<PAGE>   1
                                                                      EXHIBIT 4b



            SECOND AMENDMENT TO MORTGAGE-BACKED CREDIT AGREEMENT


        This Second Amendment to Mortgage-Backed Credit Agreement dated as of 
July 25, 1997 by and between GENERAL HOST CORPORATION, a New York corporation
and FRANK'S NURSERY & CRAFTS, INC. (collectively, "Borrowers"), and COMERICA
BANK, a Michigan banking corporation ("Bank").
        


        WHEREAS, Borrowers and Bank entered into a certain Mortgage-Backed 
Credit Agreement dated as of November 29, 1996 and an amendment thereto dated
as of June 13, 1997 (as so amended, the "Agreement"), pursuant to which
Borrowers incurred certain indebtedness and obligations and granted the Bank
certain security for such indebtedness and obligations; and
        
       WHEREAS, Borrowers and Bank desire to amend certain provisions of the 
Agreement on the terms and conditions hereof;

       NOW, THEREFORE, it is agreed:

A,     DEFINITIONS

       1.     Capitalized terms used herein and not defined to the contrary 
have the meanings given them in the Agreement.

B.     AMENDMENTS TO AGREEMENT

       1.     Section 1.2 of the Agreement is hereby amended and restated as 
follows:

              "1.2   'Applicable Interest Rate' shall mean: (a) until July 25, 
       1997, the Eurodollar-based Rate or the Prime-based Rate, as selected by 
       a Borrower from time-to-time or otherwise determined pursuant to the 
       terms of this Agreement, and (b) thereafter, the Prime-based Rate."

       2.     Section 1.11 of the Agreement is hereby amended and restated 
       as follows:

              "1.11 'Collateral Pool' shall mean the Real Property (i) for 
       which Bank has received Mortgages/Deeds of Trust, Title Insurance, 
       Insurance Policies, Surveys, Appraisals and Environmental Reports, and 
       (ii) which has not been released from the lien of the relevant 
       Mortgage/Deed of Trust pursuant to Section 11.4 hereof."

       3.     Section 1.34 of the Agreement is hereby amended and restated as 
       follows:

              "1.34 'Letter of Credit Maximum' shall mean, as of any date: (a)
       with respect to Letters of Credit generally, the Maximum Amount from 
       time to time in effect minus the aggregrate principal amount of 
       outstanding Advances; and (b)



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       with respect to Letters of Credit which are standby letters of Credit, 
       Ten Million Dollars ($10,000,000)."

       4.     Section 1.37 of the Agreement is hereby amended and restated as 
follows:

              "1.37 'Maximum Amount' shall mean Thirty Five Million Dollars
       ($35,000,000) or such lesser amount to which the Maximum Amount has then
       been reduced to pursuant to Section 4.5 hereof."

       5.     The following Section 1.44a is hereby added to the Agreement 
immediately after Section 1.44:

              " 1.44a       'Release Reduction Amount' shall, in connection 
       with the release of any Mortgage pursuant to Section 11.4 hereof, mean:
       (a) in the case of a release or discharge of a Mortgage upon any Real
       Estate identified on Exhibit "E" hereto in connection with a sale-lease
       back transaction or mortgage refinancing of such Real Estate by
       Borrowers, fifty percent (50 %) of the proceeds of such sale-lease back
       or refinancing, net of fees, commissions and other direct expenses paid
       by Borrowers in connection with such transaction, and (b) in the case of
       any other release or discharge of a Mortgage, the greater of the
       appraised value of the Real Estate for which such release is required or
       one hundred percent (100%) of the proceeds of the transaction for which
       such release is requested, net of fees, expenses, commissions and other
       direct expenses of such transaction paid by Borrowers.'
                        
       6.     Exhibit "E" is hereby added to the Agreement as Exhibit "E" to 
the Agreement.

       7.     Section 1.46 of the Agreement is hereby amended and restated as 
follows:

              " 1.46 'Revolving Credit Facility Fee' shall mean the facility 
       fee payable to Bank pursuant to Section 2.4 hereof, in an amount equal to
       three-quarters (3/4%) per annum multiplied by the Revolving Credit 
       Maximum from time to time in effect."
        
       8.     Section 1.47 of the Agreement is hereby amended and restated as 
follows: 

              "1.47 'Revolving Credit Maturity Date' shall mean December 31, 
       1997."

       9.     Section 1.48 of the Agreement is hereby amended and related to 
read:


              "l.48 'Revolving Credit Maximum' shall mean, as of any date, 
       Twenty Million Dollars ($20,000,000) or such lesser amount to which the
       Revolving Credit Maximum has then been reduced to pursuant to Section 
       4.5 hereof."

       10.    The second sentence of Section 2.5 of the Agreement is hereby 
amended to read:




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              "Thereafter, Advances shall be available to satisfy Borrower's 
       working capital needs as and when such working capital needs arise."

       11.    The second sentence of Section 3.1 of the Agreement is hereby 
amended and restated as follows:

              "Notwithstanding anything to the contrary herein: (a) Letters of
       Credit which are standby letters of credit may not be requested
       hereunder after July 25, 1997; and (b) each Letter of Credit shall
       provide that it is available by drafts drawn at sight and/or
       presentation of documents and shall have an initial expiration date (i)
       not later than one (1) year from the date of its issuance, and (ii) in
       the case of any Letter of Credit which is not a standby letter of
       credit, not later than the Revolving Credit Maturity Date."
                
       12.    Section 3.3 of the Agreement is hereby amended and restated as 
follows:

              "3.3   Letter of Credit Fees. Borrowers jointly and severally 
       agree to pay Letter of Credit Fees to Bank (i) upon the date of
       issuance, in the case of Letters of Credit that are trade letters of
       credit, in an amount equal to two and one-half percent (2.5 %) on the
       face amount of such Letters of Credit, and (ii) with respect to Letters
       of Credit which are standby letters of credit, quarterly in arrears, an
       amount equal to two and one-half percent (2.5 %) per annum multiplied by
       the average outstanding face amounts of such Letters of Credit
       outstanding during the quarterly period then ended."
        
       13.    Section 4.5 of the Agreement is hereby amended and restated as 
follows:

              "4.5   Reductions or Terminations of Commitments.

              "(a)   Scheduled Reductions. The Maximum Amount and the Revolving
                     Credit Maximum then in effect shall each be reduced (i) 
                     by the amount of Five Million Dollars ($5,000,000) on 
                     December 15, 1997, (ii) by the amount of Five Million, 
                     Dollars ($5,000,000) on December 22, 1997, (iii) by the 
                     amount of Six Million Dollars ($6,000,000) on December 29,
                     1997, and (iv) to Zero Dollars ($0) on the Revolving 
                     Credit Maturity Date.

              "(b)   Mandatory Reductions upon Releases. Upon each release of a
                     Mortgage pursuant to Section 11.4 hereof the Maximum Amount
                     and the Revolving Credit Maximum shall each be reduced by 
                     the Release Reduction Amount for the relevant Real 
                     Property.

              "(c)   Discretionary Reductions. Upon five (5) Business Days prior
                     written notice to Bank, the Borrowers may permanently 
                     reduce the Maximum Amount and/or Revolving Credit Maximum,
                     in whole or part, provided that each partial reduction of 
                     the Maximum





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                Amount and/or Revolving Credit Maximum shall be in an amount
                equal to One Million Dollars ($1,000,000).

          "(d)  Requirements for all Reductions. Upon each reduction in the
                Maximum Amount and/or Revolving Credit Maximum under this
                Section 4.5:

                "(i)   in the case of a reduction of the Revolving Credit
                       Maximum Borrowers shall, on the date of such reduction,
                       pay to Bank, (x) the Revolving Credit Facility Fee 
                       accrued on the amount of the Revolving Credit Maximum so
                       reduced through the date of such reduction and (y) the
                       amount (if any) by which the sum of Advances then
                       outstanding plus the amount of outstanding face amounts
                       Letters of Credit which are standby letters of credit 
                       would exceed the Revolving Credit Maximum (as so reduced)
                       together with accrued interest thereon, and

                " (ii) in the case of a reduction of the Maximum Amount, the
                       Borrowers shall deliver to Bank, as collateral for the
                       Borrowers' indebtedness and obligations hereunder (and
                       hereby grants and assigns to Bank a security interest
                       therein) cash and/or cash equivalents in amounts equal to
                       the amount by which the aggregate undrawn face amount
                       of Letters of Credit outstanding as of the date of such
                       reduction exceeds the Maximum Amount as so reduced."

    14.   Section 9.4 of the Agreement is hereby amended and restated as 
follows:

          "9.4   Financial Covenants. Borrowers will not permit:

          "(a)   the Fixed Charge Coverage Ratio to be less than:

                 "(i)    1.45 to 1.0, as of the last day of the Borrowers' 
                         fiscal quarter ending in August 1997, and

                 "(ii)   1.50 to 1.0, as of the last day of any subsequent 
                         fiscal quarter.

          "(b)   the Debt to Capitalization Ratio, as of the last day of any 
                 fiscal quarter, to be greater than seventy five percent (75 %).

          "(c)   Tangible Net Worth to be less than:

                 "(i)    Eighty Six Million Dollars ($86,000,000) as of the 
                         last day of the Borrowers' fiscal quarter ending in 
                         August 1997;
                                           



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                 "(ii)  Seventy Three Million Dollars ($73,000,000) as of the 
                        last day of any subsequent fiscal quarter."

           "(d)  EBITDA, as of the last day of any fiscal quarter, to be less 
                 than Thirty Million Dollars ($30,000,000)."

    15.    Section 11.4 of the Agreement is hereby amended and restated as 
follows:

           "11.4 Releases of Real Property. Provided that, upon such release and
    after giving effect thereto, (i) no Default or Event of Default exists or 
    will exist, (ii) the Collateral Amount for Real Property remaining in the
    Collateral Pool will not be less than the greater of the Maximum Amount (as
    reduced in connection with such release pursuant to Section 4.5 hereof) or
    the aggregate outstanding amount of all Advances and Letters of Credit, and
    (iii) the aggregate outstanding amount of Advances and Letters of Credit
    which are standby letters of credit will not exceed the Revolving Credit
    Maximum (as reduced in connection with such release pursuant to Section 4.5
    hereof), Bank shall release Real Estate from its Mortgage upon the
    consummation by Borrowers of a sale leaseback transaction or mortgage
    refinancing involving such Real Property provided that, simultaneously with
    such release, the requirements of Section 4.5 of this Agreement are
    complied with."
        
C.  REPRESENTATIONS

    Borrowers hereby represent and warrant that:

    1.     Execution, delivery and performance of this Amendment and any other 
documents and instruments required under this Amendment or the Agreement are
within their respective corporate powers, have been duly authorized, are not in
contravention of law or the terms of their respective Articles of Incorporation
or Bylaws, and do not require the consent or approval of any governmental body,
agency, or authority.
        
    2.     This Amendment, and the Agreement as amended by this Amendment, and 
any other documents and instruments required under this Amendment or the
Agreement, when issued and delivered under this Amendment or the Agreement,
will be valid and binding in accordance with their terms.
        
    3.     The continuing representations and warranties of the Borrowers set 
forth in Sections 7.1 through 7.6 and 7.8 through 7.13 of the Agreement
are true and correct on and as of the date hereof with the same force and
effect as made on and as of the date hereof.
        
    4.     The continuing representations and warranties of the Borrowers set 
forth in Section 7.7 of the Agreement are true and correct as of the date
hereof with respect to the most recent financial statements furnished to Bank
in accordance with Section 8.1 of the Agreement.
        




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       5.      No Event of Default, or condition or event which, with the 
giving of notice or the running of time, or both, would constitute an Event of
Default under the Agreement, has occurred and is continuing as of the date
hereof.
        
D.     MISCELLANEOUS

       1.     This Amendment may be executed in as many counterparts as Bank 
and the Borrowers deem convenient and shall become effective upon: (a) delivery
to Bank of all executed counterparts hereof; and (b) delivery to Bank, in form
and substance satisfactory to Bank of each of the documents and instruments
listed on the Checklist attached as Exhibit "A" hereto.
        
       2.     Borrowers and Bank acknowledge and agree that: (a) except as 
specifically amended hereby, all of the terms and conditions of the Agreement
and the Documents, remain in full force and effect in accordance with their
original terms; (b) the Agreement (as amended by this Amendment) and the other
Documents constitute a complete integration of the terms and conditions with
respect to the extensions of credit by Bank to Borrowers and the other
transactions described therein; and (c) no amendment to or deviation from the
terms and conditions set forth in the Agreement (as amended by this Amendment)
and Documents shall be effective or binding upon Bank unless set forth in a
duly executed writing signed on behalf of the Borrowers and Bank.
        
       3.     Borrowers shall pay all of Bank's legal costs and expenses 
(including attorneys' fees and expenses) incurred in the negotiation,
preparation and closing hereof, including, without limitation, costs of all
lien searches and financing statement filings.
        
       4.     Except as specifically set forth herein, nothing set forth in 
this Amendment shall constitute, or be interpreted or construed to constitute,
a waiver of any right or remedy of Bank, or of any default or Event of Default
whether now existing or hereafter arising.
        






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              WITNESS the due execution hereof as of the day and year first 
above written.




GENERAL HOST CORPORATION                            COMERICA BANK


By:  Robert M. Lovejoy, Jr.                       By:  Chris Georvassilis
    ------------------------                          ------------------------
      Robert M. Lovejoy, Jr.                            Chris Georvassilis
Its:  Vice President                              Its:  Vice President

1 Station Place                                   500 Woodward Avenue, M.C. 3280
Stamford, Connecticut 06902                       Detroit, Michigan 48226
Telephone: (203) 357-9900                         Telephone: (313) 222-6239
Telefax: (203) 357-0148                           Telefax: (313) 222-3330



FRANK'S NURSERY & CRAFTS, INC.


By:  Robert M. Lovejoy, Jr.
    -----------------------------       
       Robert M. Lovejoy, Jr.
Its:   Vice President

6501 East Nevada
Detroit, Michigan 48234
Telephone: (313) 366-8400
Telefax: (313) 564-2084





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                                 EXHIBIT "A"



                             SECOND AMENDMENT TO
                      MORTGAGE-BACKED CREDIT AGREEMENT
                    BETWEEN GENERAL HOST CORPORATION AND
                     FRANK'S NURSERY & CRAFTS, INC.  AND
                                COMERICA BANK



                                July _, 1997



1.     Recertification of Authority Documents


       a.   General Host Corporation
       b.   Frank's Nursery & Crafts, Inc.

2.     Second Amendment to Mortgage-Backed Credit Agreement

       a.   Exhibit "A" - Checklist
       b.   Exhibit "E" - Specified Real Estate

3.     Acknowledgement and Consents

       a.   General Host Holding Corp.
       b.   AMS Industries, Inc.
       C.   AMS Salt Industries, Inc.
       d.   Bay Resources, Inc.
       e.   Nursery Distributors, Inc.








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                                 EXHIBIT "E'



 Store #                   City
 -------                   ----

    5                      Dearborn Heights, MI
   29                      Fort Wayne, IN
   45                      Bloomington, MN
  102                      Philadelphia, PA
  112                      Blaine, MN
  117                      Tampa, FL
  181                      Norton Shores, MI
  189                      Toledo, OH
  266                      Cincinnati, OH
  272                      Westfield, MA
  622                      Milford, CT
  624                      Kenvil, NJ
  648                      Brockton, MA
  649                      Hadley, MA
  652                      Springfield, MA





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                         ACKNOWLEDGEMENT AND CONSENT



      Each of the undersigned hereby acknowledges and consents to the 
execution, delivery and performance of that certain Second Amendment to
Mortgage-Backed Credit Agreement dated as of even date herewith by GENERAL HOST
CORPORATION and FRANK'S NURSERY & CRAFTS, INC. (collectively the "Companies")
and COMERICA BANK, ("Bank"), amending that certain Mortgage-Backed Credit
Agreement dated as of November 29, 1996 between Companies and Bank (the
"Agreement").
        
        This acknowledgement and consent is executed and delivered by the under-
signed in connection with the Guaranty from the undersigned in favor of Bank 
and delivered in connection with the Agreement, which Guaranty remains in full 
force and effect with respect to indebtedness now or hereafter incurred under 
the Agreement, including, but not limited to, the Note.
        
        Capitalized terms used herein and not defined to the contrary have the
meanings given them in the Agreement.

        Executed as of the 25th day of July, 1997.
                                                                       


                                                GENERAL HOST HOLDING CORP.

                                                By:  Robert M. Lovejoy
                                                    -------------------------   

                                                Its: Vice President & Treasurer
                                                    ---------------------------
                                                
                                                AMS INDUSTRIES, INC.

                                                By:  Robert M. Lovejoy
                                                    -------------------------   

                                                Its: Vice President & Treasurer
                                                    ---------------------------
                                                
                                                AMS SALT INDUSTRIES, INC.

                                                By:  Robert M. Lovejoy
                                                    -------------------------   

                                                Its: Vice President & Treasurer
                                                    ---------------------------

                                                BAY RESOURCES, INC.

                                                By:  Robert M. Lovejoy
                                                    -------------------------   

                                                Its: Vice President & Treasurer
                                                    ---------------------------

                                                NURSERY DISTRIBUTORS, INC.      


                                                By:  Robert M. Lovejoy
                                                    -------------------------   

                                                Its: Vice President & Treasurer
                                                    ---------------------------