Plant Growth Regulators Used in Turfgrass Management

Transcription

1 1 Plant Growth Regulators Used in Turfgrass Management Tim R. Murphy 1, Ted Whitwell 2, L. B. McCarty 2 and Fred Yelverton 3 The University of Georgia 1, Clemson University 2 and North Carolina State University 3 Introduction Plant growth regulators (PGRs) or inhibitors are increasingly being used on golf courses to suppress seedheads and vegetative growth of desirable turfgrasses, enhance turfgrass quality, and manage annual bluegrass (Poa annua) growth and development. Depending upon the turfgrass and situation, PGRs may reduce mowing costs, prevent scalping, increase turf density and decrease the need to mow steep slopes. Traditionally, PGRs were used in the southern United States to suppress bahiagrass (Paspalum notatum) and tall fescue (Festuca arundinacea) seedhead production in low maintenance areas such as highway roadsides, airports, and golf course roughs. However, in recent years, new products have been registered for use in most high maintenance turfgrasses. PGRs are recommended for use only on certain turfgrass species (Table 1). Additionally, the use of a PGR is often determined by the type of turfgrass area and level of maintenance. For example, imazameth (Plateau) is recommended for use on low maintenance sites such as roadsides and airports. However, flurprimidol (Cutless), paclobutrazol (TGR, others) and trinexapac-ethyl (Primo) may be used on putting green turfgrasses. The PGR label should always be consulted for information concerning turfgrass species and application sites. Prior to the development of PGRs for fine turfgrasses, several undesirable characteristics were associated with PGRs used on low maintenance, or rough, turfgrass sites. These included: a) phytotoxicity (burn) of treated leaves for four to six weeks following applications, b) reduced recuperative potential when the PGR-treated turfgrass was physically damaged, and c) increased weed pressure due to reduced competition from treated turfgrasses. However, because most PGRs were historically used in low maintenance areas these undesirable characteristics did not pose a problem to most managers. Several growth regulating chemicals have recently been developed for use on fine turfgrasses such as fairways, tees and putting greens. Fine turfgrass PGRs suppress vertical top growth, but usually do not affect the lateral or horizontal spread of stolons. The most noticeable effect is usually a reduction in the amount of clippings, and a reduction in mowing frequency. On tee boxes and fairway landing areas, turf recovery from golf club divots and other injuries occur while vertical top growth remains suppressed. Depending upon the product, fine turf PGRs also enable superintendents to reduce mowing frequency on fairways, suppress annual bluegrass in creeping bentgrass greens, improve ball lie or playability, and suppress the growth of bermudagrass during overseeding with a cool-season turfgrass. PGR Classification

2 2 Similar to herbicides, PGRs are placed into groups based on mode-of-action, or the way they inhibit growth of turfgrasses. Classification schemes can vary; however, three distinct groups of PGRs exist (Table 2). Cell Division Inhibitors (also called Type I PGRs). Cell division inhibitors are primarily foliage absorbed and inhibit cell division and differentiation in meristematic regions. They inhibit both vegetative growth and seedhead development. Growth inhibition is rapid, occurring within four to 10 days, and lasting three to four weeks, depending on application rate. Mefluidide (Embark, Embark Lite) and maleic hydrazide (Royal Slo-Gro, others) are examples of cell division inhibitors. These products are used primarily on low and medium maintenance turfgrass areas as phytotoxicity (yellowing) can be a problem. On golf courses, cell division inhibitors may be useful to reduce mowing on steep slopes, ditches and other difficult-to-mow areas. Herbicides. Various herbicides are used at low rates to suppress growth or seedhead development of turfgrasses. Depending upon the chemical, herbicides inhibit turfgrass growth and development through interruption of amino acid synthesis (glyphosate, sulfometuron, chlorsulfuron, metsulfuron, imazameth, imazethapyr + imazapyr) or fatty acid biosynthesis (sethoxydim). Turfgrass tolerance can be marginal and is highly rate dependent. Herbicides are primarily used only on low maintenance turfgrasses to reduce mowing and control weeds. Gibberellin Biosynthesis Inhibitors (also called Type II PGRs). Gibberellin is a plant-produced hormone that is needed for cell elongation, and normal growth and development. There are numerous gibberellins needed for normal plant growth and development. When gibberellin production is inhibited, plant cells do not elongate, internodes become shortened and overall plant growth is reduced. Two types of gibberellin biosynthesis inhibitors are available for use on golf courses. Trinexapac-ethyl (Primo), a Class A Gibberellin Biosynthesis Inhibitor, is foliar absorbed and inhibits the synthesis of gibberellin late in its biosynthetic pathway. Paclobutrazol (TGR) and flurprimidol (Cutless) are root absorbed Class B Gibberellin Biosynthesis Inhibitors and inhibit gibberellin biosynthesis in the early stages of this pathway. This early blockage prevents the synthesis of numerous gibberellins. Inhibition during the early stages of gibberellin biosynthesis can lead to increased injury when environmentally-stressed turfgrasses are treated with Class B Gibberellin Biosynthesis Inhibitors. Additionally, turfgrasses may exhibit various morphological responses such as the widening of creeping bentgrass leaf blades. Studies have shown that inhibiting gibberellin biosynthesis late in the pathway, as with trinexapac-ethyl, is less physiologically disruptive and injurious to turfgrasses. Site of Absorption PGRs are absorbed, or enter the turfgrass plant, by roots, foliage, or with some products, both roots and foliage (Table 2). Root absorbed PGRs, such as paclobutrazol and flurprimidol, require irrigation or rainfall after application to move the material into the turfgrass root zone. In contrast, trinexapac-ethyl is rapidly absorbed by turfgrass foliage, and irrigation after application is not necessary. Compared to cell division inhibitors there is less likelihood of leaf burn due to improper spray pattern overlaps with the gibberellin biosynthesis inhibitor PGRs. Most foliar absorbed materials (e.g., mefluidide, maleic hydrazide, and herbicides) require uniform, even coverage to prevent phytotoxicity and must be absorbed by turfgrass leaves before irrigation or rainfall occurs. Growth Suppression

3 3 Cell division inhibitor PGRs quickly (five to seven days) suppress vegetative growth, but usually provide a shorter period of growth suppression than gibberellin biosynthesis inhibitors. However, unlike gibberellin biosynthesis inhibitors, cell division inhibitors are highly effective in suppressing seedhead development. The growth suppression activity of gibberellin biosynthesis inhibitors is often not immediately evident. Compared to cell division inhibitors, paclobutrazol and flurprimidol are slower (10 to 14 days) in suppressing turfgrass growth, but their duration of activity is usually longer, lasting from four to eight weeks, depending on application rate. However, trinexapac-ethyl has been shown to reduce common and hybrid bermudagrass clipping weights 50% at seven days after application. Depending upon application rates and schedules, trinexapac-ethyl also provides long term (four to eight weeks) growth suppression. Another key difference is that while gibberellin biosynthesis inhibitors decrease seedhead stalk height, they have little effect on the actual formation of seedheads. Application Timing Timing of application with PGRs is critical to achieve desired results. When used for seedhead suppression, the PGR must be applied before seedhead formation and emergence. Applications made after seedhead emergence will not be effective. For bahiagrass, mow the area as seedheads initially emerge (usually from May to mid-june) to provide a uniform, even appearance to the site. The PGR treatment should be applied about 10 to 14 days following mowing or just prior to new seedhead appearance. Additional applications six to eight weeks later may be required if new seedheads begin to emerge. If PGRs are being used on creeping bentgrass golf greens, applications should be made during periods of active root growth. In the southern United States, this would be during the mid-fall and spring months. Applications should not be made during mid-summer and mid-winter months. On warm-season turfgrasses, such as bermudagrass, the appropriate PGR should be applied to actively-growing turfgrasses after full spring green-up and several mowings. Applications can be repeated during summer months if additional growth regulation is needed. Weed Control Considerations An integrated weed management program must accompany any PGR use as PGRs usually do not suppress weed growth, particularly broadleaf weeds. On high maintenance turfgrasses it usually is advisable to continue preemergence herbicide use for control of annual grass weeds. After the PGR has been applied, annual and perennial weeds can become a problem as PGR-treated turfgrass often does not compete well with weeds. Normally, 2,4-D, dicamba or various two- and three-way herbicide mixtures are used to control broadleaf weeds. Other postemergence herbicides such as MSMA, for annual grass weed control, or nutsedge control herbicides may also be needed in some situations. Postemergence herbicides often cause temporary phytotoxicity to turfgrasses. Postemergence herbicides can be tank-mixed with PGRs; however, turfgrass injury is often greater than when either type of product is used alone. Therefore, on high maintenance turfgrasses, where color and appearance may be of utmost importance, it is advisable to not tank mix postemergence herbicides with PGRs. Additionally, if a postemergence herbicide has injured the turfgrass, PGR application should be delayed until the turfgrass has fully recovered. Reference to the PGR label and personal experience is the best guide to determine suitability of tank-mixing PGRs and postemergence herbicides.

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5 5 Currently Available Plant Growth Retardants Low Maintenance Bahiagrass, Bermudagrass and Tall Fescue Sulfometuron-methyl (Oust 75DG). Used in bahiagrass at 0.5 oz./a (0.02 lb. ai/acre). Sulfometuron-methyl is foliar and root absorbed and should be applied to bahiagrass in spring or early summer seven to 14 days after first mowing. Do not use a surfactant. Do not apply to wetlands or where runoff water may drain onto cultivated lands or forests. Do not apply to turf less than one year old. Treated areas may appear less dense and temporarily discolored. Only one application per year should be made as repeat application within the same year can reduce bahiagrass density. Read and follow all label recommendations before use. Often tank-mixed with Roundup Pro, Telar, Campaign and/or Velpar. DO NOT EXCEED RECOMMENDED RATE. Sulfometuron-methyl is not recommended for use on high maintenance turfgrasses. Chlorsulfuron (Telar 75 DG). Used in tall fescue, bahiagrass and bermudagrass at rates that range from 0.25 to 1.0 oz./a (0.012 to 0.05 lb. ai/acre). Chlorsulfuron is foliar and root absorbed and should be applied to well-established tall fescue prior to seedhead formation. Do not apply to turfgrasses less than one year old. A nonionic surfactant should be added to the spray mixture. A tank-mix of Telar at 0.25 oz./a plus Embark at 0.5 pt./a can be used to suppress growth and seedhead emergence in tall fescue. Apply this tank-mix after spring green-up but before tall fescue seedhead emergence. Chlorsulfuron may also be tank-mixed with glyphosate and sulfometuron-methyl. Chlorsulfuron should not be used on turfgrasses that are stressed due to drought, insects, disease, cold temperatures or poor fertility as injury may result. Chlorsulfuron is not recommended for use on high maintenance turfgrasses. Metsulfuron (Escort 60 DF). Used in tall fescue and bermudagrass at rates that range from 0.25 to 2.0 oz./a (0.009 to lb. ai/acre). Metsulfuron is foliar and root absorbed. Low rates (0.25 to 0.5 oz./a) may be used for growth and seedhead suppression in well-established tall fescue. Applications to tall fescue should be made in the spring after two to three inches of new growth but before seedhead emergence. In bermudagrass, metsulfuron is used primarily to control weeds at rates that range from 0.25 to 2.0 oz./a. A tank mix of Escort at 0.25 to 0.33 oz./a plus Embark at to 0.25 pt./a can be used to suppress growth and seedhead emergence in tall fescue. Do not apply metsulfuron to turfgrasses less than one year old. A nonionic surfactant should be added to the spray mixture. Metsulfuron should not be used on turfgrasses that are stressed due to drought, insects, disease, cold temperatures or poor fertility as injury may result. Metsulfuron is not recommended for use on high maintenance turfgrasses or bahiagrass. Sethoxydim (Vantage 1.0 lb./gal.; Poast 1.5 lbs./gal.). Apply at lbs. ai/a (Vantage pts/a; Poast pt./a) to established, low-maintenance tall fescue for seedhead suppression. Applications should be made in the early spring before tall fescue seedhead emergence. A crop oil concentrate at 2.0 pts./acre or Dash HC spray adjuvant at 1.0 pt./a must be added to Poast. It is not necessary to add a spray adjuvant to Vantage. Unlike other herbicides used for growth suppression on low maintenance grasses, sethoxydim has no herbicidal activity on broadleaf weeds. Appropriate broadleaf weed control practices are usually necessary following the use of sethoxydim. Vantage may also be used in high maintenance centipedegrass and fine fescues (creeping red, chewings, sheep and hard) for annual grass and bahiagrass control.

6 6 Maleic hydrazide (Retard 2.25 lbs./gal.; Royal Slo-Gro 1.5 lbs./gal.; Liquid Growth Retardant 0.6 lbs./gal.). Apply the respective product at 3.0 lbs. ai/acre. Maleic hydrazide is foliar absorbed. Apply to bahiagrass in spring or seven to 14 days after first mowing. Do not use a surfactant. Do not apply to turf less than three years old and do not reseed within three days after application. Treated areas may appear less dense and temporarily discolored. Do not use on St. Augustinegrass, and do not apply to bahiagrass under drought conditions. Read and follow all label recommendations before use. A 12-hour rainfree period is required for optimum activity. Maleic hydrazide is not recommended for use on high maintenance turfgrasses Glyphosate (Roundup Pro 4 lbs./gal.). Apply only to bahiagrass at 4.0 to 8.0 fl. oz./a (0.18 to 0.22 lb. ai/acre). Glyphosate is also sold in combination with 2,4-D as Campaign 2.5L. The addition of 2,4-D to glyphosate increases broadleaf weed control. Glyphosate is foliar absorbed. Note: Glyphosate is classified as a nonselective herbicide. Only low rates of glyphosate should be used in bahiagrass, or severe injury will occur. Make the initial application at a rate of 6.0 to 8.0 fl. oz./a after full greenup of bahiagrass (timing will vary according to location). Treated areas may appear less dense and temporarily discolored. Repeat applications of Roundup Pro at 4.0 to 6.0 fl. oz/a six weeks later can be used to extend the period of growth and seedhead regulation. Read and follow label recommendations prior to use. Glyphosate is not recommended for use on high maintenance turfgrasses. Imazapic (Imazapic Applicators Concentrate, Plateau). Apply to bahiagrass and bermudagrass at 2.0 to 4.0 fl. oz/a (0.031 to lb. ai/acre). Imazapic is foliar and root absorbed. Apply to bahiagrass in spring two to three weeks before seedhead formation or seven to 10 days after mowing. Imazapic also provides some broadleaf weed and annual grass control. Do not apply to wetlands. Treated areas may appear less dense and temporarily discolored. Do not use on St. Augustinegrass, tall fescue or drought-stressed bahiagrass. Add a surfactant or methylated seed oil according to label recommendations. Read and follow label directions before use. Imazapic is not recommended for use on high maintenance turfgrasses. Imazethapyr + imazapyr (Event 1.46L). Apply to low maintenance tall fescue at 8.0 to 10.0 fl oz./a (0.09 to 0.11 lb. ai/acre). Apply after turf has completed spring transition, is actively growing and has at least 2 inches of vertical growth. Add a surfactant at 0.25% v/v. Do not use on stands less than one year old or on highly managed turf. Do not reseed within three months after application. Read and follow label directions before use. Mefluidide (Embark 2S). Apply at 1.5 to 2.0 pt./a (0.38 to 0.5 lb. ai/acre) approximately 2 weeks before seedhead appearance. Do not use on turf less than four months old and do not reseed within three days after application. Treated turf may appear less dense and temporarily discolored. An eight-hour rain-free period is needed after application. The addition of 0.25 to 0.5% v/v nonionic surfactant may increase seedhead control but also may increase turf discoloration. For low maintenance bermudagrass, use 1.0 lb. ai/a application rate. Bermudagrass, Tall Fescue and St. Augustinegrass Mefluidide (Embark 2S, Embark Lite 0.2S). Mefluidide is recommended for use on turfgrasses under low to medium levels of maintenance. Apply at to 1.0 lb. ai/a (Embark 2S to 4 pts./a: Embark Lite 0.2S - 10 to 40 pts./a). Mefluidide is primarily foliar absorbed. Apply to common bermudagrass (4 pts./a Embark 2S, 40 pts./a

7 7 Embark Lite), tall fescue (1.5 pt./a Embark 2S, 15 pts./a Embark Lite), and St. Augustinegrass (1.0 pt./a Embark, 10 pts./a Embark Lite). Apply in spring after greenup until approximately two weeks before seedhead appearance. Optimum results may not be obtained if rainfall or irrigation occurs within eight hours following application. Do not apply to turf within four growing months after seeding, and do not reseed within three days after application. Treated turf may appear less dense and temporarily discolored. Adding 1 to 2 qts of a nonionic surfactant per 100 gal of spray solution may enhance suppression; however, turfgrass injury (discoloration) may also be increased. Use 0.5 pt./a of Embark or 2 to 5 pts./a of Embark Lite in the early January through February time frame to suppress Poa annua seedheads in fairways. Iron applications 10 days or less before mefluidide application may lessen discoloration. Read and follow label recommendations before use. Flurprimidol (Cutless 50WP). Apply at 0.75 to 3.0 lbs./a (0.375 to 1.5 lbs. ai/acre). Flurprimidol is root absorbed. Apply to bermudagrass or zoysiagrass golf course fairways, hard-to-mow and trimmed areas. Provides four to eight weeks of suppression. Flurprimidol must be uniformly applied and irrigated-in with 0.5 inch water within 24 hours of application. Flurprimidol does not completely control seedheads, only seedhead stalk elongation. Temporary turf discoloration may follow this treatment. St. Augustinegrass, bahiagrass, and common bermudagrass require the higher rate. Repeat applications every 4 weeks on Tifway bermudagrass with 1.0 lb/a will minimize turf injury. Trinexapac-ethyl (Primo 1EC, Primo 25WSB). Apply at rates that range from 0.1 to 0.75 lb. ai/acre (Primo 1EC to 6 pints/a; Primo 25WSB to 44 oz/a) depending upon the turfgrass species, mowing height and length of suppression desired. Trinexapac-ethyl is foliar absorbed. Low rates are for hybrid bermudagrass, centipedegrass, zoysiagrass, and St. Augustinegrass; medium rates are for common bermudagrass and tall fescue while the high rate is for bahiagrass. Primo 1EC at 3.0 to 6.0 fl.oz./a and Primo 25WSB at 1.35 to 2.7 oz./a may also be used on creeping bentgrass and hybrid bermudagrass putting greens. A one hour rain-free period is needed after application. Mowing one to seven days after application improves appearance. Repeat applications may be applied as needed or approximately three to six weeks apart to maintain growth suppression but do not exceed 21 pints/a of Primo 1EC or 174 oz./a of Primo 25WSB per year. Trinexapac-ethyl will suppress seedheads on hybrid bermudagrass, but only partial seedhead suppression is observed on other turfgrass species. Temporary turf discoloration may follow treatment. It is not necessary to add a surfactant to trinexapac-ethyl. Primo formulations may also be used to enhance the establishment of cool-season turfgrasses in bermudagrass (overseeding). Apply Primo before verticutting, spiking, scalping, etc. and 1 to 5 days before overseeding. Paclobutrazol (TGR Turf Enhancer 2SC). Apply at 2.0 to 3.0 pts./a (0.5 to 0.75 lb. ai/acre) to actively-growing hybrid bermudagrass and St. Augustinegrass. Paclobutrazol is also available on several dry fertilizer formulation carriers. Paclobutrazol is root absorbed. This product may also be used on overseeded golf greens and fairways during winter for turf enhancement, for annual bluegrass suppression and to suppress the growth of perennial biotypes of annual bluegrass in creeping bentgrass greens. Paclobutrazol should be applied in early January for seedhead suppression of annual bluegrass. Do not apply to saturated soils and treat only dry foliage. Read and follow recommendations before use. Foliar Suppression of Annual Bluegrass in Bentgrass or Overseeded Bermudagrass

8 8 Mefluidide (Embark 2S, Embark Lite 0.2S). Apply at 0.05 to lb. ai/acre (Embark 2S pt./a; Embark Lite - 2 to 5 pt./a) to suppress annual bluegrass seedhead development. Mefluidide must be applied before seedheads emerge. Application timing varies between geographical locations, but in the South is generally during the January though early March time frame (actual timing of application depends upon location and climatic conditions). Mefluidide is primarily foliage absorbed. Do not apply to turf within four growing months after seeding, and do not reseed within three days after application. Treated turf may appear less dense and temporarily discolored. Adding 1 to 2 qts of a nonionic surfactant per 100 gal of spray solution may enhance suppression; however, discoloration may also be increased. Iron applications may lessen discoloration. Mefluidide formulations are not recommended for use on golf course putting greens. Read and follow label recommendations before use. Paclobutrazol (TGR Turf Enhancer 2SC). Apply at 6.4 to 48 fl. oz./a (0.1 to 0.75 lb. ai/acre) in late winter to early spring after growth of desired grasses has resumed and one to two mowings has occurred. Do not apply after March 15 to avoid delaying greenup of bermudagrass. Paclobutrazol is root absorbed and 0.25 inch rainfall or irrigation water should be applied within 24 hours of application. Fall and spring applications of paclobutazol may also be used over a period of years to suppress the growth of perennial biotypes of annual bluegrass in creeping bentgrass greens. Repeat applications may be made three to four weeks apart. Do not use if Poa annua populations exceed 70%. Flurprimidol (Cutless 50WP). Apply at 0.25 to 0.5 lb/a (0.12 to 0.25 lb. ai/acre) to actively-growing creeping bentgrass in the spring after third or fourth mowing or in the fall months. Repeat, if necessary, at 3 to 6 week intervals, but do not exceed 2 lb. product/a per growing season. Delay overseeding two weeks after application. Make final fall application eight weeks before onset of winter dormancy. Flurprimidol is not as effective as paclobutrazol in suppressing the growth of perennial biotypes of annual bluegrass. Read and follow all label recommendations before use. Products listed are for use by professional turf managers only. Trade and brand names are used for information only.

9 9 Table 1. Labeled turfgrass species for various plant growth regulators. Plant Growth Regulator Common Name Trade Names Foliag e maleic hydrazide mefluidide sethoxydim Royal Slo-Gro, others Embark, Embark Lite Poast, Vantage Suppression Characteristics Seedhead Maintenance Level of Turfgrass Site 1 Labeled Turfgrass Species Yes Yes Low Bahiagrass, Bermudagrass, Fescues, Yes Yes Low, Medium Low, Medium, High Kentucky Bluegrass, Ryegrass Bermudagrass, Centipedegrass, Fescues, Kentucky Bluegrass, Ryegrass, St. Augustinegrass Yes Yes Low, Medium, High Centipedegrass, Fine Fescues, (roadside tall Fescue) glyphosate Roundup Pro Yes Yes Low Bahiagrass, Bermudagrass imazapic Plateau Yes Yes Low Bahiagrass, Bermudagrass imazethapyr + Event Yes Yes Low Bahiagrass imazapyr sulfometuron Oust Yes Yes Low Bahiagrass, Bermudagrass metsulfuron Escort Yes Yes Low Bermudagrass, Fescues, Kentucky Bluegrass chlorsulfuron Telar Yes Yes Low Bahiagrass, Bermudagrass, Fescues, Kentucky Bluegrass flurprimidol Cutless Yes No Low, Medium, High Bentgrass, Bermudagrass, Kentucky Bluegrass, Ryegrass, St. Augustinegrass, Zoysiagrass paclobutrazol TGR, others Yes No Low, Medium, High Bentgrass, Bermudagrass, Kentucky Bluegrass, Ryegrass, St. Augustinegrass, Zoysiagrass trinexapac-ethyl Primo Yes Partial Low, Medium, High Bahiagrass, bentgrass, Bermudagrass, Centipedegrass, Fescues, Kentucky Bluegrass, Ryegrass, St. Augustinegrass, Zoysiagrass 1 Low maintenance turfgrass sites: roadsides, airports, storage sites, hard-to-mow areas, etc. Medium maintenance turfgrass sites: industrial grounds, parks, cemeteries, golf course roughs, home lawns. High maintenance turfgrass sites: putting greens, tees, fairways, athletic fields, high quality home lawns and commercial properties.

10 10 Table 2. Characteristics of various plant growth regulators used in turfgrass management. PGR Common Names Maleic hydrazide Mefluidide Sethoxydim Glyphosate Imazameth Imazethapyr + imazapyr Sulfometuron Metsulfuron Chlorsulfuron Flurprimidol Paclobutrazol 1 Trinexapac-ethyl PGR Trade Name Absorption Site Mode of Action Comments Retard, Royal Slo-Gro, Liquid Growth Retardant, Embark, Embark Lite Poast, Vantage, Roundup Pro, Plateau, Event, Oust, Escort, Telar Cutless TGR Turf Enhancer Primo 1 Paclobutrazol is also available on dry fertilizer carriers. Foliar Foliar root Root, Foliar Foliar and Inhibit cell division (mitosis) Inhibit fatty acid biosynthesis Inhibit amino acid biosynthesis Interfere with gibberellin biosynthesis Effective seedhead suppressors. Growth inhibition is rapid, within 4 to 10 days, and lasting three to 4 weeks. Usually low in cost, but turfgrass tolerance is low and rate dependent. Use is restricted to low maintenance turfgrasses. Initial growth response is slower compared to cell division inhibitors, but duration of activity is usually longer, 4 to seven weeks. Rainfall, irrigation, or high application volumes are required for activating root absorbed PGRs.