Here are your corrected sentences:
Tundra plants are hardy but don’t grow tall. A dwarf birch is only knee-high.
Why this is correct (step by step):
- Both sentences start with a capital letter and end with a period — good!
- "don’t" is a fine contraction for talking with friends or in simple writing. If you want to be more formal, you can write "do not."
- Use a hyphen in "knee-high." When two words work together to describe how tall something is, we usually join them with a hyphen: "knee-high plant" or "is knee-high."
- Everything else (word order and meaning) is clear and correct.
Extra examples:
- The grass is ankle-high.
- A knee-high plant is easier to see than a tiny seedling.