I noticed that my strawberries were growing bigger when I watered the plants.

In this context, “strawberries” could refer to strawberry plants or to actual berries. I assume you mean the latter (the berries on the plants are getting bigger), in which case this sentence is fine as-is. But if you mean the plants are getting bigger, then it would be better to say “when I watered them,” as recommended by one of the other correctors.

You might say that's natural, but they had been suffering {from | during} the cold weather and stopped growing for a couple of weeks.

The difference between “suffering something” and “suffering from something” is hard to explain, but in many contexts “I’m suffering X” means “I’m experiencing X, which is unpleasant,” whereas “I’m suffering from something” means “I’m truly suffering, and the cause of my suffering is X.”

In this case, you want the latter: “suffering from/during the cold weather.”

I’m glad that they are alive again.

This is fine, or you could say, “I’m glad they’re beginning to thrive again.”

You might say that's natural, but they had been suffering from the cold weather and stopped growing for a couple of weeks.

I noticed that my strawberries were growing bigger when I watered the plantsm.

I noticed that my strawberries were growing biglarger when I watered the plants.

Your original way was also good👌🏿

You might say that's natural, but they had been suffering from the cold weather and stopped growing for a couple of weeks.