Yes, plagiarism includes the use of material that has been copied without citing the source. This applies to the methods section as well as to any other section.
In general, copying text, even from one of your own previously published articles, without citing the source is a violation of publication ethics, as it amounts to self-plagiarism. Similar to plagiarism of another’s work, self-plagiarism included, if discovered after publication, may result in retraction.
Studies examining the extent to which editors rely on plagiarism detection software show that they very rarely rely solely on plagiarism detection software to classify a manuscript as suspected of plagiarism. Editors consider the results of the software, but for the most part, also rely on their own judgement. These studies and general discussion forums in which CrossCheck members participate suggest that editors tend to be more tolerant of textual similarities in the methods section because most original studies use previously described methods.
However, the consensus is that it is still essential to cite the article in which the methods were first described. This is true even for frequently used methods
If you submit two manuscripts based on the same method, you must do the following to protect against self-plagiarism:
- Make sure that the studies are different enough to warrant two separate publications without this being considered a salami-slicing tactic.
- Make sure that in both manuscripts you cite the original study in which said method was first described (I am assuming here that the method itself was not developed by you and has been described previously).
- Point out to the editor in the cover letter that the duplication of content in the methods section is intentional, since you have replicated a previously published method.
- If one manuscript is published earlier than the other, the second manuscript should cite the first and indicate that the methods were previously described in one of your own papers.
- If both manuscripts are submitted around the same time, the cover letter for each manuscript should indicate that another study using the same methods is under consideration by a different journal.