Weed Controlling Chemicals
Herbicides to control unwanted plantsare one of the latest developments available togardeners. Before World War II, only a fewchemicals such as table salt, dinitro, oils andsodium chlorate were available. Since thattime, a large number of products have beenoffered for sale, and an even larger number havebeen tested by industrial and university labora-tories. These chemicals have helped to reducethe estimated 4-billion-dollar loss caused bysteeds each year.
Herbicides are widely used in commercialagriculture where large acreages are planted toa single crop but are little used in home gardens.There are several reasons for this situation.First, most gardens contain a wide variety ofplant materials which have different tolerancesto herbicides. Second, few herbicides come insmall packages. Third, directions for use insmall areas have not been available. Thesedirections are provided in Table V. The oneexception to the lack of use of herbicides in thehome garden has been in the lawn area. Alawn represents a comparatively large area ofone type of plant, and thus resembles com-mercial agriculture. Small packages of lawnherbicides and directions for their use arewidely available.
Herbicides are classified into several groups:soil sterilants, pre-emergence herbicides, post-emergence herbicides and soil fumigants. Soilsterilants are used around railroad right-of-ways, industrial sites and other locations wherevegetation can be unsightly or constitute a firehazard. These chemicals are sometimes appliedbefore a blacktop driveway is laid around ahome to prevent the growth of some of theperennial weeds. Some of the chemicals used assoil sterilants at rates of to lbs. or more peracre are widely used selective herbicides at 4lbs. or less per acre. These include Atrazine,Karmex and Simazine, and others.
Pre-emergence herbicides must be appliedbefore the weed seeds germinate, preventing thegrowth of the weeds. In addition to the areabeing weed free, there must be soil moisture forthis type of herbicide to be effective. Some ofthese herbicides must be incorporated into thesoil surface to be effective.
Post-emergence herbicides are applied afterthe weeds have appeared. This type of herbi-cide usually must be applied as a directed sprayso that the crop foliage is not contacted. Theaction of oil, dinitro and other contact herbi-cides causes a rapid discoloring or killing of thefoliage. Hormone-type sprays kill the weeds in aslower but effective manner by disrupting themetabolism of the plants after its absorption bythe leaves and stems.
If open ground is to be developed into agarden, the use of a soil fumigant such asMethyl Bromide, Vapam or VPM, or Vortexshould be considered. This is the one timewhen all major soil pests can be eliminatedwith a single treatment. There are severalrestrictions on the use of these chemicals, suchas cost of the chemical of approximatelydollar per too sq. ft., danger to the applicator,danger to roots of plants which may be growinginto the treated area, or difficulty of application.since these materials work best injected into the soil or under a plastic cover. Soil fumigants areonly effective if the soil temperature is about55° F. and with no undecomposed organicmatter in the soil. The soil should be worked intoa seed bed condition and be moist, but notsaturated, at the time of application for thetreatment to be effective. A time interval ofbetween 7 and as days must elapse betweenapplication and planting so that the chemicalmay dissipate. This time period depends on thechemical used and the weather conditionsfollowing treatment. A longer time is neededwhen cool, wet conditions prevail than when it ishot and dry.
Another less costly treatment which willeliminate most of the weeds in the soil is theuse of calcium cyanamide. This fertilizersupplies nitrogen and calcium to the soil duringits breakdown period. The rates of applicationvary from go to 8o lbs. per too° sq. ft., with thelower rates being used on light, sandy soil. Asplit application is most effective, with half thematerial worked into the upper a in. of soil andthe rest applied to the soil surface. An intervalof 3o days must elapse between application andplanting. This material is particularly suitablefor use in the construction of a new lawn, sincein many areas it is desirable to raise the pH ofthe soil before planting in order to grow goodturf. Each pound of calcium cyanamide equals apound of limestone and, in addition, suppliespound of nitrogen.
In all cases when using an herbicide, themanufacturer’s directions should be followed.State experiment station and extension servicerecommendations can be of value to selectthose treatments which have been most effectivein your area. Soil type, weeds present, amountof soil moisture, season of the year and possiblyother conditions all make a difference in theeffectiveness of an herbicide. Many weed speciesare resistant to certain herbicides. Because ofthis, an alternation or a combination of herbi-cides has been used in many cases.
Weed Control in the Lawn
Lawn areas have been the target for mosthome chemical weed-control projects to date.Control of broadleaved weeds, such as Dan-delion and Plantain, has been relatively easywith 2,4-D herbicides. Crabgrass has beencontrolled by disodium methyl arsonate,Dacthal and other compounds.
We could conclude that weeds in lawns areno longer a problem because there are chemicalmeans to control them. However, chemicalcontrol is only one of many factors in theproper maintenance of a good lawn. Adequatefertilizer and water as well as proper mowingheight and frequency for the particular grass As many common weeds such as Dandelionand Plantain have been satisfactorily controlled,other weeds have been noted as being problems.These problem weeds include Annual BlueGrass, clovers and Veronica. There are controlsfor thens weeds available but these controlmeasures may cause other problems. Dacthalwill prevent the growth of Annual Blue Grassbut it may damage existing stands of Bentgrass.It is possible to sow desirable turf grasses with-in a year of a Dacthal application, if necessary.Betasan has a longer residual life than Dacthaland is less affected by soil type and soil cover.It does not seem to damage established standsof desirable turf grasses, but should be used onlywhere the desirable turf makes up the majorityof the stand. This caution is necessary since itis not possible to establish new grass seedlingsfor an extended period.
Dicamba or Banvel D has been very effectiveon many of the harder-to-kill species of broad-leaved weeds, including the clovers. Considerablecaution must be used with this chemical since itcan damage ornamental plants both by spraydrift and soil absorption. This damage (dis-torted leaves) has not caused plant losses whereDicamba was used at rates higher than suggestedfor clover control. The plants will outgrow thesymptoms if the spray was not applied directlyto thent. Sometimes it may be necessary totolerate some damage to eliminate some difficultweed.
Veronica is the major remzining broad-leaved pest for which there is ito really successfulcontrol. Endothal is effective but is no dangerous to turf grasses that it should be avoided unlessexpert care is available.
Table 1 gives the suggested treatments forvarious problern lawn weeds.
Weed Control in Ornamental PlantingsSeveral herbicides with long residual proper-ties have been found useful in nurseries andplantings containing woody ornamental plants,since these plantings are not changed frequently.With many of these chemicals, application inthe fall will prevent weed development until thefollowing May. During this period it is oftendifficult to kill weeds by mechanical meanssince conditions are too cool and moist for thetoads to dry out after they are disturbed bycultivation.
Mulching with various organic materialssuch as sawdust, barkdust, leaves, groundcobs, cocoa bean shells and other products is aneffective way to control many annual weeds inthe home garden. Generally, mulches do notstop most perennial weeds and they tend to itup or deactivate many herbicides. Casoronseems to be one of the herbicides least affectedby mulches. If a mulching program has beenfollowed, it may be necessary to rely on herbi-cides which kill by foliage absorption. Amitrole,when used as a directed spray with repeatedapplication, will control many problem weedssuch as Quackgrass, Horsetail Rush and CanadaThistle.
The herbicides and amounts to use in orna-mental plantings are given in Table II.
Weed Control in Fruit Plantings
Weed control in fruit plantings is complicatedby the problem of achieving satisfactory controlwithout having a residue of the herbicide in thefruit. This is possible to do if the manu-facturer’s directions as to the amounts to use,method of application and time of applicationsare followed carefully. Many of the same herbi-cides used on ornamental plantings are alsoused on fruit plants.
Oils (rarely used in ornamentals) are used tokill existing weeds around fruit trees, with carebeing used not to contact the trunk of the tree.Repeated applications need to be made sincethere is no residual control with the types ofoils which can be safely used.
With the exception of the oils, most herbicideapplications are made after harvest in the fallor before bloom in the spring.
The herbicides and the amounts to use indifferent fruit crops are shown in Table III.
Weed Control in Vegetable CropsThis is the most difficult area in the homegarden in which to use herbicides. Many of the
plants remain in the soil only a short time, sothat long residual herbicides cannot be used. Avery wide variety of plants are grown whichhave different susceptibilities and tolerancesto herbicides, to that there are as many differ-ent herbicides used as there are varieties grown.Finally, and perhaps most important, oneshould avoid leasing a residue of the herbicidein the edible portion of the plant.
Herbicides and instructions for their use invegetable crops are given in Table IV.
Atnounts to Use in the Garden
(Editor’s Note)
As the author of this article states, theherbicide trade apparently is listing the applica-tion amounts of herbicides in pounds or gallonsof active ingredients per acre. The gardenershould take special note of this for if he usestables and recommendations made up this way,serious and damaging mistakes can be made ifhe figures amounts of application on any otherbasis. For instance, in Table III it is noted thatthe herbicide Simazine is recommended forcontrolling weeds in blackberries, used at therate of 4 lbs. active ingredient per acre. In TableV Simazine is noted as an 8o% wettable powder.Hence the amount of the actual product toapply per acre is figured thus: 4.o lbs. dividedby 0.8 (80`,./.) = 5 lbs. of the actual product tobe used. This, of course, would be dissolved ina sufficient quantity of water to that it could bespread evenly over the entire area concerned.To find the amount for too sq. ft., one observesin Table V that if t lb. active ingredient of Sima-zine is applied to I acre, the amount for too sq.ft. is 1.25 grams, or of a teaspoonful. Since therecommendation for blackberries is for 5 lbs.per acre, the amount for too sq. ft. of black-berries would be 1.25 grams x 5 = 6.25 grams,dissolved, of course, in a sufficient amount ofwater to cover evenly the area involved.One proceeds similarly with the emulsifiableconcentrates. Note that 3-6 lbs. of Eptam isrecommended for use in controlling weeds inbeans (Table IV). In Table V one sees thatEptam is listed as an emulsifiable concentrate.It is listed SEC which simply means that 6 lbs.of active ingredient of the herbicide are in t gal.of the concentrate. Hence when 3-6 lbs. of theEptam (active ingredient) are recommended foran acre of beans, it means o qts. to t gal. of6 EC Eptam concentrate, mixed, of course, insufficient water to cover the area evenly. Fortoo sq. ft. of beans it would be (from Table V)1.5 ml. of the concentrate, mixed in sufficientwater to cover the area evenly.
This does seem complicated to the gardener.
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