UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): November 6, 2015

PLUM CREEK TIMBER COMPANY, INC.

(Exact Name of Registrant as Specified in Charter)

(206) 467-3600

Registrant’s Telephone Number, including area code

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Item 8.01. Other Events.

On December 10, 2015, Weyerhaeuser Company filed a Registration Statement on Form S-4 (which contains a preliminary joint proxy statement/prospectus) (the “Registration Statement”) in connection with the proposed transaction, as contemplated by the Agreement and Plan of Merger, dated as of November 6, 2015, by and between Weyerhaeuser Company and Plum Creek Timber Company, Inc.

The Registration Statement may be found on the SEC’s website which is located at www.sec.gov, under the name or “Weyerhaeuser Company.” The information contained therein is subject, in its entirety, to completion or amendment as described therein.

Forward Looking Statements

This document contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, as amended. Some of these forward-looking statements can be identified by the use of forward-looking words such as “believes,” “expects,” “may,” “will,” “should,” “seeks,” “approximately,” “intends,” “plans,” “estimates,” “projects,” “strategy,” or “anticipates,” or the negative of those words or other comparable terminology.

The accuracy of such statements is subject to a number of risks, uncertainties and assumptions including, but are not limited to, the following factors: (1) the risk that the conditions to the closing of the transaction are not satisfied, including the risk that required approvals from the shareholders of Weyerhaeuser or Plum Creek for the transaction are not obtained; (2) litigation relating to the transaction; (3) uncertainties as to the timing of the consummation of the transaction and the ability of each party to consummate the transaction; (4) risks that the proposed transaction disrupts the current plans and operations of Weyerhaeuser or Plum Creek; (5) the ability of Weyerhaeuser and Plum Creek to retain and hire key personnel; (6) competitive responses to the proposed transaction; (7) unexpected costs, charges or expenses resulting from the transaction; (8) potential adverse reactions or changes to business relationships resulting from the announcement or completion of the transaction; (9) the combined companies’ ability to achieve the growth prospects and synergies expected from the transaction, as well as delays, challenges and expenses associated with integrating the combined companies’ existing businesses; and (10) legislative, regulatory and economic developments. These risks, as well as other risks associated with the proposed transaction, will be more fully discussed in the joint proxy statement/prospectus that will be included in the Registration Statement on Form S-4 that will be filed with the SEC in connection with the proposed transaction.

In addition, actual results are subject to other risks and uncertainties that relate more broadly to Plum Creek’s overall business, including those more fully described in Plum Creek’s filings with the Securities and Exchange Commission (“SEC”) including its annual report on Form 10-K for the fiscal year ended December 31, 2014, and its quarterly reports filed on Form 10-Q for the current fiscal year, and Weyerhaeuser’s overall business and financial condition, including those more fully described in Weyerhaeuser’s filings with the SEC including its annual report on Form 10-K for the fiscal year ended December 31, 2014, and its quarterly reports filed on Form 10-Q for its current fiscal year. Forward looking statements are not guarantees of performance, and speak only as of the date made, and neither Plum Creek nor its management undertakes any obligation to update or revise any forward-looking statements.

Additional Information And Where To Find It

This document does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities

laws of any such jurisdiction. The proposed transaction will be submitted to the shareholders of each of Plum Creek and Weyerhaeuser for their consideration. Weyerhaeuser will file with the SEC a Registration Statement on Form S-4 that will include a joint proxy statement/prospectus of Plum Creek and Weyerhaeuser. Each of Plum Creek and Weyerhaeuser will provide the joint proxy statement/prospectus to their respective shareholders. Plum Creek and Weyerhaeuser also plan to file other documents with the SEC regarding the proposed transaction. This document is not a substitute for any prospectus, proxy statement or any other document which Plum Creek or Weyerhaeuser may file with the SEC in connection with the proposed transaction. INVESTORS AND SECURITY HOLDERS OF PLUM CREEK AND WEYERHAEUSER ARE URGED TO READ THE JOINT PROXY STATEMENT/PROSPECTUS AND ANY OTHER RELEVANT DOCUMENTS THAT WILL BE FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. You may obtain copies of all documents filed with the SEC regarding this transaction, free of charge, at the SEC’s website (www.sec.gov). In addition, investors and security holders will be able to obtain free copies of the joint proxy statement/prospectus and other documents filed with the SEC by the parties on Plum Creek’s Investor Relations website (www.plumcreek.com/investors) (for documents filed with the SEC by Plum Creek) or on Weyerhaeuser’s investor relations page on its corporate web site (www.weyerhaeuser.com) (for documents filed with the SEC by Weyerhaeuser).

Participants in the Solicitation

Plum Creek, Weyerhaeuser, and certain of their respective directors, executive officers and other members of management and employees, under SEC rules may be deemed to be participants in the solicitation of proxies from Plum Creek and Weyerhaeuser shareholders in connection with the proposed transaction. Information regarding the persons who may, under the rules of the SEC, be deemed participants in the solicitation of Plum Creek and Weyerhaeuser shareholders in connection with the proposed transaction will be set forth in the joint proxy statement/prospectus when it is filed with the SEC. You can find more detailed information about Plum Creek’s executive officers and directors in its definitive proxy statement filed with the SEC on March 26, 2015. You can find more detailed information about Weyerhaeuser’s executive officers and directors in its definitive proxy statement filed with the SEC on April 1, 2015. Additional information about Plum Creek’s executive officers and directors and Weyerhaeuser’s executive officers and directors can be found in the above-referenced Registration Statement on Form S-4 when it becomes available.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

DATED: December 10, 2015