I wanted to intentionally reference a famous Dylan Thomas poem in a poem of my own. I wondered whether there is a convention for how to do this or if it is 'allowed' at all.
rolfedh's concise answer seems sufficient, but the following long answer may add some value.
The question of how to quote another's work in one's poetry has two components: how the quotation should be demarcated and how attribution should be expressed.
Demarcation of Quotations
Using quotation marks is the most straightforward way of indicating a quotation and is the most appropriate for expressing a distinction between the quoted material and the statements of the poem's speaker. This separation may be desired when the quoted text is argued against, used an appeal to authority, or set up for more abstract consideration. Quotation marks, like other punctuation, can also influence the pacing of the poem, slowing the reader. This slowing is desirable if one wishes the reader to consider the quote briefly and natural when placed at an intended pause but would be particularly problematic in the middle of an intense, rapid flow.
Italics provide a subtler demarcation. Italics are commonly used in fiction to distinguish thoughts from speech and thus can provide a sense of internal connection. Italics are also more likely to be appropriate for very short quotations (e.g., two words), especially when such are broadly distributed throughout the poem. Italics do introduce the danger of implying emphasis more than just distinction; this is particularly likely for very short quotations. Moderating the implied emphasis can be accomplished by earlier establishing the use (e.g., length or familiarity can establish italicized words as quotes) or ensuring that the tone and flow would not naturally support interpretation as emphasis. Interpretation as mere light emphasis may not detract substantially from the poem since italicized quotations are generally more internalized; the distinction between a moving quotation and the speaker's own passionate thoughts is not critical.
The use of single quotation marks — outside of nested quotations — seems to provide a separation and formality intermediate between italics and double quotation marks. Single quotation marks might be preferred for quoting a folk saying when using such as statement of truth; the folk saying is less formal and likely more internalized than a classic quotation but is meant to have an authority.
Not specifically indicating a quotation is an attractive option when the quotation is familiar, deeply integrated into the speaker's expression, or paraphrased or otherwise not strictly quoted. Allusions and paraphrases (unless the source is in a different language) should not use quotation marks since such indicates precise quotation. Italics or single quotation marks might be acceptable for casual, approximate quotations, especially when the speaker is using such as a reference or the difference from the true quote is being emphasized. Even use of double quotation marks may be appropriate if used as an ironic indication of accuracy, for example:
I learned a lot from Hamlet's solemn plea: "to live a life in tights or not to be"
This mangling might, with adequate context (possibly beyond the "solemn plea"/"soliloquy" wordplay and "life in tights"), be recognized as a humorous misquotation, possibly thought by the speaker to be accurate.
Extensive inclusion of quotations can also argue against specific demarcation; peppering one's poem with quotation marks is more likely to be distracting than edifying. Even moderate but regular use of quotations may justify lack of markings. For example, if the fourth line of every eight line stanza is a quotation, even italics may be too prominent, especially if the intended effect is to subtly unify the quotations and the poem.
Poetry is granted significant license to violate conventional grammar, but this freedom should be used to enhance expression — emotion, context, pacing, art — not as an indulgence for sloppiness.
Attribution of Quotations (and Allusions)
Attributions serve multiple purposes. They endorse the source, hinting that the reader may enjoy the source or other works by the source's author. The enthusiasm of this endorsement influences the reader's perception of the poem. If reader is familiar with the author or the specific source, the reader's positive or negative attachments to that source are linked to the writer of the poem and to the poem itself.
Attributions influence the tone with which the poem is perceived. Using the title "Wonderment: Considerations on Gerard Manley Hopkins' 'The Windhover'" gives a very different impression from "Wonderment¹" with the footnote "¹ Composed after reading Gerard Manley Hopkins 'The Windhover'". Excluding explicit citation can present a subtly intimate tone with readers familiar with the source, like accidentally learning that one shares a special interest with an acquaintance. Leaving out a citation, especially when other works are cited, may be taken as a slight, especially if appreciation is not expressed in the poem or a quotation is easily mistaken as one's own creation.
Attributions also borrow associations from the source or the source's author. Beyond the tone implied in the form — or absence — of citation, an attribution may appeal to authority ("I am only repeating what Homer wrote"), stir affection ("like Elizabeth Barrett Browning, I will speak of love"), or hint of beauty ("John Keats' bright words flow into my not unlovely stream"). Citation emphasizes the connection. This emphasis may seem arrogant, fatuous, reverent, precisely correct, or formally dull, substantially depending on how the relationship to the source is handled in the work.
Finally, attributions give credit for inspiration or content. This formal purpose of attribution is somewhat less a matter of artistic choice; attribution may be required by licensing, publication policies, or mere courtesy. This aspect also often constrains the expression of attribution
The purposes of attributions provide some guidance for how attributions should be handled.
Including attributions in the text of a poem is often contrary to the expression of the poem, potentially drawing the reader out of the text (which is usually undesired). However, such can be natural expression when the quote is presented by the speaker as a quotation, especially when the identification of the source is significant for the speaker. Self-talk and speech among those with common interests typically elides citation, so explicit citation may introduce significant emphasis.
Externalizing the subject in thought or address provides opportunity for citation. "Miss Barrett's words still stand that love, mere love,/is beautiful indeed and worthy of" and "Shakespeare, were you so dull as to believe/that love can be 'an ever fixed mark/That looks on tempests and is never shaken'?" are somewhat natural inclusions of attributions.
Including attribution in the title is less likely to interfere with the poetry and may provide appropriate context for the poem, which is one role of titles anyway. This attribution can be precise and clear ("On Reading Shakespeare's 13th Sonnet"), general and vague ("Byronic Wit"), or even cryptic and playful ("Comfort from Liberty's Poet" — referring to Emma Lazarus' "Assurance").
A common technique for establishing context is to use an opening quotation distinct from the poem. While such can be used merely to provide general context, it can also indicate the source for more subtle references or quotations without attribution within the poem. For example:
Wonderment Brute beauty and valour and act, oh, air, pride, plume, here Buckle! AND the fire that breaks from thee then, a billion Times told lovelier, more dangerous, O my chevalier! Gerard Manley Hopkins "The Windhover"
The use of a footnote attached to the title — if all references are to a single source — is perhaps the least obtrusive of methods of attribution.
More explicit attribution is more important when the quotation may be unfamiliar to the reader or the source less easily discovered via web search. Brief quotes, particularly using common words, quotes which are not explicitly marked as such, and quotes from less prominent sources are more difficult to identify.
Whether attribution should be included is a question similar to whether translation of words from other languages should be included. Attributions are less important for many quotations of classics; the quotation may be broadly recognized or at least the source be easily discovered, just as one would typically not need to translate mon ami or sic transit gloria mundi. Unlike translation, attribution involves credit and, sometimes, endorsement (in either direction) and not just identification. In addition, like translation, the broader one's readership in time or culture, the more helpful clear attribution becomes.
Publication Context and Attribution
Publication context significantly influences the form that attribution can and should take when it is not integrated into the poem or its title. On one's personal website, reduced space and formatting constraints facilitate extensive notes on works: inspirations, artistic critiques, personal significance, etc. Using links, hide/show buttons, or sidebars (when targeting wider displays) can keep extensive comments from distracting from the poetry itself.
For one's own printed publication, while length is only money and formatting is limited only by the medium, one still should consider the effect on the reader. For a poem meant only to be read and enjoyed, one would prefer not to draw attention away from the text. Attaching a footnote to the title of the text would be preferred for this use; e.g., [title]'Feeling the D.T.s°', [at the foot of the page]'° Quotations and references are from Dylan Thomas' ""'. In a scholarly presentation, drawing attention away from the text on the first reading is unexceptional.
On a poetry sharing forum, length is usually not a significant constraint, but formatting often prevents reducing the prominence of commentary. Such contexts often also lack support for internal linking. For shortish single poem postings, end-of-post comments have sufficient proximity to the relevant text of a poem that this is not especially problematic. For longer poems, the use of unlinked endnotes may be the best choice, with endnote references and clarifications placed before more extensive and more general commentary.
For a printed poetry journal, space, pagination, and formatting style significantly restrict the length and placement of comments. On the other hand, journals may have formal policies about attribution. If all the quotations are from a single text or a single author, one attributing footnote for the first quotation or attribution in the title may suffice.
An online poetry journal, while free from most physical constraints, will have stylistic formatting constraints and expectations about attribution.
Poetry journals, being more formal, are likely to have established policies with respect to attribution. If such policies are not found or are unclear, a brief email exchange can answer questions about a journal's policies and preferences.
Consider the following fragment:
My case I thus did firmly press; She questioned then my eagerness, "The grave's a fine and private place, But none I think do there embrace?" I answered with my bounded wit, "If you could make our sun but sit, I would not love at lower rate. I can but on your bidding wait. Time’s wingèd chariot hurrying near Should never be a cause for fear; Beyond all hasty worldly strife, Past heaven's golden gate of life, Gardens of vast eternity Can but enshrine your rich beauty. Yet love in truth can know a haste Which would see no time left to waste."
This is so crammed with quotes and references to Andrew Marvell's famous "To His Coy Mistress" that no one would think the references accidental, incidental, or mere plagiarism. ("The grave’s a fine and private place,/But none, I think, do there embrace."; "Thus, though we cannot make our sun/Stand still, yet we will make him run."; "Nor would I love at lower rate."; "Time’s wingèd chariot hurrying near;"; "Through the iron gates of life:"; "Deserts of vast eternity.") Because Marvell's poem is so well known and the references so extensive — though not marked as such — explicit attribution is less necessary, but the title could easily provide attribution — e.g., "After Sharing 'To His Coy Mistress'" (which sets the context and explains the density of the references for both speakers as well as clearly providing attribution).
Note that none of the quotes are marked by quotation marks or italics. Quotation marks would have been awkward given the quoting of speech in the fragment. In addition, the conversational and somewhat playful tone make formal quoting less appropriate. This integration also points to a natural use of poetic references for the speakers. If an argument rather than an appeal was being made, marking quotations would give some sense of appeal to an authority.
The next fragment uses italics:
*But that which fairest is, but few behold,* Why then should I be granted this bright gold Whose heart is iron and clay, all intermixed?
Using italics to mark the quotation (from Edmund Spencer's, Amoretti, Sonnet 15), presents it as a remembrance in the speaker's thoughts. The use of quotation marks would have introduced a greater sense of distance ([it is said that] "..."); italics present the quote as an intimate, tender heart-truth. The length (and demarcation) of the quote make web search easy, but the reference is probably too obscure (even if the work was a Spencerian sonnet) for attribution to be omitted. In some publication contexts a brief side note, likely in a reduced font, would be sufficient: (Amoretti). A title such as "I am not Edmund Spencer" might be sufficient attribution while also providing the usual function of a title.
Here is another example of using italics:
Now steadfast husband, fate-tried hero stand Be *strong in will to strive, to seek, to find,* *and not to yield* nor quarter ever grant
Italics are used in this case to avoid the disruption of pacing quotation marks would introduce while also indicating the inclusion of a quotation. The poem would presumably make other references to Alfred Lord Tennyson's "Ulysses" and use that connection to enhance the tone, so denoting such references is important to the expression.
In this case, using a fully attributed quotation from the source before the poem text would probably be most appropriate for communicating attribution. Such establishes some context and marks the significance of that source to the poem's text. This can be especially powerful and clear when the quotation preceding the text is partially repeated in the poem text, assuming sufficient separation that the reuse is subtly resonant rather than dully repetitive. While a title such as "Encouragement from Tennyson's 'Ulysses'" would also work, a preceding quotation gives greater significance to the source as an inspiration and as a great work. The latter significance may also be added in part to the poem itself.
The following example uses quotation marks:
"What oft was thought," but that is not quite true; The author must be given her full due. ... The nature of a few might so be dressed, But love, before, was "ne'er so well expressed."
The quotation in the first line is clearly presented as both authoritative ('it is well known that') and distinct. Using italics would have given such a softer tone (both less distinct and less argumentative).
The quotation in the last line forms a closing agreement and intensification of the body. By closing the work with the latter half of the quotation, the writing experiences a compression, as if the intervening lines were but a light digression on a well-established truth. Beginning with "Alexander Pope did not go far enough in this case when he wrote 'True wit is nature to advantage dressed/What oft was thought but ne'er so well expressed.'" and ending with "Nevertheless, Pope rightly states that extraordinary expression of meaning, as exemplified in our subject, distinguishes the excellent poem." — this parenthesis gives a sense of weight to the internal content.
The lesser familiarity of the source also urges the use of quotation marks. The quote is sufficiently unique that explicit attribution could be avoided. (On my website, attribution is provided in the comments on the work.) Internal attribution of the quote would have weakened the presentation both by length and by lessening such to merely one source indicating a truth rather than a bald statement of fact. Poetry often favors vigor over rigor.
This fragment inlines explicit attribution and uses quotation marks:
The Bard wrote well and true: "love is not love Which alters when it alteration finds". Affection like a spring-clock downward winds, But true love knows no stay nor stop. Above All movements of the human heart its end Is set eternally and will not bend.
In this case, the speaker is using the quotation as a well-known, authoritative statement. The explicit internal attribution contributes to the argument and the demarcation sets apart the quote as a mathematician might set apart the statement of a base theorem before presenting proofs of its derived lemmas.
This fragment uses no demarcation of the quotation:
To be, or not to be, I will not ask; Outrageous fortune's not my enemy. Though troubles rage about me like the sea, I will not turn from my appointed task.
Because the source is extremely well known, at least for "To be, or not to be", attribution is unnecessary. Since the quotes are short (and intermingled with the text), demarcation of the quotes would be more distracting than helpful. The lack of demarcation also personalizes the quotations. Formal quotation would be awkward, even silly:
"To be, or not to be," I will not ask; "Outrageous fortune"'s not my enemy. Though "troubles" rage about me like the "sea", I will not turn from my appointed task.
The following example likewise lacks demarcation:
I doubt and hope: Is love as strong as death? And oaths of love, do they retrieve as well?
The work's title, "Reading Sonnets after Loss", hints at the source (Elizabeth Barrett Browning's Sonnets from the Portuguese, specifically Sonnet 27 — "That Love, as strong as Death, retrieves as well.") but does not declare it. The attribution of the reference is important to more fully appreciating the work, and a web search is more likely to link to Song of Songs. However, any rabid fan of Sonnets from the Portuguese — for whom the significance would be greater — would recognize the reference and feel a more intimate appreciation.
Attribution does not add substantially to the emotion of this work, particularly if the reader does not have a fondness for Sonnets from the Portuguese. I do include attribution on my website both because I encourage people to read Sonnets from the Portuguese and because I generally provide some commentary for better understanding or appreciating the work.
In this case explicit demarcation, besides being somewhat awkward, would mar the sad but strong tone and distance the words that she treats as her lover's own, bound deeply in her heart.
The manner in which quotations are indicated influences the formality, importance, and separation from the speaker with which they are perceived. Poetry is allowed to violate conventions for the sake of emotional or artistic expression, but conventions establish expectations and influence the reader's perception.
Attribution is not always necessary but increases in importance based on expected unfamiliarity, which is influenced by the breadth of the expected audience. The form that attribution takes influences the reader's perception of the poem; attributions can flow naturally in the poem itself, but often such would detract from the poem and be better placed in the title or a footnote.