Nice game.
It could me more exiting to add some mechanic to knock out the bugs for a while. Also making the plants grow stronger the more you water them, so the bugs will take more time to make them sick.
I see you didn't collect enough time energy from diamonds to unlock the way.
In the game, when the player kills diamonds, they drop some time energy. This time energy is what reload the bars. Once the bars are reloaded, they retract and free the way. To reload the bars, you must be in their yellow circle and have enough time energy (bottom center of the screen).
The farer the bars are from 0 minute, the more energy they consume. And that is the problem of the first bar. It is too close from the 59minute's and the bar of 59 consume too more.
The safe area won't make you survive to the laser. It show where you should be to avoid wasting you energy on the 59minute bar.
You can affect a spell to a specific key.
Exple : time=> Q; bomb => W, plant =>E, fire=>R, wind=>T.
Or if you still want to use the grid system, you can affect a specific key to each cell.
I think the first method is better because you wont have to pause the game while the player is casting a spell. He types a combination of keys and the spell casts at the mouse position.
I'm not familiar with resource management games, so it took me a little time to understand what I should do; the tutorial helps. The gameplay was clean and I liked the mood that the music puts. I had some problem with the UI : some times, the panel of information about the planet appear an disappear quickly.
Nice game!