Not an issue

With an if.

Total Times Completed 14151

That doesn't seem unsolvable at all. There are several posts below about that too.

It simply means replacing digits 0-4 with '0' and 5 to 9 with '1' in the string~~

you must square digit by digit not the complete entry

Calculate "the nth day (as an integer) on which the evaporator will be out of use".

Assert.AreEqual(1,kata.GrowingPlant(10,9,4));

If it is that test, you can see it reaches 4 inches mark the first day when going up.

I passed the kata with no problem in C#. Read this and give useful information about the problem, state the language, post your code with markdown formatting and marking your post as having spoiler content.

A problem with your code is not a kata issue. There are already more than 400 solutions in C#.

If the string is not one of the specified characters, the function should return null (throw an ArgumentException in C#).

Then you return null/nil/None.

No, the kata is ok and you didn't understand the instructions, read them again:

you are asked to square every digit of a number.

Not an issue. Your solution is wrong.