I would like to know what it means to say "lia avino sxatas kreski plantojn" compared to this sentence.
Also, it would mean that his grandmother likes herself to get bigger, and she is somehow doing that to plants.
(If I'm correct) -- this is an ambiguity that is possible and common in English and impossible in Esperanto: "His grandmother likes growing plants" means either that she likes plants that are growing, "lia avino sxatas kreska plantojn" or that she likes the activity of growing plants, "lia avino sxatas kreskigi plantojn."
Kreski La knabo kreskas. - He grows on his own.
Kreskigi La farmisto kreskigas plantojn - He makes them grow.
Kreskiĝi I really cannot come up with an example, for I have rarely heard/used it. Anyway when you encounter a situation where the subject makes itself grow, you can use kreskiĝi there.
Occam's Razor. The simplest solution is 'la planto kreskis', no matter what reason. I remember (memorigi) it as ~Iği = to BECOME ~, ~igi = to MAKE something ~.
isnt -iĝ to indicate a change? if so doesnt kreskas already indicate a change? then kreskiĝi would be redundant