"Plants are growing from the earth."
Translation:El la tero kreskas plantoj.
Possession is just one use of de.
A bit like "of" in English - in "the son of my mother's brother", it shows possession, while in "lots of love" or "he is afraid of the dark" or "because of the rain", it does not.
The basic difference between de and el as spatial prepositions is that el is "from out of" while de is more generally "from", or perhaps "from next to".
What do you mean with "they are to X"?
Do you mean obligations, as in "They are to read this book by tomorrow and write an essay on it" ?
No, that doesn't work with "ili estas legi" or the like - you need something like "ili devas legi" (they have to read....) or "Oni ordonis al ili legi" (one has ordered them to read) or similar.
And future plans such as "They are to build a new hospital there next spring" would be "Ili konstruos novan malsanulejon tie la venontan printempon" with plain future (konstruos = will build).
Or do you mean something else?
Esperanto verbs are the same in the singular and the plural. So "My plant grows" is "Mia planto kreskas", and "My plants grow" is "Miaj plantoj kreskas". The verb is "kreskas" in both cases. The only exception to this is in compound verb forms such as "estis krekanta" ("was growing"), which if the subject were plural, would be "estis kreskantaj".
I think "could be" would be more appropriate, because the word order can be varied in Esperanto without changing the meaning, so "Plantoj kreskas el la tero", "El la tero kreskas plantoj", "Plantoj el la tero kreskas" and "El la tero plantoj kreskas" all mean the same thing. Of course there are limits to this freedom of word order - we couldn't for instance say, "La kreskas el tero plantoj".