herbicide-manufactured by Reemay), also improves weed control.
Chemical control In general, you use pre-emergence herbicides to control annual grasses, while post-emergence products are effective for perennial-grass control. Thus, for example, management strategies are quite different for controlling crabgrass vs. quackgrass. This is why understanding grass life cycles is important.
* Pre-emergents. You have several choices for annual grass control in ornamental beds, and you must base your application timing on the emergence pattern of the grass. Early spring applications are best for summer weeds, while a late-summer timing is appropriate for winter weeds. Your choice of material depends on the ornamental species present. Several effective chemicals pose little hazard to a wide range of herbaceous and woody ornamentals (see table, below). Some of these chemicals are available in both sprayable and granular formulations. I prefer granular formulations because they can provide a wider margin of safety over sprayable forms, especially in herbaceous plantings such as annual bedding plants. Check the label to determine which species are tolerant of the chemical you're using.
In woody-ornamental plantings, you have additional choices for annual grass control. In addition to other pre-emergents, dichlobenil and oxadiazon, and the combination products O-O, RegalStar, Rout, OH II and Snapshot also control annual grasses. These products are best for woody plantings due to their potential for injuring herbaceous plants. Dichlobenil controls annual weeds and also will control certain perennial grasses such as quackgrass. Because dichlobenil affects common turfgrass species such as tall fescue, you must prevent granules from reaching turf areas when you apply this product to ornamental beds. You also must not use dichlobenil in herbaceous ornamentals.
In general, the products listed above, with the exception of dichlobenil, do not control established perennial grasses. Pre-emergence crabgrass herbicides do control perennial grasses germinating from seed, and some inhibit pegging down of creeping perennials. However, to really control perennial grasses in landscape beds you have to use a post-emergence product.
* Post-emergents. The post-emergence products you can use for perennial grasses include both selective and non-selective materials. Non-selective products affect all plants, including desirable ornamentals, so you must apply them with caution. Selective products pose much less risk of crop injury, and you often can apply them directly over the top of ornamentals.
* Selective post-emergents. The selective grass-control products, generally referred to as post-emergence grass herbicides, are the compounds of choice in broadleaf ornamentals due to their high degree of safety to desirable plants. This group of selective herbicides includes sethoxydim, fluazifop, fenoxaprop and clethodim. These products are systemic, so they affect the rhizomes and stolons of perennial grasses. Repeat treatments are generally necessary for long-term control of established grasses, however. Members of this herbicide class only control grasses and have basically no effect on most broadleaves, sedges and other non-grass monocots such as wild onion and wild garlic.
Certain grasses can tolerate some of these products. Cool-season turfgrasses such as tall fescue, Kentucky bluegrass and perennial ryegrass tolerate fenoxaprop, and tall fescue has limited tolerance to fluazifop. Clethodim is the only product in this group that controls annual bluegrass. This highlights the importance of weed identification when choosing a herbicide. Also, remember that nutsedge, sometimes referred to as nutgrass, is not a grass and is unaffected by this group of chemicals. Obviously, you must not apply these materials over the top of ornamental grasses. In addition, be careful to avoid drift onto adjacent turf areas when applying these products to ornamental beds.
Sethoxydim and fluazifop have been used extensively for annual- and perennial-grass control in a wide range of herbaceous- and woody-broadleaf ornamentals. An important use for these two herbicides is control of established Johnsongrass, bermudagrass and quackgrass-tenacious weeds with few control options when growing among ornamentals. Fenoxaprop effectively controls annual grasses, with suppression of certain perennial grasses. The advantage of fenoxaprop is that you can use it for crabgrass control in cool-season turf as well as broadleaf ornamentals. Clethodim is the newest compound in this group, and we know less about ornamental tolerance to this chemical than sethoxydim and fluazifop. However, it does control both annual and perennial grasses.
Check the label to determine which broadleaf ornamentals you can treat with over-the-top applications of a given post-emergence grass herbicide. Although most broadleaf species have excellent tolerance to these chemicals, occasionally a species will experience damage from a post-emergence grass herbicide, so you should check labels carefully for plant tolerances. If you have any doubt about potential damage, test spray a small area before a more wide-scale application.
Do not apply to grasses under drought stress-in drought conditions, weeds do not absorb and translocate herbicides as well. Treat annual grasses when they are small (before tillering) for optimum results. Make sure that perennial grasses are actively growing when you treat them. Repeat applications usually are necessary for long-term control. For certain products, you will need an adjuvant for optimum control-check the label.
* Non-selective post-emergents. The non-selective herbicides glyphosate, glufosinate, diquat and pelargonic acid also are options for grass control in ornamentals. Diquat and pelargonic acid are contact herbicides, so they will not affect the root system of perennials. Glufosinate has limited translocation in plants, while glyphosate is systemic. Due to the potential for non-target injury-with any of these products but especially those that translocate-you must apply them carefully around desired plants. Shielded sprays or wiper applications minimize the potential for systemic injury.
Identify the grassy weeds that infest the landscape beds you maintain. Determine if they are annual or perennial and then develop a control strategy that accommodates the ornamentals you maintain and grasses you need to control. Depending upon the situation, you may need cultural, chemical or a combination of strategies to control your grass problems. Or perhaps you could be creative when a client asks, "Is this an ornamental grass or a weed?"
Dr. Jeffrey Derr is associate professor of weed science at Virginia Polytechnic Institute and State University (Virginia Beach, Va.).
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