The esp8266/network/thingspeak part has changed alot "lately" and the code is not very clear any more.
Making updating of thingspeak to a separate module, would make the code more readable. Perhaps firstly change that part to a class and then change that to separate module (or "tab").

This could be done prior to the timer change.

Add automatic watering, based on soil moist and time of day.

Add support for lamp, activated based on amount of light and time of day.

In spring time, it is dark very long time in the morning. There is also a possibility of temperatures below zero (centigrade). A lamp could speed up the growth and decrease the possibility of plants freezing...

The periodical pushes to thingspeak, could be done with timers instead of checking in loop, has time passed enough --> cleaner code.
Check which approach to use. Simple timer.h or something more HC? Look for example: http://playground.arduino.cc/Code/Timer https://learn.adafruit.com/multi-tasking-the-arduino-part-2/timer

It should be possible to check state of window, and the forcedly open or close the window.
Same for watering and lamp.

• ESP might not be stable enough.
• We are on limits of Arduino pro mini
  • add more arduinos or replace by Mega
  • check if raspberry-pi or (coming) Omega2 could be used as companion.