This root and ones like it were connected to flowers for all of Tolkien’s life. The earliest manifestation of this root was ᴱ√LOHO or ᴱ√LO’O from the Qenya Lexicon of the 1910s; the entry for ᴱ√LOHO appears immediately below ᴱ√LO’O, and Tolkien indicates they are related roots, both extended from ᴱ√OLO “tip” (QL/55). These roots include derivatives like ᴱQ. lōte “flower”, ᴱQ. lotōrea “flourishing” and ᴱQ. lokta- “sprout, bud, put forth leaves or flowers”. There are also derivatives of these roots in the contemporaneous Gnomish Lexicon: G. lost “blossom, bloom”, G. lothli “floret”, G. luitha- “to bloom” (GL/54-55), though G. lôs “flower” was said to be unrelated, connected to G. lass “leaf” instead (GL/55). ᴱQ. losse “rose” probably had a similar derivation (QL/56).
This confusion of √LOT(H) and √LOS carried forward into Tolkien’s later writings. In The Etymologies of the 1930s ᴹ√LOT(H) was given as the root for “flower” (Ety/LOT(H)), but this entry originally included a variant ᴹ√LOS (EtyAC/LOT(H)). Tolkien then said ᴹQ. losse “blossom” (< ᴹ√LOT(H)) was “usually, owing to association with olosse snow, only used of white blossom” (Ety/LOT(H)), where ᴹQ. olosse was derived from ᴹ√GOLOS “✱snow, white” (Ety/GOLÓS). This intermingling carried forward into etymological notes from the late 1950s or early 1960s, where Tolkien said (PE17/26):
> The stems √LŎS, √LOTH, √LOT are much entangled both for formal reasons, and because of actual associations of meaning (probably from beginning of Primitive Quendian and explaining the approach of the forms). Quenya word for “flower, a single bloom” is lóte, but S loth (< lotho/a), but Quenya also has lōs. Q. for snow is losse (S los).
These associations were also mentioned in etymological notes on roots for flowers from this same period, where Tolkien clarified that √LOT, √LOTH were the roots for “flower” and √LOS for “snow” (PE17/160-161). These roots were mentioned again in notes associated with The Rivers and Beacon-hills of Gondor from the late 1960s (VT42/18):
> S. loss is a derivative of (G)LOS “white”; but loth is from LOT. Sindarin used loss as a noun, but the strengthened form gloss as an adjective “(dazzling) white”. loth was the only derivative of LOT that it retained, probably because other forms of the stem assumed a phonetic shape that seemed inappropriate, or were confusible with other stems (such as LUT “float”), e.g. ✱lod, ✱lûd. loth is from a diminutive lotse and probably also from derivative lotta-.
In this last note, Tolkien seems to have abandoned √LOTH, explaining S. loth “flower” as derived from √LOT via ✱lotse. In any case, starting in the 1930s Tolkien was consistent that the roots for “flower” and “snow” were distinct but often confused, and that snow-words were derived from roots like √(G)LOS and flower words from roots like √LOT(H), though he waffled a bit on the exact details.
This root appeared in The Etymologies of the 1930s as ᴹ√ROS “distil, drip” with derivatives like ᴹQ. rosse “fine rain, dew” and N. rhoss “rain”, the latter an element in N. Celebros “Silver-rain” (Ety/ROS¹). In later versions of The Silmarillion, the name S. Celebros was translated “Foam-silver” (WJ/151), indicating a shift in meaning, though the element still meant “rain” in other later names like S. Silivros “Sparkling Rain” (MR/155) and S. Dimrost “Rainy Stair” (S/220).
Tolkien discussed this root at length in a 1968 essay labeled The Problem of Ros (PM/367-371). He indicated that by this point, the intended meaning of the root √ROS was “spindrift, spray”, but he felt this meaning was problematic due to its conflict in Sindarin with S. ross “red haired” and the similarity of the root to Latin “rōs” = “dew” (PM/368). He then launched into a lengthy discussion formulating a new theory whereby the element -ros was actually Beorian to explain its use in various Beleriandic names, only to remember at the last minute that he had also used this element in the Third Age place name S. Cair Andros “Ship of Long Foam”, rendering his alternate theories unviable (PM/371). Presumably at this point the original meaning of the root was restored.