Abbreviations – Can ‘PM’ Be Used for ‘Pro Memoria’?

1) A three years old seedling.

2) Three-years-old seedling

3) 3 yr old seedling.

The phrase #1 is grammatically incorrrect because when you use the word "old" to refer to the age of a person or thing, it's a postpositive adjective. For example, you can say "This seedling is three years old, but you cannot say "This is three years old seedling.

The phrase #2 is also incorrect. When you combine the words in the phrase "three years old" by hyphenating them to make it an adjective or a noun, you must use the singular noun (year). So the correct phrase is:

Three-year-old seedling.

As for the phrase #3, it should also be framed in the same pattern as #2 mentioned above;

3-yr-old seedling.

"Sched." could be used for scheduled. Almost any word can be abbreviated by shortening it, given sufficient context, but ask yourself is it really important to save those few letters.

Any form of abbreviation will harm readability. The word "scheduled" appears as part of a sentence. I can't see much benefit in writing (for example) "meeting sched. at 4:30". I wouldn't use the abbreviation as part of a paragraph.

I've never seen skd as an abbreviation, even though I pronounce the word as skedule. It would seem to be even less likely where shedule is the pronunciation.