You can Bury Wild Growth in a /castsequence line as Cym recommended and force it to cast Swiftmend before it will allow Wild Growth to cast.
/castsequence Swiftmend, Wild Growth
by itself that line would cast Swiftmend and then Wild Growth and then freeze until Swiftmend comes off CD. Castsequence lines require that the previous spell in the line cast before it can move on to the next. which is why lines like:
/castsequence spell 1, spell 2, null
work. null is an invalid cast and thus once the sequence reaches null that line freezes and cannot move past it. Putting a reset=combat condition on something like that makes it cast that sequence line once per combat period.
The issue is the second part off your request to just cast Wild Growth when Swiftmend is in CD. you can try something like this:
/castsequence Swiftmend, Wild Growth, Essence of G’Hanir
/cast Wild Growth
most of the time that will do what you want, but occasionally due to mathematical probabilities, GCD and other factors you may sit with Swiftmend off CD for a cycle or 2