The ramp / difficulty curve is definitely off! I really need to improve my game balancing skills.
As for the plants, when plants have a low pH they start pulsing in size a bit. Which can help if colors can't be distinguished. Also, when the plant is close to dying there is an alert sound effect that plays for a few seconds and some radial particles that come out of the base. I actually had number of other accessibility ideas but just didn't have enough time to implement them all :(