Not sure overgrowth works.

@@ -23 +23 @@
-<gloss>overgrowth</gloss>
+<gloss>growing lushly</gloss>

繁茂	81625	  
繁茂し	41638	  
繁茂する	16767
繁茂さ	1901	

eijiro has lots of glosses, but:
繁茂する
形
flourishing（植物が）
繁茂して
副
flourishingly
luxuriantly

繁茂
exuberance（植物の）
luxuriance（植物などの）〔【形】luxuriant〕
luxuriant growth（植物などの）
overgrowth〔不可算〕
...

https://context.reverso.net/translation/japanese-english/繁茂
... flourish in the warm Gulf
... flourish in summer
... lush tropical gardens (繁茂した熱帯の庭園)
...  caused by an overgrowth of the yeast fungus ... (酵母菌の繁茂で引き起こされた
... lush forests
... lush aquatic plants
... a landslide or a trail overgrown with brush (繁茂した登山道など)

sankoku and wiktionary's definitions are both "おいしげること".  Our gloss for 生い茂る is: "to grow thickly; to be overgrown; to thrive; to grow in abundance"

usually verbal(almost 75% of ngrams).  reverso often has "lush" especially, then "flourish" and "overgrowth"

Unfortunately I don't see how to get "lush" in here, which seems quite common. Maybe as a separate [exp] entry "繁茂した" if it were worth it.

@@ -19,0 +20,2 @@
+<gloss>flourishing (of plants)</gloss>
+<gloss>thriving</gloss>
@@ -20,0 +23 @@
+<gloss>overgrowth</gloss>

Meikyo vt and vi additions to n,vs entries
-*- via bulkupd.py -*-

@@ -18,0 +19 @@
+<pos>&vi;</pos>