<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-10-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000825542</issuerCik>
        <issuerName>SCOTTS MIRACLE-GRO CO</issuerName>
        <issuerTradingSymbol>SMG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001505744</rptOwnerCik>
            <rptOwnerName>Johnson Stephen L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THE SCOTTS MIRACLE-GRO COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>14111 SCOTTSLAWN ROAD</rptOwnerStreet2>
            <rptOwnerCity>MARYSVILLE</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>43041</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2014-10-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>114</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>54.91</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>114</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>114</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The deferred stock units were granted, together with an equal number of related dividend equivalents, in lieu of a portion of the reporting person's annual cash retainer, under the LTIP, and subject to the terms and conditions of an individual award agreement.  The deferred stock units equal the quarterly deferral amount divided by the closing price of a common share on the grant date (any fractional deferred stock units are rounded up to the nearest whole deferred stock unit).  Each whole deferred stock unit represents a contingent right to receive one common share of The Scotts Miracle-Gro Company.  Each dividend equivalent represents the right to receive additional deferred stock units in respect of dividends that are declared and paid during the period beginning on the grant date and ending on the settlement date with respect to the common shares of The Scotts Miracle-Gro Company represented by the related deferred stock unit.</footnote>
        <footnote id="F2">The deferred stock units are 100% vested upon grant and shall be distributed in shares as soon as administratively practicable, but no later than 90 days, following the earliest to occur of:  (i) reporting person's cessation of service as a director of The Scotts Miracle-Gro Company; (ii) reporting person's death; (iii) reporting person becomes disabled; or (iv) January 31, 2017.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Kathy L. Uttley as attorney-in-fact for Stephen L. Johnson</signatureName>
        <signatureDate>2014-10-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
