I am not on top with my Sindarin phonetics. Would a causative (luñgu-tā) 'make heavy' would result in a different form from an inceptive luñgu-tă 'be made heavy, become heavy'? (PE22:157)  - or would they collide in an identical form?

+Tamas Ferencz My mistake, of course I meant it short. But it seems that the derivation of verbs from adjectives can perform two ways:
• with ta in the inceptive meaning 'become smth.'
• with causative tā in the meaning 'make smth.' (PE22:135). Tolkien seems to play with the idea that inceptives are made with -s forms, but in 1969 materials we get, for example, Q. ninquita meaning both 'to make white' and 'to become white', coalescing in all itw Eldarin derivatives. In the 'Common Eldarin' concept, the 'become heavy' idea would rather result in luñgusjā form, which in S. would yield I'm unsure what exactly!
UPDATE: *lœngœchia, if not lengeichia, apparently.

Couldn't you express this as "grow heavy" hence gala long "grow heavy" or even tuia long "swell heavy"?

+Leonard W. gala is used of plants only. Ola lung < olā lungu 'become heavy', if to pursue this line?

I'm fairly sure that Tolkien used 'al in the context of becoming, but I can't recall if it is a phrase in PE17, PE22 or VT50. I'll have a look.

But then, maybe 'ala isn't derived from the same root as gala-? I honestly dont' know much about derivation.

How did you arrive at olā?

Ah, OL "grow" [PE22:103]. :) It seems to me to be the best way to go. I'm still hunting for that other quote, though. 

+Leonard W. If there were any associations of al with 'becoming' (as there definitely were with 'goodness'), they are all obsoleted by selecting al for negation. CE verb galā is derived from the stem GAL and would indeed yield ala in Quenya, but that does not shift the 'plant' concept.
olā is described, for example, at PE22:134.

Rather contradictory to my own statement, I see PE17:153 that it is explicit, as you said, "of vegetable growth only."  My mistake.

OL seems to be the better way to go.

+Leonard W. If we address to the 'Common Eldarin: Noun Structure', we find that Tolkien explicitly separated the two roots, OL for 'become' and GAL (as in PE17) for 'grow'. Curiously, the concept of s-inceptives from this work is absent from any earlier or later work, and in 1969 Tolkien readily accepts ninkwita for both 'become white' and 'make white' with only different conjugation of ninkwinten inceptive against ninkwitanen causative.

+Александр Запрягаев Interesting to be sure. I suppose the verb would be contextual, but I can imagine it becoming confusing if you're not careful.  :)

Out of curiosity, what would OL become in Sindarin? ol- > elin "I [am] becoming"?

+Leonard W. That's an inherently continuous verb, so I'd prefer not to make aorists and settle on olad, olon, olanthen (úlen?), olathon, olol, úliel (eliel?), olanthen.

All right! That makes sense. Thank you!

+Александр Запрягаев
"Curiously, the concept of s-inceptives from this work is absent from any earlier or later work"

I wonder if the unglossed tankarya (PE17:76) would beg to differ, though that's from arya and probably the 1950s.