Is it mentioned anywhere in Valmiki Ramayana that there are plants or trees on the sides of road in Ayodhya?
When King Dasaratha decides to coronate Sri Rama as the Prince (would be king), and requests his priest Vasistha to inform Sri Rama and Sita to fast on that day, next day being the auspicious day for coronation, and follow certain rituals, Vasistha goes to Sri Rama's palace.
It was mentioned in Ayodhya Kanda that all roads were cleaned with water and city was being decorated for the next day's celebration. At that juncture it was mentioned about trees on either side of the roads.
सिक्तसंमृष्टरथ्या हि तदहर्वनमालिनी | आसीदयोध्या नगरी समुच्छ्रितगृहध्वजा || २-५-१८
On that day, in the city of Ayodhya, all the roads were cleaned and sprinkled with water. There were rows of trees on both sides of the roads and flags hoisted on houses.