101 WEED CONTROL IN GROUND COVER PLANTINGS Clyde L. Elmore Cooperative Extension, University of California The use of chemical weed control of annual weeds in ground covers has progressed rapidly in recent" years. Because of availability of new herbicides And the knowled:",e of the use in ornamentals of these materials, good control of most weeds can be achieved. Good answers for problem weeds such as burclover, marestail, sowthistle and most perennial weeds remain unanswered. As leisure time (shorter work weeks, less fuel thus less travel, ~tc.) increases, grer.ter attention will be eiven to the environment, more particularly the landscape. This can be observed by the growth of the nursery inctustry and its present great expansion. Herbicides presently registered for ground covers and comments on their use are prepared in the followinc table.
102 Table 1. Herbicides Registered for Ground Cover Ornamentals Preemergence herbicides Herbicide DCPA (DACTHA:r ) Ornamentals Most ornamentals, after planting of rooted cuttings of Carpobrotus edule (large iceplant). Remarks Ajuga reptans (carpet bugle may be injured). diphen~id..16\ ( DYMII)!9, ENID.!!,"->') Safe on most ground cover species, new and established plantings. Will stunt plants when used preplant and incorporated. EPTC.. '15\. ( EFT.AJV?U"J nitralin.tm.. (PLANAVI!'P'J nitro.1en (TO~ triflurali_n (TREF~ diphenamid + trifluralin May be used on Ajuga, Gazania, Hypericum, iceplant, ivy, Pachysandra, periwinkle, Sedum, and strawberry. Safe on new and established plantings after the first irrigation. Safe on most new and established plantings. Almost all established plantings safe on new plantings after first irrigation. Most ornamental plantings. Should not be used just prior to planting, short residual (2-4 weeks), (must have immediate incorporation). Needs incorporation (may be water incorporated), good grass and some broadleaf control. Follow application with irrigation. Misses chickweed and burclover. Must be incorporated. Combination lengthens grass control and broadens spectrum.
103 Table 2. Herbicides Registered for Ground Cover Ornamentals Postemergence herbicides Herbicide ammonium sulfate bittern dalapon_/6\ (DOWPONW'J magnesium chloride MgC~ nitrofen (TO~ Iceplant. Iceplant. Ivy. Iceplant. Ornamentals Most new and established plantings. Remarks Annual weed control requires 7() F. temperature or greater for best control. Requires warm temperatures 70 F. or greater, good on annuals. Grass control, requires repeat treatments for perennial grasses. Requires warm temperatures 70 F. or greater, good annual weed control. Weeds must be in 2-3 leaf stage,or smaller. Follow application with irrigation. NOTE: The remaining herbicides mentioned in this report are not registered for use in ground cover ornamentals at this time.
104 The principal work and thus the major information presently available is o~he use of preemergence herbicides. The herbicide diphenamid (DYMI~ ENID~ has been widely used in many ground cover species and shrubs. At rates of 8-10 lb. a.i./a of diphenamid, preemergence control of many annual weeds has been excellent. Some annual broadleaf weeds (marestail, fleabane, burclover and others) are not controlled. Annual grass control from a single application generally lasts from 4-6 months. This is ~ticularly true in the Central Valleys where commonly trifluralin (TREFLA~ is added at 2 lb. a.i./a as a tank mix in a combination with 10 lb. a.i./a diphenamid. This combination provides a longer period of control of annual grasses and is safe on a wide range of ornamentals. Many of the previously mentioned broadleaf plants are not controlled. Nitralin (PLANAVIi+ID} can also be used on ornamental plants with the same high degree of selectivity as trifluralin. It has a distinct advantage over trifluralin in that volatility of nitralin is approximately 100 times less. This means that nitralin may be left on the soil surface without mechanical incorporation or leaching for up to 14 days without significant loss. By contrast, trifluralin should be moved into the soil in 24 hours or less. Both herbicides should be applied to a dry soil surface and followed by an irrigation. DCPA (DACTHA~ has been used extensively as a preemergence herbicide for control of annual grasses and some broadleaves. Control has been most effective in the southern part of the state. DCPA is selective on almost all ground cover species with only occasional injury to Ajuga reptans. Nitrofen (TO~ appears to be one of the more versatile herbicides for weed control in ground cover plantings. Nitrofen applied preemergent will control most weeds except chickweed, mustard, and burclover. Application on newly transplanted liner stock has not exhibited injury. Nitrofen should be followed by an irrigation for best control. Nitrofen will also control weeds when applied early posternergence from the cotyledon to 2 leaf stage. Cheeseweed (Malva parviflora) can be controlled at a larger growth stage when sprayed with 4 lb. a.i./a. Control is poor if grasses and most broadleaf weeds have developed past the 3 to 5 leaf stage. Several new herbicides were evaluated in a study at the South Coast Field Station in 1973. Weed control efficacy as well as costs to maintain a weed free environment with each of the herbicides were evaluated (tables 3 and 4) for one year. A study in 1970-71 indicated that during the first 2 months of establishment, costs of $2,941 per acre were incurred from hand weeding when two locations were averaged (Davis and San Jose area). The costs to establish ground covers for one year, including cost of herbicides, application and additional hand weeding, ranged from $6.82 to $30.21 per 1,000 ft2 in a South Coast Field Station trial. Diphenamid plus trifluralin at 10 lb.+ 2 lb. a.i./a gave excellent weed control except that white clover appeared six months after treatment. Total hand weeding and
105 herbici~costs for the year waa._$15 per 1,000 square feet. Oxadiazon (RONST~ or oryzalin (SURF~ at 4 lb. a.i./a each provided the best weed control and the least cost ($7.78 and $6.82 repectively). Most herbicides, with the exception of nitralin at 2 lb. a.i./a and oryzalin at 1 lb. a.i./a, gave acceptable control for 2 months. Both herbicides are weak on common groundsel (Senecio vulgaris) which accounted for the relatively poorer control. After a hand weeding at 2 months, all herbicides gave commercially acceptable control for 3 additional months. White clover was controlled with oxadiazon and oryzalin at 4 lb. a.i./a when evaluated six months after application (table 3).
106 Table 3. Weed Control In Ground Covers - South Coast Field Station 1972-73. Herbicide Rate l month 3 months 6 months lb. a.i./a Annual weeds White clover nitralin 2 4.8 8.o 4.5 diphenamid 10 + 2 9.5 9.3 2.8 + trifluralin nitrofen 4 6.8 7.5 2.0 oxadiazon 1 6.5 9.1 5.5 oxadiazon 2 9.2 8.8 6.5 oxadiazon 4 9.8 9.9 9.1 alachlor 2 8.6 8.8 3.5 alachlor 4 9.1 8.8 2.8 alachlor 8 9.4 9.8 4.o oryzalin 1 5.8 8.5 4.o oryzalin 2 7.0 9.5 7.2 oryzalin 4 9.1 9.9 9.6 butralin 4 7.0 8.5 4.2 butralin 8 9.0 9.7 4.8 nitrofen + nitralin 2 + 1 7.5 8.1 5.1 nitrofen + nitralin 4 + 2 9.0 9.1 5.5 hand weeded 10.0 6.5 2.5 nonweeded 0 0 1.2 *Weed control: 0 = no control; 10 = complete control.
107 Table 4. Costs of Hand Weeding Ground Covers Per Year - South Coast Field Station 1972-73. Additional weeding Herbicide lb. a.i./a Herbicide cost!/ costs/a Total cost/a Total 2 cost/1000 ft nitralin 2 $36 $1275 diphenamid 10 + 2 82 551 + trifluralin nitrofen 4 38 1139 oxadiazon 1 2i.Y 1292 oxadiazon 2 40 396 oxadiazon 4 72 267 alachlor 2 28 880 alachlor 4 38 853 alaehlor 8 68 621 oryzalin 1 22Y 1095 oryzalin 2 36 515 oryzalin 4 64 233 butralin 4 32!/ 955 butralin 8 56 612 nitrofen + nitralin 2 + 1 37 956 nitrofen + nitralin 4 + 2 66 533 hand weeded 5070 $1311 632 1177 1316 435 339 908 890 689 1117 550 297 988 668 993 600 5070 $ 30 15 27 30 10 8 21 20 16 26 13 7 23 15 23 14 116 Herbicides applied 7-13-72; 3-9-73 ($7.50 application cost).!/ extimated price (no market price established)
108 Table 5. Relative Phytotoxicity of Preemergence Herbicides on Six Newly Planted Ground Cover Species. - tq ~ ->, Ul Ul ri 0 111 0 "O,-... Ul ri Cl) ri +> bo... 8 ::s s:: ~ ::s s:: S-t ri s:: n.0 s:: Cl) ft.t s:: ri... 1111+>... 11!... >, ~ 0 al... ~ > E! '" 11! ri Ct,..t p. S-t S-t $.i n 111 -~ Cl) s:: '" ~ +> 0 Cl) > p. 0 111 s:: S-t.,., p. Cl) 0 ~ 0 11! Cl) s:: 111 $.i $.i Cl) '" 11! E! s::.0 0... Ill 0 tq... S-t Cl> 0 n... "O..-I.µ ft.i m s:: ~~ 0 "O s~ +> Cl) Ul ::c: 0 t/l Rate 111 Ill G) bo Cl) S-t lb. a.i./a Herbicide i! G) < tq < ~ 0 nitralin 2 * * * * * diphenamid 10 + 2 * b. b. + trifluralin nitrofen 4 * * * * oxadiazon 1 * 0 oxadiazon 2 b. * * 0 oxadiazon 4 6 * * * 0 alachlor 2 * * * * alachlor 4 * * * alachlor 8 * oryzalin 1 * * * * * * oryzalin 2 * * * oryzalin 4 * * * * * butralin 4 * * * * * butralin 8 * * * * nitrofen + nitralin 2 + 1 * * * * * * nitrofen + nitralin 4 + 2 * * * * * * no effect - treatment on new plantings, after first irrigation on dry soil surface, then followed by another irrigation. b. early stunting - followed by recovery. 0 excessive injury b.
109 Of the new herbicides tested oxadiazon at 2 and 4 lb. a.i./a was phytotoxic to Sedum brevifolium (table 4). Early stunting also occurred on Delasperma alba and Carpobrotus edule however growth was normal by 3 months. None of the other new herbicides injured the three previously mentioned ground covers or Hedera canariensis, Osteos ermum fruticosum and Vinca major. Diphenamid plus trifluralin (10 lb.+ 2 lb.a.i. A) stunted Vinca major and Sedum brevifolium at 1 mo. however no effect was observed by 3 months. With the exception of oxadiazon on Sedum brevifolium the herbicides tested could be used on any of the ground covers in this test if they were registered. Alachlor (LAssdID, and butralin (formally A-820) gave good weed control at 1 month and at 3 months after a hand weeding at 2 months. No injury was observed with either herbicide treatment. No new postemergence herbicides have been tested. The major need for weed control herbicides and practices is for a selective postemergence herbicide for grasses and/or broadleaves in ground cover ornamentals. Dalapon, 2,4-D and amitrole are not sufficiently selective to be used on a broad spectrum of ornamentals. Amitrole has been used on Carpobrotus edule at 1 lb. a.i./a with selective ;wntrol of annuals. Other ornamentals are injured however. Dalapon (DOWPO~ has registration for selective control of grasses in ivy, and 2,4-D water soluble amine has been safely used on Vinca major. These are examples of selective postemergence use, but there are no broad spectrum (for mixed grasses, and broadleaf weeds) selective treatments in these ornamentals. Control of perennial weeds, selective postemergence control of grasses and certain broadleaf weeds is the challenge. Also some broadleaf weeds still are not controlled well with preemergence herbicides.